LUVOIR

National Aeronautics and Space Administration

www.nasa.gov
Credit for cover art: NASA GSFC / Adriana Manrique Gutierrez (CI Labs)
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report

LUVOIR Study Team

Science and Technology Definition Team – Voting Members

Debra Fischer (Yale University), Co-Chair

Bradley Peterson (Ohio State University / STScI), Co-Chair

Jacob Bean (University of Chicago)

Daniela Calzetti (University of Massachusetts – Amherst)

Rebekah Dawson (Penn State)

Courtney Dressing (University of California – Berkeley)

Lee Feinberg (NASA GSFC)

Kevin France (University of Colorado – Boulder), LUMOS Instrument Lead

Olivier Guyon (University of Arizona / Subaru Telescope)

Walter Harris (University of Arizona / LPL), Solar System Working Group Lead

Mark Marley (NASA Ames), Exoplanet Working Group Lead

Victoria Meadows (University of Washington)

Leonidas Moustakas (JPL)

John O’Meara (St. Michael’s College), Cosmic Origins Working Group Lead

Ilaria Pascucci (University of Arizona / LPL)

Marc Postman (STScI), HDI Instrument Lead

Laurent Pueyo (STScI), ECLIPS Instrument Lead

David Redding (JPL), Technology Working Group Lead

Jane Rigby (NASA GSFC)

Aki Roberge (NASA GSFC), Study Scientist

David Schiminovich (Columbia University)

Britney Schmidt (Georgia Institute of Technology)

Karl Stapelfeldt (JPL)

Christopher Stark (STScI)

Jason Tumlinson (STScI), Simulations Working Group Lead

Science and Technology Definition Team – International Non-Voting 

Members

Martin Barstow (University of Leicester, UK)

Lars Buchhave (DTU Space, National Space Institute, Denmark)

Nicolas Cowan (McGill University, Canada)

José Dias do Nascimento Jr. (Brazilian Federal University, Brazil)

Marc Ferrari (Laboratoire d’Astrophysique de Marseille, France)

Ana Gomez de Castro (Universidad Complutense de Madrid, Spain)

Kevin Heng (University of Bern, Switzerland)

Thomas Henning (Max Planck Institute for Astronomy, Germany)

Michiel Min (Netherlands Institute for Space Research, Netherlands)

Antonella Nota (European Space Agency)

Takahiro Sumi (Osaka University, Japan)
LUVOIR The Large UV Optical Infrared Surveyor

The LUVOIR Interim Report

Science and Technology Definition Team – Ex-Officio Non-Voting Members

Shawn Domagal-Goldman (NASA GSFC), Deputy Study Scientist

Mario Perez (NASA HQ), Program Scientist

Michael Garcia (NASA HQ), Deputy Program Scientist

Susan Neff (NASA GSFC), COR Program Chief Scientist

Erin Smith (NASA Ames), COR Deputy Chief Scientist

LUVOIR Study Office & Engineering Team

Julie Crooke (NASA GSFC), Study Manager

Matthew Bolcar (NASA GSFC), Lead Engineer

Jason Hylan (NASA GSFC), Deputy Lead Engineer

Giada Arney (NASA GSFC), Science Support Analysis Team Lead

Steve Aloezos (NASA GSFC)

Adrienne Beamer (NASA GSFC)

Carl Blaurock (Night Sky Systems)

Vince Bly (NASA GSFC)

Ginger Bronke (NASA GSFC)

Christine Collins (NASA GSFC)

Knicole Colón (NASA GSFC)

James Corsetti (NASA GSFC)

Don Dichmann (NASA GSFC)

Jean-Etienne Dongmo (NASA GSFC)

Lou Fantano (NASA GSFC)

Thomas Fauchez (NASA GSFC / USRA)

Joseph Generie (NASA GSFC)

Gene Gochar (NASA GSFC)

Qian Gong (NASA GSFC)

Tyler Groff (NASA GSFC)

Kong Ha (NASA GSFC)

Bill Hayden (NASA GSFC)

Andrew Jones (NASA GSFC)

Roser Juanola Parramon (NASA GSFC / USRA)

Ravi Kopparappu (NASA GSFC / University of Maryland – College Park)

Irving Linares (NASA GSFC)

Alice Liu (NASA GSFC)

Eric Lopez (NASA GSFC)

Avi Mandell (NASA GSFC)

Bryan Matonak (NASA GSFC)

Sang Park (Smithsonian Astrophysical Observatory)

Shannon Rodriguez (NASA GSFC)

Lia Sacks (NASA GSFC)

Lisa Smith (NASA MSFC)

Hari Subedi (NASA GSFC, Princeton University)

Steve Tompkins (NASA GSFC)
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report

Geronimo Villanueva (NASA GSFC)

Garrett West (NASA GSFC)

Neil Zimmerman (NASA GSFC)

POLLUX Team

Coralie Neiner (Observatoire de Meudon, France), Co-Lead

Jean-Claude Bouret (Laboratoire d’Astrophysique de Marseille, France), Co-Lead

Stéphane Charlot (IAP, France), Extragalactic Science Working Group Lead

Jean-Yves Chauffray (LATMOS, France), Solar System Science Working Group Lead

Chris Evans (University of Edinburgh, UK), Stellar Physics Science Working Group 

Co-Lead

Luca Fossati (Space Research Institute, Austria), Exoplanet Science Working Group 

Lead

Ana Inès Gomez de Castro (Complutense University of Madrid, Spain), Stellar Physics 

Science Working Group Co-Lead

Cecile Gry (LAM, France), ISM/IGM Science Working Group Lead

Pasquier Noterdaeme (IAP, France), Cosmology Science Working Group Lead

Eduard Muslimov (LAM, France), Lead Engineer, Optical Designer

Arturo Lopez Ariste (IRAP, France), Polarimeter Designer

Martin Barstow (University of Leicester, UK), Detector Working Group Manager

David Montgomery (UK Astronomy Technology Center, UK), Mechanical Designer

Pierre Royer (Catholic University of Leuven, Belgium), Calibration Unit Manager

Udo Schuehle (MPS, Germany), Coatings Working Group Manager

Louise Lopes (CNES, France), CNES Manager

Marc Ferrari (LAM, France), CNES Representative on LUVOIR STDT

Dietrich Baade (ESO, Germany)

Richard Desselle (CSL and University of Liège, Belgium)

Boris Gaensicke (University of Warwick, UK)

Miriam Garcia (CAB/CSIC, Madrid, Spain)

Serge Habracken (University of Liège, Belgium)

Jon Lapington (University of Leicester, UK) 

Vianney Lebouteiller (AIM/CEA, France)

Maelle Le Gal (LESIA, France)

Frédéric Marin (Strasbourg Observatory, France)

Fabrice Martins (University of Montpelier, France)

Yael Nazé (University of Liège, Belgium)

Hadi Rahmani (GEPI, France) 

Hugues Sana (Catholic University of Leuven, Belgium)

Steve Shore (University of Pisa, Italy)

Daphne Stam (TU Delft, The Netherlands)

Luca Teriaca (MPS, Germany) 

Jorick Vink (Armagh Observatory, UK)

Huirong Yan (DESY, Germany)
LUVOIR The Large UV Optical Infrared Surveyor

The LUVOIR Interim Report

Community Working Group Members / Additional Contributors

Cosmic Origins

Nate Bastian (Liverpool John Moores University)

Brendan Bowler (University of Texas – Austin)

Charlie Conroy (Harvard University)

Paul Crowther (University of Sheffield)

Selma de Mink (University of Amsterdam)

Ruobing Dong (University of Arizona)

Bruce Elmegreen (IBM)

Steven Finkelstein (University of Texas – Austin)

Brian Fleming (University of Colorado – Boulder) 

Michele Fumagalli (Durham University)

Jay Gallagher (University of Wisconsin – Madison)

Melissa Graham (University of California – Berkeley)

Eva Grebel (University of Heidelberg)

Gregory Herczeg (KIAA / University of Peking)

Benne Holwerda (University of Louisville) 

Christopher Howk (University of Notre Dame)

Mark Krumholz (Australian National University)

Søren S. Larsen (Radboud University) 

Tod Lauer (NOAO)

Janice Lee (IPAC)

Yoshiki Matsuoka (Ehime University)

Stephan McCandliss (Johns Hopkins University)

Stella Offner (University of Texas – Austin)

Ian Roderer (University of Michigan)

Elena Sabbi (STScI)

P. Christian Schneider (University of Hamburg)

Vicky Scowcroft (University of Bath)

Paul Scowen (Arizona State University)

Warren Skidmore (Thirty Meter Telescope)

Linda Smith (ESA / STScI)

Russell Smith (Durham University)

Rachel Somerville (Rutgers University)

Harry Teplitz (IPAC)

Christy Tremonti (University of Wisconsin – Madison)

Kate Whitaker (University of Massachusetts – Amherst)

Gerard Williger (University of Louisville)

Rosemary Wyse (Johns Hopkins University)

Allison Youngblood (NPP / NASA GSFC) 

Exoplanets

Eric Agol (University of Washington)

Daniel Apai (University of Arizona)
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report

Natalie Batalha (NASA Ames)

Ruslan Belikov (NASA Ames)

Luca Fossati (Austrian Academy of Sciences)

Yuka Fujii (Tokyo Institute of Technology)

Claire Marie Guimond (McGill University)

Eric Hébrard (University of Exeter) 

Tiffany Jansen (Columbia University)

Eliza Kempton (Grinnell College / University of Maryland) 

Brianna Lacy (Princeton University)

Andrew Lincowski (University of Washington)

Jacob Lustig-Yeager (University of Washington)

Timothy Lyons (University of California Riverside)

Gijs Mulders (LPL)

Stephanie Olson (University of California Riverside)

Ramses Ramirez (Tokyo Institute of Technology)

Christopher Reinhard (Georgia Tech)

Tyler Robinson (Northern Arizona University)

Edward Schwieterman (University of California – Riverside)

Stoney Simons (National Institutes of Health)

Dillon Teal (NASA GSFC)

Guadalupe Tovar (University of Washington)

Sara Walker (Arizona State University) 

Solar System

Alvaro Alvarez-Candal (Observatorio Nacional, Brazil)

Gerbs Bauer (JPL)

Dennis Bodewits (University of Maryland – College Park)

Valeria Cottini (NASA GSFC) 

Lori Glaze (NASA GSFC) 

Jeff Morgenthaler (Planetary Science Institute)

Alex Parker (Southwest Research Institute)

Noah Petro (NASA GSFC)

Noemi Pinilla-Alonso (University of Central Florida)

Silvia Protopapa (University of Maryland – College Park)

Andrew Rivkin (Johns Hopkins University – Applied Physics Lab)

Technology and Design

Lynn Allen (Harris Corporation)

David Allred (Brigham Young University)

Jon Arenberg (Northrop Grumman)

Kunjithapatham Balasubramanian (JPL)

Scott Basinger (JPL)

Ruslan Belikov (NASA Ames)

Ray Bell (Lockheed Martin)

Paul Bierdan (Boston Micromachines)
LUVOIR The Large UV Optical Infrared Surveyor

The LUVOIR Interim Report

Ron Broccolo (Harris Corporation)

Eric Cady (JPL)

Kerri Cahoy (Massachusetts Institute of Technology)

Jeff Cavaco (Northrop Grumman)

Alberto Conti (Northrop Grumman)

Jim Contreras (Ball Aerospace)

Laura Coyle (Ball Aerospace)

Brendan Crill (JPL)

Javier Del Hoyo (NASA GSFC)

Larry Dewell (Lockheed Martin)

Jeanette Domber (Ball Aerospace)

Matthew East (Harris Corporation)

Mike Eisenhower (Smithsonian Astrophysical Observatory)

Michael Feinberg (Boston Micromachines)

Greg Fellers (Lockheed Martin)

James R. Fienup (University of Rochester)

Don Figer (Rochester Institute of Technology)

Brian Fleming (University of Colorado – Boulder)

Kevin Fogarty (STScI)

Pascal Hallibert (ESA)

Erika Hamden (California Institute of Technology)

Alex Harwit (Ball Aerospace)

Michael Helmbrecht (Iris AO)

John Hennessy (JPL)

Peter Hill (Zygo / Ametek)

Joe Ho (Ball Aerospace)

Sona Hosseini (JPL)

Tony Hull (University of New Mexico)

Jeffrey Jewell (JPL)

Jeremy Kasdin (Princeton University)

Scott Knight (Ball Aerospace)

Mary Li (NASA GSFC)

Paul Lightsey (Ball Aerospace)

Chris Lindensmith (JPL)

Sarah Lipscy (Ball Aerospace)

John Lou (JPL)

Makenzie Lystrup (Ball Aerospace)

Gary Matthews (ATA Aerospace)

Stephan McCandliss (Johns Hopkins University)

Ted Mooney (Harris Corporation)

Dustin Moore (JPL)

Didier Morancais (Airbus)

Mamadou N’Diaye (Université Côte d’Azur)

Shouleh Nikzad (JPL)

Joel Nissen (JPL)
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report

Alison Nordt (Lockheed Martin)

Jim Oschmann (Ball Aerospace)

Sang Park (Smithsonian Astrophysical Observatory)

Enrico Pinna (Istituto Nazionale di Astrofisica)

Bill Purcell (Ball Aerospace)

Manuel Quijada (NASA GSFC)

Bernie Rauscher (NASA GSFC)

Norman Rioux (NASA GSFC)

Michael Rodgers (Synopsys / JPL)

Garreth Ruane (Caltech)

Derek Sabatke (Ball Aerospace)

John Sadleir (NASA GSFC)

Babak Saif (NASA GSFC)

Eric Schindhelm (Ball Aerospace)

Gene Serabyn (JPL)

Stuart Shaklan (JPL)

Chris Shelton (JPL)

Fang Shi (JPL)

Evgenya Shkolnik (Arizona State University)

Nick Siegler (JPL)

Remi Soummer (STScI)

Joe Sullivan (Ball Aerospace)

Phil Stahl (NASA MSFC)

Kathryn St Laurent (STScI)

Kiarash Tajdaran (Lockheed Martin)

John Trauger (JPL)

Steve Turley (Brigham Young University)

John Vayda (Northrop Grumman)

Michael Werner (JPL)

Scott Will (University of Rochester)

Marco Xompero (National Institute of Astrophysics, Italy)

David Yanatsis (Harris Corporation)

John Ziemer (JPL)

Acknowledgements

The STDT would like to thank Daniel Apai (University of Arizona), Steven Finkelstein (University 

of Texas – Austin), Luca Fossati (Austrian Academy of Sciences), and Tyler Robinson (Northern 

Arizona University) for their major contributions to the science cases that appear in this report. 

We also thank Brian Fleming (University of Colorado – Boulder), John MacKenty (STScI), and 

Stephan McCandliss (Johns Hopkins University) for their extensive participation in instrument 

design. We are very grateful to our industrial cooperative agreement partners: Northrop 

Grumman, Lockheed Martin, Ball Aerospace, and Harris Corporation. We express sincere 

appreciation to the LUVOIR Senior Advisors group, Natalie Batalha (NASA Ames), Julianne 

Dalcanton (University of Washington), Alan Dressler (Carnegie Observatories), Jay Gallagher 

(University of Wisconsin – Madison), Jonathan Fortney (University of California – Santa Cruz), 
LUVOIR The Large UV Optical Infrared Surveyor

The LUVOIR Interim Report

and Garth Illingworth (University of California – Santa Cruz), for their valuable guidance and 

feedback. Finally, we thank Pat Tyler, Jay Friedlander, T. Britt Griswold (NASA GSFC), Chad 

Smith, and Graham Kanarek (STScI) for their hard work laying out this document, creating 

graphics, and writing simulation tools. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report i

Contents

1 LUVOIR: Telling the Story of Life in the Universe ................................................... 1-2

2 Introduction.......................................................................................................... 2-2

2.1 The worlds around us .................................................................................. 2-3

2.1.1 The census of Earth-like exoplanets................................................. 2-3

2.1.2 The search for life............................................................................. 2-7

2.1.3 The solar system’s place in the planet family .................................... 2-9

2.2 The evolution of the cosmos ...................................................................... 2-10

2.2.1 The lives of galaxies ....................................................................... 2-11

2.2.2 The far reaches of space and time ................................................. 2-12

2.2.3 The origins of stars and planets ..................................................... 2-12

2.3 A big observatory for big goals................................................................... 2-13

2.3.1 The LUVOIR mission concepts....................................................... 2-14

2.3.2 Designing for serviceability ............................................................. 2-15

2.3.3 Enabling LUVOIR ........................................................................... 2-16

2.4 What is the difference between LUVOIR and HabEx?................................. 2-17

2.5 The LUVOIR Interim Report........................................................................ 2-18

References......................................................................................................... 2-19

3 Is there life elsewhere? Habitable exoplanets and solar system ocean worlds....... 3-2

3.1 A habitable and inhabited planet: The Earth through time............................. 3-4

3.2 Signature science case #1: Which worlds are habitable?.............................. 3-9

3.2.1 Habitable planetary environments .................................................. 3-10

3.2.2 Ocean/icy worlds of the solar system............................................. 3-17

3.2.3 Strategy for identifying habitable exoplanets................................... 3-20

3.2.4 Exoplanet yields for the baseline LUVOIR-A concept...................... 3-22

3.3 Signature science case #2: Which worlds are inhabited? ........................... 3-26

3.3.1 The search for biosignatures .......................................................... 3-27

3.3.2 False positives for life ..................................................................... 3-30

3.4 The strategy for habitability and biosignatures confirmation ........................ 3-33

3.5 Summary ................................................................................................... 3-38

References......................................................................................................... 3-40

4 How do we fit in? Planetary systems and comparative planetary science ............. 4-2

4.1 Signature science case #1: Comparative atmospheres ................................ 4-4

4.1.1 Diversity of composition ................................................................... 4-5

4.1.2 Photochemistry and hazes............................................................... 4-8

4.1.3 Clouds ........................................................................................... 4-11

4.1.4 Atmospheric evolution and escape ................................................ 4-12

4.2 Signature science case #2: Planetary system architectures ........................ 4-15

4.2.1 Mature exoplanet systems ............................................................. 4-16

4.2.2 Young exoplanet systems .............................................................. 4-18

4.2.3 Solar System comets, asteroids, and KBOs................................... 4-23
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ii The LUVOIR Interim Report

4.3 Summary ................................................................................................... 4-27

References......................................................................................................... 4-29

5 How do galaxies evolve?...................................................................................... 5-2

5.1 Signature science case #1: The cycles of matter.......................................... 5-3

5.1.1 The essential ultraviolet .................................................................... 5-5

5.1.2 Gas flows in absorption: More sources, deeper limits, earlier times .. 5-7

5.1.3 Mapping the cycles of matter with the faintest light in the universe. 5-11

5.1.4 Gas flows and quenching............................................................... 5-15

5.1.5 Mapping inflow and outflow with down-the-barrel spectroscopy .... 5-15

5.2 Signature science case #2: The multiscale assembly of galaxies ................ 5-19

5.2.1 Galaxy assembly at the faint frontier............................................... 5-19

5.2.2 Seeing inside galaxies as they form and transform ......................... 5-22

5.3 Dissecting galaxies one star at a time......................................................... 5-25

References......................................................................................................... 5-27

6 What are the building blocks of cosmic structure?................................................ 6-2

6.1 Signature Science Case #1: Connecting the smallest scales across 

cosmic time ................................................................................................. 6-4

6.1.1 Dwarf galaxies in the local universe: testing dark matter models....... 6-6

6.1.2 Dwarfs in the distant universe: probing the impact of reionization... 6-10

6.2 Signature Science Case #2: Constraining dark matter via high precision 

astrometry.................................................................................................. 6-13

6.3 Signature Science Case #3: Tracing ionizing light over cosmic time............ 6-17

References......................................................................................................... 6-22

7 How do stars and planets form? .......................................................................... 7-2

7.1 Signature science case #1: How do stars form? .......................................... 7-2

7.1.1 Very massive stars ........................................................................... 7-4

7.1.2 Stellar multiplicity ............................................................................. 7-6

7.1.3 The low-end of the IMF in unresolved populations............................ 7-7

7.2 Signature science case #2: How and in what environments do planets 

assemble? ................................................................................................. 7-10

7.2.1 Detect and characterize protoplanets............................................. 7-11

7.2.2 Trace the composition, evolution, and dispersal of planet-forming 

material.......................................................................................... 7-12

7.2.3 Map protoplanetary material dispersed via disk winds.................... 7-13

7.2.4 Star and planet formation in Orion.................................................. 7-13

7.3 Signature science case #3: The rise of the periodic table ........................... 7-15

7.3.1 The first stars and the first metals................................................... 7-15

7.3.2 The origin of elements heavier than iron ......................................... 7-16

References......................................................................................................... 7-19

8 The LUVOIR architectures .................................................................................... 8-2

8.1 Study philosophy ......................................................................................... 8-2

8.2 Architecture A .............................................................................................. 8-4
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The LUVOIR Interim Report iii

8.2.1 Design overview............................................................................... 8-4

8.2.2 Design drivers ................................................................................ 8-13

8.2.3 Design alternatives......................................................................... 8-22

8.3 Looking ahead ........................................................................................... 8-26

References......................................................................................................... 8-27

9 The LUVOIR telescope and instruments ............................................................... 9-2

9.1 Optical telescope element (OTE) .................................................................. 9-2

9.1.1 OTE overview................................................................................... 9-2

9.1.2 OTE design drivers........................................................................... 9-3

9.1.3 OTE design implementation ............................................................. 9-3

9.2 Backplane support frame (BSF).................................................................. 9-12

9.2.1 BSF overview................................................................................. 9-12

9.2.2 BSF design drivers......................................................................... 9-12

9.2.3 BSF design implementation ........................................................... 9-12

9.3 Science instruments................................................................................... 9-15

9.3.1 Overview........................................................................................ 9-15

9.3.2 Extreme Coronagraph for Living Planetary Systems (ECLIPS)......... 9-17

9.3.3 LUVOIR Ultraviolet Multi-Object Spectrograph (LUMOS) ................ 9-24

9.3.4 High-Definition Imager (HDI) ........................................................... 9-29

9.4 Alternate instruments and design implementations..................................... 9-33

References......................................................................................................... 9-34

10 POLLUX: European Study of a UV Spectropolarimeter....................................... 10-2

Study leads and contact information .................................................................. 10-2

Participating institutes ........................................................................................ 10-2

10.1 Executive summary.................................................................................... 10-2

10.2 Science overview ....................................................................................... 10-3

10.2.1 Stellar magnetic fields across the HR diagram................................ 10-3

10.2.2 What are the characteristics of exoplanet atmospheres and how 

do planets interact with the host stars?.......................................... 10-4

10.2.3 The various phases of the ISM and extragalactic IGM .................... 10-6

10.2.4 Beyond the unresolvable regions of active galactic nuclei: 

revealing accretion disk physics, dust composition and magnetic 

field strength with UV polarimetry ................................................... 10-7

10.2.5 Testing fundamental physics and cosmology using absorption 

lines towards quasars .................................................................... 10-9

10.2.6 Solar system: Surfaces, dust scattering, and auroral emissions.... 10-10

10.3 Design drivers .......................................................................................... 10-11

10.4 Design overview & implementation........................................................... 10-11

10.4.1 Baseline optical architecture and specifications............................ 10-12

10.4.2 Polarimeters................................................................................. 10-14

10.4.3 Mechanical architecture ............................................................... 10-15

10.4.4 Detectors..................................................................................... 10-17

10.4.5 Operation modes ......................................................................... 10-17
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10.4.6 Performance evaluation................................................................ 10-18

10.4.7 Future developments for POLLUX................................................ 10-19

10.5 Engineering and technology challenges.................................................... 10-19

References....................................................................................................... 10-23

11 LUVOIR technology development....................................................................... 11-2

11.1 Introduction................................................................................................ 11-2

11.2 Coronagraph technology: High contrast imaging with an obscured 

aperture ..................................................................................................... 11-3

11.2.1 Coronagraph architectures............................................................. 11-4

11.2.2 Deformable mirrors ........................................................................ 11-9

11.3 Active optics: An architecture for ultra-stability ......................................... 11-11

11.3.1 Finding the minimum-cost, maximum-performance solution......... 11-12

11.3.2 Wavefront sensing and control concurrent with coronagraph 

observations: LOWFS, OBWFS, and artificial guide stars ............. 11-12

11.3.3 Metrology of the telescope optics: Edge sensors and laser 

trusses......................................................................................... 11-15

11.3.4 Non-contacting disturbance isolation ........................................... 11-18

11.3.5 Disturbance avoidance: Micro-thrusters for attitude control.......... 11-20

11.3.6 Active control of the telescope wavefront .................................... 11-20

11.3.7 Primary mirror segments .............................................................. 11-22

11.3.8 UV reflective coatings................................................................... 11-24

11.4 Instrument technologies ........................................................................... 11-25

11.4.1 Detectors for LUVOIR instruments ............................................... 11-25

11.4.2 Microshutter arrays ...................................................................... 11-29

11.5 The technology path ahead...................................................................... 11-29

References....................................................................................................... 11-30

12 LUVOIR Cycle 1 ................................................................................................. 12-2

12.1 Doing the impossible.................................................................................. 12-2

12.2 A transformational first year of science ....................................................... 12-2

13 Appendix A: Further LUVOIR science cases ......................................................... A-1

13.1 Observations of Venus with LUVOIR............................................................. A-2

13.2 Exo-cartography for terrestrial planets.......................................................... A-6

13.3 Extragalactic massive stars .......................................................................... A-9

13.4 Ultraviolet haloes around edge-on galaxies................................................. A-12

13.5 Prospects for mapping terrestrial exoplanets with LUVOIR......................... A-15

13.6 Spatially resolved maps of star-forming gas with LUVOIR........................... A-18

13.7 The quiescent UV spectra of cool dwarf stars ............................................ A-20

13.8 Exoplanet diversity in the LUVOIR era ........................................................ A-22

13.9 Detecting exomoons with LUVOIR ............................................................. A-24

13.10 LUVOIR for stars, stellar evolution, and the local universe........................ A-29

13.11 UV characterization of exoplanet host stars: keys to atmospheric 

photochemistry and evolution.................................................................. A-35

13.12 White dwarfs as probes of fundamental astrophysics .............................. A-38
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13.13 Small bodies of the inner solar system..................................................... A-42

13.14 Resolved photometry of young super star clusters .................................. A-45

13.15 The detection of terrestrial planets in the habitable zones of A-stars ........ A-48

13.16 Quest for the first quasars ....................................................................... A-50

13.17 Protostellar outflows/jets ......................................................................... A-53

13.18 Exploring the high energy processes in microquasars as an exemplar 

case for high time resolution astrophysics................................................ A-56

13.19 Comets and minor planets: the importance of the small things ................ A-58

13.20 Detecting liquid surface water on exoplanets........................................... A-63

13.21 Geology and surface processes in the solar system ................................ A-68

13.22 Transit spectroscopy of Earth-sized planets around M-dwarfs ................. A-70

13.23 The transmission spectra of rock atmospheres on magma worlds........... A-73

13.24 A statistical search for global habitability and biospheres beyond Earth ... A-76

14 Appendix B: Science calculation and simulation details ........................................B-1

14.1 Estimating LUVOIR’s exoplanet yield ............................................................B-1

14.2 The LUVOIR science simulation tools .........................................................B-12

15 Appendix C: Complete science traceability matrix for LUVOIR-A ..........................C-1

16 Appendix D: Further technical information ............................................................D-1

16.1 LUVOIR telescope temperature considerations ............................................D-2

16.2 Getting to orbit: launch vehicles ...................................................................D-9

16.3 Starlight suppression with coronagraphs....................................................D-12

16.4 Impacts of optical coatings on polarization and coronagraphy....................D-15

16.5 UV coatings and short-wavelength cutoff ...................................................D-18

16.6 Ultraviolet detectors for cosmic origins and exoplanet science with 

LUVOIR......................................................................................................D-21

17 Appendix E: Additional LUVOIR instrument concepts ........................................... E-1

17.1 Starshades for LUVOIR................................................................................ E-2

17.2 Optical/Near-Infrared Spectrograph (ONIRS) ................................................ E-7

17.3 Extension of LUVOIR’s capabilities to 5 mm or beyond ................................. E-9
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Chapter 1
LUVOIR The Large UV Optical Infrared Surveyor

1-2 The LUVOIR Interim Report

1 LUVOIR: Telling the Story of Life in the Universe

Interim Report Executive Summary

Over four hundred years have passed 

since Galileo’s first observations of the 

heavens fundamentally transformed 

humanity’s understanding of the cosmos. 

We have learned that we live not in a static 

eternal universe, but in an expanding one, 

comprised of matter, radiation, dark matter, 

and dark energy, with the latter two still poorly 

understood. We exist not in an island universe, 

but instead within one of hundreds of billions 

of galaxies, comprised of tens of trillions of 

stars. We have seen that our solar system 

is not alone, but instead one of unknown 

billions of planetary systems in the Milky 

Way. These discoveries have emerged from 

a history of advances in astronomical theory, 

observation, and technology culminating 

in telescopes on the ground and in space 

covering every observable wavelength of 

light, new frontiers in gravitational wave and 

neutrino observations, and entire universes 

residing in simulations on supercomputers. 

Despite all these triumphs, many basic, 

fundamental, and essential questions 

remain: Is there life elsewhere? Is our world 

unique? How do stars form? What are 

the building blocks of structure? How did 

our galaxy, solar system, and Earth arise 

and evolve? These questions demand an 

observatory beyond any in existence or in 

development to answer. They require a large 

aperture to capture the very faintest objects 

across cosmic time. They require spatial 

resolution high enough to separate planets 

from their host stars and the ability to block 

out the latter’s blinding light. They require 

access to a range of wavelengths broad 

enough to read the atomic fingerprints of 

matter across all temperatures and densities. 

In this document, we describe such an 

observatory: The Large Ultraviolet / Optical / 

Infrared Surveyor (LUVOIR) mission concept. 

LUVOIR represents an ambitious yet feasible 

observatory designed to answer not only 

the questions astronomy presents today, 

but also the as of yet unknown questions of 

tomorrow. 

LUVOIR’s main features are:

• A large (15-m LUVOIR-A, 8-m LUVOIR-B), 

segmented aperture designed to fit in a 

single launch vehicle.

• Scalable, serviceable architecture design 

for an observatory at Earth-Sun L2.

• A total wavelength range of 100 nm–2.5 

μm.

• ECLIPS: An ultra-high contrast coronagraph with an imaging camera and integral field spectrograph spanning 200–

2000 nm, capable of directly observing a 

wide range of exoplanets and obtaining 

spectra of their atmospheres.

• HDI: A near-UV to near-IR imager covering 

200–2500 nm, Nyquist sampled at 400 

nm and diffraction limited at 500 nm, with 

high precision astrometry capability.

• LUMOS: A far-UV imager and far-UV + 

near-UV multi-resolution, multi-object 

spectrograph covering 100 nm–400 nm, 

capable of simultaneous observations of 

up to hundreds of sources.

• POLLUX: A UV spectropolarimeter covering 100–400 nm with high-resolution 

point-source spectroscopic capability. 

A consortium of European institutions, 

with leadership from the French Space 

Agency, is contributing this instrument 

concept study.

LUVOIR continues the vision of the Great 

Observatories as a community-driven facility 

with the power and capability to answer 

scientists’ questions across the full NASA 
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portfolio of Cosmic Origins, Physics of the 

Cosmos, Exoplanet, and Solar System 

Exploration science. These science areas 

include:

• Exploring the full diversity of exoplanets.

• Discovering and characterizing exoplanets in the habitable zones of Sun-like stars 

across a range of ages and searching for 

biosignatures in their atmospheres, in a 

survey large enough to provide evidence 

for (or against) the presence of habitable 

planets and life.

• Remote sensing of the planets, moons, 

and minor bodies of the solar system.

• Exploring the building blocks of galaxies 

both in the local universe and at their 

Figure 1.1. Illustration of LUVOIR’s range of science and capability as compared to other 

facilities. LUVOIR dramatically increases the sample size and diversity of exoplanets that can be 

studied, pushing to dozens of Earth-like planets around Sun-like stars. LUVOIR’s sensitivity and 

spatial resolution opens access to the ultra-faint and ultra-distant regime, enabling observations 

of individual stars in the full variety of galaxies. Those capabilities also create new opportunities 

in remote sensing of solar system bodies. Credit: NASA / New Horizons / J. Friedlander & T. B. 

Griswold (NASA GSFC)

emergence in the distant past, and 

elucidating the nature of dark matter.

• Understanding how galaxies form and 

evolve from active to passive, both by 

studying their stars and their gaseous 

fuel across all temperatures and phases.

• Following the history of stars in the local 

volume out to tens of megaparsecs to 

understand how they form and how they 

depend on their environment.

• Observing the birth of planets and 

understanding how the diversity of 

planetary systems arises.

A transformative facility like LUVOIR with 

its large aperture and need for ultra-high 

contrast observations will require maturation 
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of some key technologies, including 

coronagraph design, wavefront sensing and 

control, and ultra-stable structures. With 

input from NASA, the aerospace industry, 

and commercial partners, this study has 

identified the technological gaps and has 

defined a path to realize LUVOIR. The 

observatory is being designed to leverage:

• Segmented mirror technologies from 

the ground and the James Webb Space 

Telescope (JWST)

• Coronagraph development from the 

ground and the Wide Field Infrared 

Survey Telescope (WFIRST)

• Serviceable design heritage from the 

Hubble Space Telescope (HST) and other 

missions

• Detector and optics technologies from 

multiple missions

• Industry and commercial innovations in 

vibration isolation and launch vehicles

LUVOIR’s power, flexibility, and 

serviceability ensure that it can perform not 

only the “signature” science examples in 

this report, but also the unknown science 

of the future. It is an observatory to serve 

the astronomical community not just for the 

2030s, but also for many decades to follow.
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Chapter 2
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2 Introduction

Humanity is defined by the drive to know 

about our place within the world around us. 

The very name homo sapiens captures what 

we prize in ourselves, and our never-ending 

quest for wisdom leads us ever further 

outward. The value of that quest for the 

betterment of our species is immeasurable. 

Ages-old questions and investigations 

earned us the revelations that the stars are 

Suns swirling in a vast galaxy, itself one of 

a myriad of islands in a boundless cosmos. 

Now we have crossed another threshold of 

discovery: there are planets around other 

stars. Tracing a path from the dawn of the 

universe to life-bearing worlds is within our 

grasp. The further revelations we will find on 

that path have long belonged to the realm of 

speculation—now we begin to find out if our 

imaginary musings are real. 

This monumental objective demands 

powerful and flexible new tools, and different 

ways of combining our scientific skills. We 

have found that the range and diversity of 

worlds is far greater than we imagined, yet 

we see planetary systems reminiscent of 

the solar system as well. At the moment, 

the vast majority of known exoplanets are 

“small black shadows” indirectly detected 

through their effect on their host stars. Our 

knowledge of exoplanet properties is largely 

limited to orbits, masses, and sizes. We 

have just begun to measure the atmospheric 

constituents of gas giant exoplanets; such 

studies will greatly expand in the coming 

years.

The next frontier is to extend these 

capabilities to rocky planets and find the “pale 

blue dots” in the solar neighborhood (Figure 

2.1). We can determine whether habitable, 

Earth-like conditions are common or rare 

on nearby worlds and probe them for signs 

of life. Concurrently, we will nurture a new 

discipline—comparative exoplanetology—by 

studying a huge range of exoplanets, thereby 

gaining invaluable information for placing 

the solar system in the broader context of 

planetary systems. A vital part of establishing 

that context is deeper understanding of the 

bodies in our own system.

Our drive to know goes beyond asking the 

question “what exists?” to “why does it exist?” 

and pushes us to understand the origins of 

all we see around us. The boundary of what 

we can see now stretches all the way to the 

dawn of the universe, but like our first steps 

in the study of new worlds, our vision lacks 

both completeness and context. We seek to 

understand the environments and processes 

that gave rise to a life-supporting cosmos, 

from the formation of the earliest structures 

and the rise of the elements essential for life 

Figure 2.1. Imaging Earth 2.0. Simulation of the 

inner solar system viewed at visible wavelengths 

from a distance of 13 parsec with LUVOIR. The 

enormous glare from the central star has been 

suppressed with a coronagraph so the faint 

planets can be seen. The atmosphere of each 

planet can be probed with spectra to reveal 

its composition. Credit: L. Pueyo, M. N’Diaye 

(STScI)/A. Roberge (NASA GSFC)
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Figure 2.2. From galaxy evolution to star and planet formation to habitable worlds. Credits: 

NASA/ESA/Hubble; ESO/L. Calcada; NASA Ames/SETI Institute/JPL-Caltech

as we know it, to the assembly and evolution 

of galaxies, to the detailed mechanisms 

of star and planet formation (Figure 2.2). 

The boundaries of physics will be tested 

and stretched while exploring the birth and 

evolution of the cosmos.

In subsequent chapters of this report, 

we will delve into far greater detail on all of 

these scientific questions and investigations, 

which define the tool we need to leap 

forward. That is the Large Ultraviolet/Optical/

Infrared Surveyor (LUVOIR), a concept for 

an ambitious future space telescope in the 

tradition of NASA’s Great Observatories. 

LUVOIR had its genesis in the 2013 NASA 

Astrophysics Roadmap “Enduring Quests, 

Daring Visions.” The current LUVOIR study 

builds upon telescope concepts going back 

decades (e.g., ATLAST, the High Definition 

Space Telescope) but goes far beyond 

them in both scientific and technical detail. 

LUVOIR will provide the powerful observing 

capabilities required to execute the 

revolutionary investigations we will discuss. 

Furthermore, LUVOIR’s power, flexibility, 

and longevity will allow it to answer the asyet-unknown questions of the 2030s and 

beyond.

2.1 The worlds around us

We have long speculated about the existence of planets outside the solar system. 

The names of imagined worlds are familiar 

to us: Tatooine, Vulcan, Arrakis. Our own 

planetary system is largely bimodal in nature, 

with small rocky planets close to the Sun and 

massive gas giants in the cold outer reaches. 

Scientists naturally expected that other systems would be similar—if they existed at all. 

In the mid-1990s, decades of persistence 

and technological innovation were finally rewarded with the discovery of the first exoplanets. We have been both shocked and 

delighted to find that exoplanets are both 

common and diverse—reality far surpassed 

our scientific predictions. We can now look 

into the sky and realize that most of the stars 

we see harbor worlds. Soon, we will more 

fully reveal the character of giant planets, 

which will further stretch the bounds of our 

theories. The next steps in this quest are 

even more amazing.

2.1.1 The census of Earth-like 

exoplanets

Indirect planet discovery techniques, like 

the transit method used by NASA’s Kepler 

Mission, have shown that small rocky planets 

are not rare. Kepler indicates that about 24% 

of sun-like stars have roughly Earth-size 

planets in their habitable zones (Kopparapu 

et al. 2018, and references therein). These 

zones span the range of distances from the 

stars where we think rocky planets can have 

liquid water—an essential material for all life 

on Earth—on their surfaces. This revelation 

drives us to proceed to the next step: build 

the capabilities to reveal the character of 
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rocky exoplanets and measure the fraction 

that are truly Earth-like. We need to probe 

rocky planet atmospheres and determine 

their thermal/chemical states by measuring 

their molecular abundances, including water 

vapor and other greenhouse gases. 

Astronomers and planetary scientists 

have calculated that to achieve this goal 

for Earth-like planets around Sun-like stars, 

we must collect and analyze light from the 

planets themselves, i.e., obtain direct images 

and spectra (e.g., Kaltenneger & Traub 

2009, Snellen et al. 2015). But doing this for 

habitable planet candidates around Sun-like 

stars is made extremely challenging by the 

fact that the Earth is 10 billion times fainter 

than the Sun and orbits close to its host star. 

When viewed from an interstellar distance of 

10 parsecs, the apparent separation of the 

Earth from the Sun is only about the width of 

a human hair at the distance of two football 

fields.

The first direct images of exoplanets 

have been obtained with ground-based 

observatories, revealing the relatively faint 

planets by suppressing the bright light 

from the central stars using coronagraph 

instruments (Figure 2.3). High-performance 

coronagraphs on future 30-meter-class 

ground-based telescopes (the Extremely 

Large Telescopes, or ELTs) may be able 

to study the atmospheres of rocky planets 

orbiting in the habitable zones of the nearest 

cool red dwarf stars (e.g., Wang et al. 2017). 

However, direct observations of rocky 

exoplanets around Sun-like stars demand 

the ultra-high contrast possible only with a 

space-based telescope coupled to a highperformance starlight suppression system 

(Figure 2.1).

Furthermore, measuring the frequency of 

Earth-like conditions requires observations 

of a large number of candidate exoplanets 

(dozens; Figure 2.4). To find and study that 

many candidates, an even larger number of 

stars must be surveyed (hundreds). This goal 

of quantitatively measuring the frequency 

of habitable conditions on rocky worlds 

is a strong driver on the required space 

telescope size. By executing a systematic 

study of a large number of rocky exoplanets, 

we can guarantee that whatever we find 

will be of scientific value. In the absence of 

positive detections of water vapor, we would 

know that global-scale surface oceans 

are relatively rare on rocky worlds in the 

habitable zone. This null result would still 

transform our understanding of habitability 

as a planetary process, and further confirm 

the special nature of our home. On the other 

hand, if we find that Earth-like conditions are 

common on rocky worlds near our Sun, then 

a stunning vista of hospitable new worlds will 

be unveiled.

Figure 2.3. Direct image of four young, warm 

super-Jupiters orbiting the star HR8799. The 

bright light from the central star has been 

suppressed with a coronagraph. Credit: NRCHIA/C. Marois/Keck Observatory. 
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LUVOIR Frequently Asked Questions

How big is LUVOIR?

We are developing 2 architectures, LUVOIR-A and LUVOIR-B. LUVOIR-A has a primary mirror 

with a diameter of 15 meters. This size corresponds to the largest observatory that can be 

launched in a SLS Block 2 vehicle. LUVOIR-B has a primary mirror with a diameter of 8 meters, 

which is the largest observatory that can be launched in a 5-meter fairing similar to those in use 

today). [Chapter 8]

What if the SLS Block 2 launch vehicle is not available in the 2030s?

LUVOIR’s segmented design facilitates scalability, as will be demonstrated with LUVOIR-B. 

LUVOIR can take advantage of future opportunities in heavy lift commercial launch vehicles with 

large fairings (e.g., Blue Origin’s New Glenn rocket and SpaceX’s BFR). [Appendix D]

How long will LUVOIR last? 

The prime mission is 5 years, which is the standard prime mission duration for large space 

missions, with 10 years of on-board consumables. Furthermore, LUVOIR is designed to be 

serviceable and upgradeable, with a lifetime goal of 25 years for non-serviceable components. 

[Chapter 8]

What are LUVOIR’s key science goals?

This report describes a number of “signature science” programs that demand LUVOIR. 

Additionally, we are actively collecting community input on science with LUVOIR and have 

developed a number of public on-line tools to help create science programs (https://asd.gsfc.

nasa.gov/luvoir/tools/). Finally, LUVOIR is being designed to be flexible and powerful enough to 

enable the as-of-yet unknown science of the 2030s and beyond. [Chapters 3–7, Appendix A]

How much of LUVOIR’s time will be for community observers?

LUVOIR is envisioned as a facility in the tradition of NASA’s Great Observatories (Hubble, 

Compton, Chandra, Spitzer), with Guest Observer-driven operations. [Chapter 12]

Why is direct spectroscopy of habitable exoplanet candidates a key goal for LUVOIR?

Both scientists and the public are strongly motivated to find habitable planets around a range of 

stars and see if any show signs of life as we know it. Achieving this goal for the exoplanet systems 

most like the solar system—ones with Earth-like planets around Sun-like stars—requires direct 

spectroscopy of light coming from the planets after suppression of starlight with a coronagraph 

or starshade. [Chapter 3]

Why is LUVOIR focused on exoplanet spectroscopy at near-ultraviolet/visible/nearinfrared wavelengths?

This wavelength range contains key molecular markers of habitable surface conditions (e.g., water 

vapor, CO2), atmospheric biosignatures (e.g., O2, ozone), and major atmospheric constituents for 

a wide range of exoplanets (e.g., methane). [Chapters 3 & 4]

Can high-performance coronagraphy be done with an obscured, segmented telescope?

While most coronagraphs perform better with unobscured telescopes, recent advances via 

ground-based instruments and the WFIRST coronagraph technology demonstration program 

have shown that high-performance coronagraphs can be designed for obscured apertures. 

Other studies have shown that telescope segmentation has only a minor impact on coronagraph 

performance. [Chapters 9 & 11]
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LUVOIR Frequently Asked Questions Continued

Can you do high-performance coronagraphy with a UV-capable telescope?

Yes. Recent lab studies have shown that UV-compatible coated aluminum mirrors provide better 

coronagraph performance over broad wavelength ranges than UV-incompatible silver mirrors. 

[Appendix D]

Why is the telescope warm?

High-performance coronagraphy demands extreme stability of the wavefront entering the 

instrument, which is easier to achieve by actively stabilizing the telescope temperature. Maintaining 

a temperature near or above the freezing point of water also avoids risk of optics contamination 

that would harm the telescope’s ultraviolet sensitivity. [Chapter 8, Appendix D]

Why make LUVOIR capable of observing so far into the ultraviolet (< 1150 Å)? 

To understand the history of atoms in the universe, they must be observed over a wide range 

of physical conditions. Many key atomic and molecular transitions, spanning the full range of 

temperatures and densities, lie at wavelengths < 1150 Å. [Chapters 5–7]

What unique capabilities will LUVOIR offer compared to ground-based observatories in 

the 2030s?

LUVOIR will a) provide continuous wavelength coverage from the far-UV to the near-IR, b) achieve 

the 10-10 contrast levels required for direct spectroscopy of Earth-like planets around Sun-like 

stars, c) provide sensitive observations of faint objects below the sky background, d) provide 

diffraction limited imaging resolution over wide fields of view. [Chapters 3–7]

Why is there no high-resolution visible/near-IR spectrograph in the first-generation 

LUVOIR instruments?

The LUVOIR team developed a preliminary concept for such an instrument (called ONIRS). 

Given a limited number of instrument bays (four on LUVOIR-A) and some uncertainty about 

the performance of similar instruments planned for the future ground-based Extremely Large 

Telescopes (ELTs), ONIRS is currently considered an “alternate or second-generation instrument.” 

We are also studying extending the bandpass of a first-generation UV spectrograph to cover the 

visible. [Appendix E, Chapter 9]

What is LUVOIR’s primary technological challenge?

Achieving the extreme wavefront stability needed for direct observations of Earth-like exoplanets 

using a large telescope. [Chapters 8, 9, & 11]

How much will LUVOIR cost?

Determination of an accurate cost for LUVOIR is a critical desired result of the study. The LUVOIR 

team will begin internal cost and risk estimates in mid 2018. Furthermore, NASA will contract 

for independent cost and risk analysis, to take place in early 2019. The LUVOIR study aims 

to achieve greater cost fidelity though increased design and technological detail, since cost 

estimates based on single-parameter scaling relations have been shown to be inadequate 

for large space missions. A technology development plan, with schedule and cost, will also 

be presented in the Final Report. Commercial and industry partners will also contribute cost, 

technology development, and risk analyses. The results of all these analyses will appear in the 

LUVOIR Final Report.
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2.1.2 The search for life

The tool to achieve the exoEarth census can 

also probe the atmospheres of those planets 

for biosignature gases and find the first signs 

of life outside our home planet. The spectrum 

of the Earth contains features arising 

from gases of biological origin, like O2 and 

methane, in addition to strong features from 

water vapor and greenhouse gases (Figure 

2.5). The Earth is the only planet we know of 

teeming with surface life that strongly affects 

the planet’s atmosphere. Thus, there are 

good reasons to focus serious searches for 

life outside the solar system on exoplanets 

that are as much like the Earth as possible. 

By targeting exoplanets with sizes and orbits 

similar to Earth’s and host stars similar to 

the Sun, we hope to increase our chances 

of finding—and recognizing—extraterrestrial 

life.

However, it is important to understand 

that no one molecule (or pair of molecules) is 

automatically a biosignature in every context. 

The key is to calculate the abundances of 

molecules in the whole atmosphere, and 

then compare those abundances to the 

production and destruction rates predicted 

by physics and chemistry. To calculate the 

production/destruction rates, a good deal 

of knowledge about the atmosphere’s 

context will be needed, including the planet’s 

orbit and the characteristics of the host 

star. If the abundance of a molecule is too 

high, then we know the production rate is 

underestimated and an additional biological 

source is indicated. In essence, we first will 

try to explain the character of an atmosphere 

with physics and chemistry. If we fail, then 

we succeed at finding signs of life.

Therefore, confident life-detection has 

at least three fundamental requirements: 

1) assess a wide range of atmospheric 

molecules, 2) actually measure their 

abundances with some precision, and 3) 

understand the planet’s context within its 

system. The first requirement demands direct 

spectra with broad wavelength coverage 

from the near-UV to the near-IR. The 

Figure 2.4. How many rocky habitable zone exoplanets need to be observed? The bar chart 

shows the number of candidates needed to discover one potentially habitable planet (at the 95% 

confidence level), for different values of the percentage of rocky habitable zone planets that have 

atmospheric water vapor. If the percentage is 10%, then 29 candidates need to be observed, as 

illustrated in the planet montage on the right. To put it another way, if 29 candidates are observed 

and no water vapor is detected, then we learn that < 10% of rocky habitable zone planets are 

actually Earth-like. Credit: C. Stark (STScI)/A. Roberge (NASA GSFC)
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LUVOIR telescope and starlight suppression 

system can span these wavelengths of 

light and access the important constituents 

of planetary atmospheres, including 

water, carbon dioxide, molecular oxygen 

(O2), ozone, and methane. The second 

requirement demands good quality spectra 

to measure abundances, which drives the 

required telescope and instrument sensitivity. 

Additional secondary signs of life, such as 

seasonal variations in a planet’s spectral 

features, will increase confidence in any 

life-detection claim. The third requirement 

demands collection of a range of supporting 

information, including far-UV spectra of stellar 

activity-driven emission from the host stars. 

LUVOIR’s great sensitivity and flexibility will 

enable the variety of observations needed to 

properly place a planet in context.

The dozens of rocky worlds LUVOIR will 

explore will inevitably have a range of sizes, 

orbits, and chemical compositions, and orbit 

various stars with different ages. Earth has 

harbored life for most of its history, but in 

the earlier stages, the atmosphere was very 

different from that of the modern Earth (e.g., 

low O2 abundances). Recognizing such a 

planet as inhabited also demands the careful 

assessment of atmospheric abundances 

and planetary context described above. If 

LUVOIR confirms the habitability of rocky 

exoplanets, or finds signs of life, we will learn 

how those traits respond to different planet 

and star characteristics. This will turn the 

Figure 2.5. The character of a living world. This spectrum of visible and near-infrared light 

reflected from the Earth reveals the composition of its atmosphere, including gases arising from 

life. The steep rise in brightness on the left end is the sign of our blue sky. Deep absorption 

features of water vapor are ubiquitous. A sharp feature comes from molecular oxygen, produced 

by photosynthesis. And a portion of a broad methane feature is visible on the right. Methane in 

Earth’s atmosphere comes from bacteria in the guts of livestock and in swamps. Credit: NASA 

GSFC

Methane

Water vapor

O2

0.4 microns 2.4 microns
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habitable zone from a theoretical concept 

based upon a single planet to one that is 

empirically derived from multiple worlds, and 

usher in a new era of planetary science: one 

of comparative astrobiology.

The search for life also takes place closer 

to home. Over the last decades, we have 

discovered that several moons of the outer 

solar system have liquid water beneath their 

icy surfaces. These sub-surface oceans 

must be heated from below, which may 

also provide the energy needed for life. The 

geysers recently observed from Enceladus 

and Europa (Figure 2.6) may allow glimpses 

into their deep oceans. LUVOIR has an 

important role to play by determining the 

currently unknown strength and frequency 

of this phenomenon, through high resolution 

monitoring of icy moons. Such information 

will be a valuable partner for future spacecraft 

visiting these other ocean worlds to search 

for signs of life.

2.1.3 The solar system’s place in 

the planet family

When we look at the solar system, it appears 

delicately balanced to produce a living world, 

with our largest planet—Jupiter—playing a 

dominant role. However, our exoplanet discoveries show that the planet formation process is robust and leads to a wide range of 

outcomes. How do we understand the solar system in this context? By exploring the 

character of many kinds of exoplanets, including types that do not exist in the solar 

system, like warm Jupiters, sub-Neptunes, 

and super-Earths (Figure 2.7). Only then can 

we truly know what is typical about our own 

system and what is unusual. The capabilities 

that will allow LUVOIR to investigate dozens 

of potentially habitable worlds will also allow 

Figure 2.6. Spectroscopic imaging of Europa and its water jets. The left panel shows an aurora 

on Europa observed with HST (Roth et al. 2014). This UV hydrogen emission (Lyman-alpha) 

comes from dissociation of water vapor in jets emanating from the surface. The right panel 

shows a simulation of how hydrogen emission from Europa would look to a 15-m UV telescope. 

The moon’s surface is bright due to reflected solar Ly-alpha emission. With LUVOIR, one could 

monitor the ocean worlds of the outer solar system for such activity and image the individual jets. 

Credit: G. Ballester (LPL)

HST LUVOIR-A
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comprehensive study of a huge range of exotic exoplanets, which by and large will be 

easier to observe than Earth-like exoplanets.

Beyond individual exoplanets, LUVOIR 

will study the architectures of planetary 

systems, which bear witness to formation 

processes that operated eons ago. The 

relations and interactions between all 

the bodies in a system are also vital for 

understanding their character. For example, 

the upper atmosphere of Jupiter is polluted 

by impacts of comets, which continue to this 

day. We can also study in real time the violent 

formation of rocky worlds and final sculpting 

of planetary systems by observing massive 

belts of colliding asteroids and comets 

around nearby young stars (aka. debris 

disks).

There are many things still to be 

discovered and understood about the 

bodies within the solar system itself. LUVOIR 

can provide up to about 25 km imaging 

resolution at Jupiter in visible light, permitting 

detailed monitoring of atmospheric dynamics 

in Jupiter, Saturn, Uranus, and Neptune over 

long timescales. Sensitive, high resolution 

imaging and spectroscopy of solar system 

comets, asteroids, moons, and Kuiper Belt 

objects that will not be visited by spacecraft 

in the foreseeable future can provide vital 

information on the processes that formed 

the solar system ages ago. These exciting 

studies only scratch the surface of the solar 

system remote sensing that LUVOIR can do.

2.2 The evolution of the cosmos

The scope of astrophysics investigations 

that LUVOIR can do is truly vast, covering all 

the topics addressed with the Hubble Space 

Telescope (HST) and more. Experiments 

extremely difficult or impossible to execute 

with Hubble and its 2.4-meter diameter 

telescope become routine with LUVOIR. A 

detailed discussion of every part of LUVOIR’s 

astrophysics portfolio would be far beyond 

the scope of this report. We have therefore 

chosen to focus on a few major areas, 

spread throughout the history and hierarchy 

of the universe. The LUVOIR team is actively 

soliciting further contributions from the 

Figure 2.7. Artists conceptions of an assortment of sub-Neptunes and super-Earths discovered 

with the Kepler Mission. Credit: NASA/Ames/JPL-Caltech

Kepler-22b Kepler-69c Earth

Kepler-452b 

Kepler-62f 

Kepler-186f 
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broader science community, in the areas of 

astrophysics, comparative exoplanetology, 

and solar system remote sensing (see 

Appendix A).

2.2.1 The lives of galaxies

The destinies of stars and planets are 

controlled by the formation and evolution of 

their home galaxies. These are not uniform, 

static assemblages. The range of galaxy 

masses and structures is enormous, from 

massive giant elliptical galaxies, to complex 

spirals with super-massive black holes at 

their centers (sometimes seen as active 

galactic nuclei), to tiny but massive dwarf 

galaxies housing only dozens of stars. 

Throughout their lives, galaxies interact with 

both their neighbors and their neighborhood. 

Some galaxies are born small but become 

the building blocks of giant spirals like our 

own Milky Way. Most mysteriously, we see 

that some galaxies are actively forming new 

stars, while others are undergoing a slow 

death as their stars age and no new ones 

are born. Our current understanding of this 

complex interplay is highly incomplete.

Gas flows from the space between the 

galaxies (intergalactic medium), into the 

region around a galaxy (circumgalactic 

medium), then into the galaxy itself where 

it can become the fuel for new stars. 

Eventually, massive stars, supernovae, and 

active galactic nuclei drive material back out. 

Much of this cycling of matter is currently 

unobserved, since the gas flows are hot and 

tenuous. To understand the rates of star 

formation in galaxies, we need to measure 

the balance of these inward and outward 

flows. This demands sensitive ultraviolet 

spectroscopy, which can only be done from 

space. Galaxy mergers result in bursts of star 

formation as well but may also be responsible 

for the ultimate “death” of merged galaxies. 

Understanding the role this process plays in 

giving rise to the massive galaxies of today 

demands deep, high-resolution visible and 

near-infrared imaging of all types of galaxies 

(Figure 2.8) at earlier times.

Finally, stars produce essentially all the 

elements heavier than hydrogen and helium, 

the materials out of which planets are 

built. The births and deaths of stars within 

galaxies enrich the universe in elements 

needed for life (carbon, oxygen, nitrogen) 

and our technology (true metals). The global 

enrichment of the universe is intimately tied 

to star formation and the cycling of matter 

into and out of galaxies. LUVOIR will provide 

Figure 2.8. Simulation of a z~2 low mass galaxy imaged with the Hubble Space Telescope and 

LUVOIR. The images are 5.84 arcseconds across. Credit: G. Snyder (STScI)

2.4	m	 15.1	m:	LUVOIR-A	

HST LUVOIR-A
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2-12 The LUVOIR Interim Report

the ultraviolet spectroscopic capability and 

the high-resolution ultraviolet, visible, and 

near-infrared imaging necessary for major 

advancement in all these areas.

2.2.2 The far reaches of space and 

time

How an initially amorphous universe grew 

to host the diverse structures we see today 

is a driving question for astrophysicists. 

We understand that the complex interplay 

between normal matter and dark matter 

during the rapid early expansion of the 

universe was vital for producing a cosmos 

with galaxies, stars, and planets. Clusters 

of galaxies are strung out along dark matter 

webs like jewels in space (Figure 2.9). But in 

some ways, we know very little. What is the 

nature of dark matter? At the moment, we 

roughly know what it does on the largest size 

scales but not what it is. Constraining the 

nature of dark matter requires probing scales 

and masses smaller than can be probed by 

any facility today. 

Do our models of galaxy formation still 

work at smaller size scales? The models 

were built to explain the largest structures we 

can see today and they provide constraints 

on the fundamental physical parameters 

of the universe. Whether they can explain 

smaller structures is a key test that will reveal 

whether we have captured the physics 

of interactions between normal matter, 

dark matter, and ionizing light. LUVOIR will 

facilitate detailed studies of the population 

of ultra-faint, low mass dwarf galaxies in the 

furthest reaches of time and space all the 

way to today. These studies are enabled by 

visible and near-infrared imaging surveys 

that take advantage of LUVOIR’s sensitivity, 

high spatial resolution, and high-precision 

astrometry capabilities. Its UV spectroscopic 

capabilities will provide a critical tool towards 

understanding of the emergence of structure 

and light from the cosmic dark ages.

2.2.3 The origins of stars and 

planets

Descending even further in size scale, stars 

form through the collapse of dense cores 

within cold clouds of interstellar gas and dust. 

Planets then form in rotating disks of remnant 

gas and dust around the newborn stars 

(protoplanetary disks). But the fact is we do 

not have comprehensive theories for either 

process that can explain what we see, much 

less predict the currently unseen. We are 

unable to explain the fundamental outcome 

of the star formation process—the initial 

stellar mass function (IMF), which describes 

how many stars of which masses are born. 

How the cores of interstellar clouds relate to 

the stars they form and how the IMF varies 

with time and environment are complexities 

that we have only begun to wrestle with. 

With ultraviolet/visible imaging at exquisite 

spatial resolution and powerful multi-object 

spectroscopy, LUVOIR will follow high-mass 

stars out to unprecedented distances, hunt 

for binaries down to small separations, and 

Figure 2.9. Theoretical simulation of galaxy 

cluster formation in the presence of the dark 

matter web. Credit: Illustris Collaboration
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The LUVOIR Interim Report 2-13

provide access to the lowest-mass ends of 

the IMF. 

Planet formation theories were originally 

built to describe the birth of the Solar System. 

As we learned more about protoplanetary 

disks by observing them in star forming 

regions (Figure 2.10), even the solar system 

became difficult to explain. When it comes 

to exoplanets, our planet formation theories 

did not predict and cannot explain the wide 

range we have already discovered. We are 

not able to trace the planet formation process 

all the way from its beginning in gas and 

tiny dust grains to any fully formed planet, 

and it is abundantly obvious that important 

processes are missing from our theories. 

The fundamental problem is that we 

need better theories to explain what we see 

but cannot formulate those theories without 

information that we do not currently have. 

Stars and protoplanetary disks are small; 

currently, we can only study them in detail 

when they are relatively nearby. To increase 

the numbers we can study in different 

environments and thereby provide fodder for 

our theories, observations at higher spatial 

resolution are needed.

Furthermore, several key observational 

tests must be done in the ultraviolet. For 

example, massive stars are rare but play a vital 

role in star formation. Finding more of them 

requires extending our ultraviolet observing 

capabilities to the galaxies farther beyond the 

Milky Way. Ultraviolet spectroscopy is also 

needed to observe molecular hydrogen—

the most important gas in protoplanetary 

disks—at the critical time when disk gas is 

starting to disappear and planet formation 

is ending. The high spatial resolution and 

ultraviolet capability provided by LUVOIR will 

be vital for advancing our theories of star and 

planet formation.

2.3 A big observatory for big goals

Each of these revolutionary science investigations demands distinct observations, but 

they share a common need for a large space 

telescope that can collect light from the far-ultraviolet to the near-infrared. Further, that observatory must have a diverse and powerful 

toolkit of instruments. The Hubble Space 

Telescope, which has been one of the most 

productive scientific tools ever built, shares 

these characteristics. But our transformative 

goals demand a great leap in capability over 

Hubble and every other current or planned 

observatory. The design of LUVOIR is driven 

by our need for greater sensitivity, finer resolution, and higher contrast. At the same time, 

the design draws on a decades-long wealth 

of expertise and technology development 

garnered from several current and near-future space missions.

Figure 2.10. Planet-forming systems in the 

Orion Star Forming Region. Credit: NASA/

ESA/Hubble.
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2-14 The LUVOIR Interim Report

2.3.1 The LUVOIR mission concepts

NASA tasked the LUVOIR team with 

considering space telescopes in the 8-to16-meter diameter range. We are studying 

two distinct observatory architectures, 

both designed to operate at the Sun-Earth 

Lagrange 2 point (L2) with 5-year prime 

missions. By developing two observatory 

concepts, we gain better understanding of 

a complex trade space, reveal how science 

return scales with different technical choices, 

and establish robustness to uncertainties 

such as future launch vehicle capabilities and 

budget constraints. LUVOIR Architecture A 

(LUVOIR-A) features a 15-m diameter primary 

telescope aperture and four serviceable 

instruments (Figure 2.11). It was designed 

for launch on NASA’s planned Space Launch 

System (SLS) in a Block 2 vehicle. The team 

has completed the bulk of the study-phase 

design work for LUVOIR-A; refinements are 

in progress.

Architecture B (LUVOIR-B) has an 8-m 

telescope aperture and 3 instruments; 

design work on this concept began in Sept 

2017. LUVOIR-B is being designed to fit 

within a 5-m diameter fairing, similar to those 

in use today. Call-out boxes throughout this 

chapter present the high-level characteristics 

of LUVOIR-A and its instruments. Full details 

of the Architecture A design and plans for 

Architecture B appear in Chapters 8–10.

Figure 2.11. Preliminary rendering of the LUVOIR Architecture A observatory, which has a 

15-meter diameter primary mirror and four instrument bays. An animation of the observatory 

deployment may be viewed at https://asd.gsfc.nasa.gov/luvoir/design/. Credit: A. Jones (NASA 

GSFC)

The LUVOIR Mission Characteristics

Community driven observing program

Operating at Earth-Sun L2

Prime mission lifetime of 5 years

Serviceable and upgradeable

Lifetime goal of 25 years for non-serviceable 

components
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The LUVOIR Interim Report 2-15

2.3.2 Designing for serviceability

The LUVOIR-A science program described 

in this report can be achieved within a 

5-year prime mission using the as-designed 

instruments. LUVOIR should be judged 

on the strength of that science, without 

relying on an extended mission. However, 

Congress has mandated that all future large 

space telescopes be serviceable to the 

extent practicable. The LUVOIR Science 

and Technology Definition Team (STDT) has 

embraced this requirement, since gaining 

the greatest scientific return from a strategic 

mission with an extended development 

time and significant cost demands long 

operational lifetimes. LUVOIR’s serviceability 

is a key part of enabling a series of “Greater 

Observatories” in the tradition of Hubble, 

Compton, Chandra, and Spitzer.

Our preliminary observatory designs incorporate standardized mechanical components (e.g., valves, latches, rails) and allow 

replacement of many spacecraft elements 

(e.g., reaction wheels, gyros, batteries, solar arrays, computers). Consumables (e.g., 

propellant, cryogen) can be replenished and 

science instruments easily removed for replacement. Incorporating these features in 

the LUVOIR design from the outset will also 

result in faster and easier Integration and Test 

procedures on the ground before launch, 

advantages that will be realized regardless of 

whether LUVOIR is actually serviced.

We have considered the question “how 

will LUVOIR be serviced?” We first note that 

a design commitment to serviceability is not 

a commitment to servicing. If LUVOIR were 

to launch in the late 2030s, presumably the 

first servicing mission would be desired in 

the 2040s, so a decision to service would 

not have to be made in the near future. 

Whether or not LUVOIR would be serviced 

will depend on political and budgetary 

realities that are unpredictable. If history is 

a guide, it is doubtful that NASA’s Science 

Mission Directorate on its own will have the 

The LUVOIR Telescopes

Architecture A: 15-m diameter, 4 instrument bays

Architecture B: 8-m diameter, 3 instrument bays

Segmented, deployable, ultra-stable telescopes

Diffraction limited at 500 nm

Far-UV to near-IR bandpass (100 nm to 2500 nm)

LUVOIR’s Heritage

HST for serviceability

JWST for deployable telescopes, segmented 

wavefront control

WFIRST for high-performance coronagraphy

Sounding rockets and cubesats for UV 

technology development
LUVOIR The Large UV Optical Infrared Surveyor

2-16 The LUVOIR Interim Report

resources to create the infrastructure for 

servicing LUVOIR.

Servicing LUVOIR will therefore have 

to be approached opportunistically. As an 

illustrative example, the Human Exploration 

and Operations Mission Directorate is 

considering a Deep Space Gateway in cislunar space which, in principle, could be 

used for servicing scientific assets on-orbit. 

The energy required to move LUVOIR from 

Sun-Earth L2 to Earth-Moon L1 or L2 and 

back is small compared to that required for 

launch of the observatory. Returning the 

fully deployed LUVOIR to cis-lunar space for 

servicing by some combination of astronauts 

and/or robots is a technical challenge, but 

not an impossible task (Chapter 8).

2.3.3 Enabling LUVOIR

Revolutionary science goals require a 

technologically challenging observatory. The 

large LUVOIR apertures demanded by our 

sensitivity and spatial resolution goals must 

be segmented and deployable to fit within a 

launch vehicle. Fortunately, the James Webb 

Space Telescope (JWST) to be launched in 

2021 has paved the way in this area. Our 

most technically challenging observational 

goal is the extreme starlight suppression 

needed to directly observe exoplanets. 

There are two basic instruments to do this: 

coronagraphs and starshades. The latter are 

deemed unfeasible for telescopes as large 

as LUVOIR-A (details appear in Appendix 

E). Current coronagraphs cannot reach the 

contrast values needed to detect and study 

small exoplanets. For LUVOIR to achieve its 

goal of directly studying rocky exoplanets 

in the habitable zones of Sun-like stars, 

coronagraph performance must be improved 

in several ways.

As part of addressing this challenge, 

NASA has embarked on several development 

programs for starlight suppression technology. 

The Segmented Coronagraph Design Study, 

under the leadership of NASA’s Exoplanet 

Exploration Program, has demonstrated 

that coronagraphs can be designed for 

Instrument : LUMOS-A

UV multi-object spectrograph + imager

Total bandpass: 100 – 400 nm

Resolution 500 < R < 63,000

Microshutter array: 0.14” x 0.07” 

shutters, 3’ x 1.6’ field of view

Far-UV imaging channel

R = 30,400 and SNR = 5 

at 120 nm in 1.2 hours for 

F = 3 x 10-17 erg s-1 cm-2 Å-1

Instrument : ECLIPS-A

Ultra-high contrast coronagraph (10-10)

Total bandpass: 200 nm – 2000 nm

Inner working angle ~ 3.5 λ / D

Outer working angle ~ 64 λ / D

Imaging and imaging spectroscopy 

(Vis R = 140, NIR R = 70, 200)
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 2-17

segmented, obscured apertures (a point of 

doubt a decade ago). The most important 

current effort is the Wide-Field Infrared 

Survey Telescope (WFIRST) Coronagraph 

Instrument (CGI) technology demonstration 

program currently working to design a 

high-performance coronagraph coupled to 

a complex, obscured telescope aperture. 

Further, a coronagraph’s performance is 

directly linked to stability of the wavefront of 

light entering it. The CGI program has made 

great strides in developing technologies and 

algorithms to stabilize the wavefront coming 

from the telescope (low-order wavefront 

sensors, deformable mirrors). The highest 

contrast measured to date (~10-9) was 

demonstrated in testbeds at JPL through the 

CGI program (details appear in Chapter 11). 

The LUVOIR coronagraph must achieve 

about another 1–2 orders of magnitude 

improvement in contrast beyond the current 

record. To do this, the LUVOIR observatory 

was designed with wavefront stability, 

sensing, and control in mind at all times. 

Mechanical and thermal disturbances 

have been minimized and several layers of 

wavefront control implemented. As part of 

this mission study, we have assessed the 

technological maturity of each piece of the 

starlight suppression system; all are currently 

at Technology Readiness Level 3 or higher. 

The remaining major challenge is to show 

that this whole design works together as a 

system to provide the needed performance. 

In Chapter 11, we present our technology 

assessments and preliminary development 

plan.

2.4 What is the difference 

between LUVOIR and HabEx?

The Habitable Exoplanet Imaging Mission 

(HabEx) is another concept currently 

being studied in preparation for the 2020 

Astrophysics Decadal Survey. Here we 

answer a question frequently asked about 

the relationship between the LUVOIR and 

Instrument : High-Definition Imager-A

2’ x 3’ imaging camera

Total bandpass: 200 nm – 2500 nm

Nyquist sampled at 400 nm

Simultaneous UV/Vis and NIR channels

~ 70 filter slots

High-precision astrometry mode

SNR = 5 in 1.4 hours for V = 32

Instrument : POLLUX

Instrument study from European 

consortium, under leadership of CNES

Point-source UV spectropolarimeter

Total bandpass: 100 nm – 400 nm

Resolution R = 120,000

Circular + linear polarization capability
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2-18 The LUVOIR Interim Report

HabEx concepts. The two teams developed 

the text of this section together.

LUVOIR and HabEx share two primary 

science goals: 1) studying habitability 

and biosignatures in the atmospheres 

of exoplanets around Sun-like stars and 

2) executing a broad range of general 

astrophysics studies. The two mission 

concepts are distinguished by a difference 

in focus. For LUVOIR, Goals 1 and 2 are on 

equal footing. It will be a general purpose 

“Great Observatory,” a successor to Hubble 

and JWST in the 8–16 m class. HabEx will be 

optimized for exoplanet direct observations 

(Goal 1) but will also enable a range of general 

astrophysics. It is a more focused mission in 

the ~ 4 m class.

The two missions do have similar goals 

for exoplanet science but differ in their 

quantitative levels of ambition. HabEx will 

explore the nearest stars to “search for” 

signs of habitability & biosignatures via direct 

detection of reflected light. LUVOIR will survey 

more stars to “constrain the frequency” of 

habitability and biosignatures and produce a 

statistically meaningful sample of temperate 

terrestrial planets. In sum, the two studies will 

provide a continuum of options for a range of 

futures.

2.5 The LUVOIR Interim Report

This document begins by describing the 

Signature Science Cases of the LUVOIR 

mission identified by the STDT with input from 

the broader scientific community. In each 

Figure 2.12. Example observations with LUVOIR as a function of increasing distance and how 

they drive telescope and instrument needs.

SCIENCE

The 

Dynamic 

Solar 

System

Diverse 

Worlds

Living 

Worlds

Stellar 

Formation 

and History

The 

Lifecycle of 

Galaxies

The Building 

Blocks of 

Structure

Example

Observation

Spatially 

resolved 

geysers

on icy moons

Imaging & 

spectroscopy of 

hundreds of 

exoplanets 

Imaging & 

spectroscopy of 

dozens of rocky 

habitable zone 

planets

Main sequence 

turn-off out to 10 

Mpc

Spatially 

resolved 

imaging/

spectroscopy of 

the CGM

Ultra-faint dwarf 

galaxies out to 

z > 7

Observatory 

Needs

Large aperture 

FUV to NIR 

telescope

Ultra-stable, 

large aperture 

NUV to NIR 

telescope

Ultra-stable, 

large aperture 

NUV to NIR 

telescope

Large aperture 

FUV to NIR 

telescope

Large aperture 

FUV to NIR 

telescope

Large aperture 

NUV to NIR 

telescope

Instrument 

Needs

FUV+NIR 

imaging 

spectrometer

[LUMOS, 

ECLIPS]

High contrast 

NUV to NIR 

coronagraph 

with imaging 

spectrometer

[ECLIPS]

High contrast 

NUV to NIR 

coronagraph 

with imaging 

spectrometer 

[ECLIPS]

Sensitive high 

resolution

NUV to NIR 

imager & 

spectrograph 

[LUMOS, HDI, 

POLLUX]

Multi-object FUV 

to VIS 

spectrograph & 

imager

[LUMOS, HDI, 

POLLUX]

NUV to NIR 

imager with 

precision 

astrometric 

capabilities [HDI]

10 AU

10 pc

10 Mpc

10 Gpc

1 Gpc
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 2-19

science chapter, we explain the motivations 

for choosing these science cases, identify 

the key measurements that must be made, 

and describe the observations needed to 

obtain those measurements. The telescope 

and instrument characteristics required for 

the observations are identified. A high-level 

flow down of some example science cases 

appears in Figure 2.12. Simplified summary 

science traceability matrices appear at the 

end of each science chapter and a complete, 

detailed science traceability matrix appears 

in Appendix C.

The Signature Science Cases discussed 

in this report represent some of the most 

compelling types of observing programs that 

scientists might do with LUVOIR at the limits 

of its performance. As compelling as they 

are, they should not be taken as a complete 

specification LUVOIR’s future scientific 

potential. We have developed concrete 

examples to ensure that the point designs 

can execute this compelling science. We fully 

expect that the creativity of the community, 

empowered by the revolutionary capabilities 

of the observatory, will ask questions, 

acquire data, and solve problems beyond 

those discussed here—including those that 

we cannot envision today.

Chapter 3 addresses the question “Is 

There Life Elsewhere?” by motivating and 

describing a strategy to first find habitable 

exoplanets then search them for signs of life. 

Chapter 4—“How Do We Fit In?”—delves 

into the richness of planetary systems and 

comparative exoplanetology. In both chapters, key contributions from LUVOIR observations of solar system bodies are also discussed. The dynamic lives of galaxies are 

discussed in Chapter 5—“How Do Galaxies 

Evolve?” The far reaches of the universe and 

its earliest times are the topic of Chapter 

6—“What Are the Building Blocks of Cosmic 

Structure?” In the final science chapter, 

Chapter 7—“How Do Stars and Planets 

Form?”—the complex and mysterious births 

of stars and planetary systems are investigated.

An overview of the LUVOIR observatories and mission-level considerations 

are given in Chapter 8—“The LUVOIR 

Architectures.” Details on the LUVOIR-A 

telescope and each of the US-studied instruments (ECLIPS, LUMOS, and HDI) appear in Chapter 9—“The LUVOIR Telescope 

and Instruments.” Chapter 10 describes 

both the science and design of POLLUX, an 

instrument being studied by a consortium 

of European institutions, with support from 

the French Space Agency (CNES). Chapter 

11—“LUVOIR Technology Development” 

presents LUVOIR’s key technologies, their 

current status, and initial plans for advancing 

them. Finally, Chapter 12—“LUVOIR Cycle 

1”—contains an exciting vision of what the 

scientific community might chose to do with 

LUVOIR in its first year of operations in the 

late 2030s. LUVOIR is an observatory not 

just for the science of the 2030s, a selected 

sample of which appears in this report, but 

also for decades after. Just as Hubble today 

is doing science never envisioned at the time 

of its design or launch, LUVOIR is designed 

to be both flexible and powerful enough to 

do the as-of-yet unknown science of the 

2040s and beyond.

References

Kaltenegger, L. & Traub, W. A. (2009). ApJ, 

698, 519

Kopparapu, R. K., Hébrard, E., Belikov, R., 

et al. (2018). ApJ, 856, 122

Roth, L., Saur, J., Retherford. K. D., et al. 

(2014). Science, 343, 171

Snellen, I., de Kok, R., Birkby, J. L., et al. 

(2015). A&A 576, A59

Wang, J., Mawet, D., Ruane, G., et al. 

(2017). AJ, 153, 183
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Chapter 3
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3-2 The LUVOIR Interim Report

3 Is there life elsewhere? Habitable exoplanets and solar system 

ocean worlds

After millennia of wondering whether humanity is alone in the universe, LUVOIR’s large 

aperture and compelling instrument suite 

will enable the search for habitability and life 

on dozens of nearby worlds. LUVOIR's exoplanet survey will be both broad and comprehensive, which will revolutionize our understanding of planetary habitability, and will 

provide the first estimates for the frequency 

of habitable conditions and life in the local 

Solar neighborhood (Figure 3.1). LUVOIR’s 

detailed study of the orbital parameters, atmospheric compositions, surface properties, 

and temporal variability of many rocky planets in the habitable zones (HZs) of different 

stars will reveal environmental conditions for 

a diversity of worlds at different stages of 

evolution, and potentially usher in a new era 

of comparative astrobiology.

LUVOIR will address three profound 

scientific questions for exoplanets:

• How common are habitable 

environments on worlds around other 

stars?

• How common is life beyond the solar 

system?

• How does life co-evolve with its 

exoplanetary environment?

LUVOIR’s ability to measure fundamental 

environmental properties (e.g., atmospheric 

composition) for many exoplanets will help 

us answer the first two questions, and answering the last question will require detailed, 

in depth study of LUVOIR’s most promising 

worlds to understand the interactions between the biosphere and its environment. 

LUVOIR will also revolutionize the search 

for habitability and life closer to home, 

in our outer solar system. With spatial 

resolution comparable to flyby missions, UV 

capability, and its unprecedented spacebased collecting area, LUVOIR will provide 

sensitive global (i.e., planet-wide) imaging 

and spectroscopy of a diverse population 

of potentially habitable solar system worlds, 

particularly Europa and Enceladus. These 

observations, which could be made available 

on a long temporal baseline with an extended 

mission lifetime, will complement NASA’s in 

situ exploration by robotic flyby, orbiter, and 

Figure 3.1. Expected exoplanet yields for 

different types of stars. We anticipate observing 

~50 exoEarth planet candidates (green bars) 

with the LUVOIR-A 15-m architecture during 

an initial 2-year habitable planet survey. Other 

planets shown here are detected “for free” 

during the 2-year search. Color photometry and 

orbits are obtained for all planets. Two partial 

spectra are obtained for all planets in systems 

with exoEarth candidates. Red, blue, and ice 

blue bars indicate hot, warm, and cold planets, 

respectively. Planet class types from left to right 

are: exoEarth candidates, rocky planets, superEarths, sub-Neptunes, Neptunes, and Jupiters. 

Credit: C. Stark (STScI)
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The LUVOIR Interim Report 3-3

State of the Field in the 2030s

We anticipate significant advances in understanding solar system habitability and in small 

exoplanet detection and characterization between now and the mid 2030s. Exoplanet 

advances will necessarily focus almost exclusively on the more readily detectable planets 

orbiting M dwarf stars, whose worlds undergo early evolution and star-planet interactions 

that are not experienced by planets orbiting Sun-like stars. A mission like LUVOIR is 

essential for the study of planetary systems more like our own—i.e., around Sun-like stars.

The Europa Clipper: The planned Europa Clipper mission (2022–2025) will deliver insitu (mass spectroscopy) and directed spectroscopic measurements of this interesting 

potentially habitable icy world in the outer solar system. 

Frequency of habitable planets: On average, early M dwarfs harbor ~0.25 potentially 

habitable planets per star (Dressing & Charbonneau 2013, 2015). Recent detections of 

Proxima Cen b (Anglada-Escudé et al. 2016), LHS 1140 b (Dittmann et al. 2017), the 

TRAPPIST-1 planets (Gillon et al. 2017), and Ross 128b (Bonfils et al. 2017) indicate 

that many terrestrial M dwarf habitable zone planets are nearby, with further discoveries 

expected. 

Transiting Exoplanet Survey Satellite (TESS): NASA’s TESS (Ricker et al. 2014) will 

launch in 2018 and survey the entire sky for transiting habitable zone planets around 

nearby M dwarfs, with an anticipated yield of 10–20 potentially habitable M dwarf planets 

(Sullivan et al. 2015), which will be prime targets for transit spectroscopy and possibly 

direct imaging and transit spectroscopy with LUVOIR.

James Webb Space Telescope: NASA’s JWST is a 6.5-m infrared telescope that will 

launch in 2019. Its early exoplanet observations will likely be dominated by hot Jupiters 

and sub-Neptunes, but JWST may also provide our first opportunity to probe the 

atmospheres of a few transiting M dwarf HZ planets and search for biosignatures. These 

transit transmission measurements, while an important step forward, may be stymied by 

a lack of observable features (e.g., Kreidberg et al. 2014) and the instrumental noise floor 

(Greene et al. 2016). 

Planetary Transits and Oscillations of stars (PLATO): ESA’s PLATO mission is 

scheduled to launch in 2026 and will use planetary transits to search for exoplanets 

around stars ranging from M to G dwarfs (Rauer et al. 2014). PLATO’s closest targets may 

be observable with LUVOIR.

Ground-based observations: Ground-based Extremely Large Telescopes (ELTs; 30–

40-m diameter) will complement and enhance space-based observatories (Brogi et al. 

2016), including LUVOIR. Ground-based observations will likely focus efforts on M dwarf 

stars, which have a more favorable planet-star contrast ratio (10–8 for an M5 star with an 

Earth), and because adaptive optics systems perform best in the NIR (e.g., Bouchez et al. 

2014). Upgrades to instrumentation (e.g., Lovis et al. 2017) may allow observations of the 

nearest M dwarf planets over 3–4 years with high-resolution transit spectroscopy and/or 

direct spectroscopy (e.g., Snellen et al. 2013; Crossfield et al. 2011). ELT direct imaging at 

3–10 mm of terrestrial planets orbiting Sun-like stars may provide information on thermal 

emission that is complementary to LUVOIR’s direct imaging in reflected light (Quanz et al. 

2015).
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3-4 The LUVOIR Interim Report

lander spacecraft. LUVOIR will therefore also 

address the scientific question: 

• Are there habitable environments and life 

elsewhere in the solar system?

To answer these four scientific questions 

concerning habitability and life beyond the 

Earth, we have identified two Signature 

Science Cases to address the overarching 

questions: #1 Which Worlds are Habitable? 

and #2 Which Worlds are Inhabited? 

In the sections that follow we will briefly 

summarize the anticipated state of the field 

of exoplanets and solar system habitability 

during LUVOIR’s operation lifetime, and we 

describe the science motivation and required 

measurements that will enable LUVOIR to 

discover small habitable zone (HZ) planets. 

We then describe example observing 

sequences that illustrate how LUVOIR will 

conduct the Signature Science Cases and 

enable the search for habitability and life on 

planets in and beyond the solar system. 

The “Signature Science” cases discussed 

in this chapter represent some of the most 

compelling types of observing programs on 

habitable exoplanets, potentially habitable 

solar system worlds, and the search for life that 

scientists might do with LUVOIR at the limits 

of its performance. As compelling as they 

are, they should not be taken as a complete 

specification LUVOIR’s future potential in 

these areas. We have developed concrete 

examples to ensure that the nominal design 

can do this compelling science. We fully 

expect that the creativity of the community, 

empowered by the revolutionary capabilities 

of the observatory, will ask questions, 

acquire data, and solve problems beyond 

those discussed here—including those that 

we cannot envision today.

3.1 A habitable and inhabited 

planet: The Earth through time

What does an exo-biosphere look like? 

LUVOIR’s search for habitable worlds and 

life beyond the solar system will sample 

habitable zone planets orbiting stars of 

spectral type from F to M (the habitable 

zone is defined as that region around a 

star in which a rocky planet can support 

surface liquid water; Kasting et al. 1993; 

Kopparapu et al. 2013). These planetary 

systems will be at different ages, potentially 

providing a window into the different stages 

of habitable planet evolution and possibly 

with dramatically different biospheres from 

our own planet. This rich diversity of worlds 

requires a mission that is versatile and 

capable of exploring and characterizing 

environments unlike the modern Earth’s, that 

may nonetheless be habitable. To motivate 

and enhance the scientific footing of this 

search, we consider the different habitable 

environments known to exist throughout 

Earth’s 4.6 billion-year history, and the 

significant impact that our evolving biosphere 

had on these environments. These different 

environmental conditions and dominant 

biospheres generated significantly different 

spectral features. The observable features 

created by life acting on its environment are 

called “biosignatures.” By understanding 

the likely observable characteristics of these 

varied environments (e.g., Kaltenegger et 

al. 2007; Rugheimer & Kaltenegger 2017), 

we can identify measurements needed to 

understand habitable exoplanet evolution. 

While modern Earth offers detailed, high 

fidelity information on a currently inhabited 

world, the diverse environments and 

biospheres of early Earth provide valuable 

data on “alien” globally habitable or inhabited 

environments.
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Figure 3.2 shows globally-averaged 

spectra of Earth throughout its history. The 

environments of the Hadean, Archean, 

Proterozoic and Phanerozoic eons of Earth 

history are outlined below, and their distinctive 

remote observable properties are discussed 

briefly, with Sections 3.2 and 3.3 providing 

a more in-depth analysis of the spectral 

Figure 3.2. The atmospheric composition of Earth has changed significantly over its history, as 

has its spectral features. For instance, the canonical biosignatures of modern Earth (e.g., O2, 

O3 ) cannot be observed in the Archean eon. A wide wavelength range enables higher fidelity 

spectral characterization by providing observations of many atmospheric species, and often 

multiple absorption bands of a given species. This breaks degeneracies between overlapping 

bands (e.g., overlapping CH4 and H2O absorption features, as can be seen in the near-infrared 

in the Archean Earth spectra) and enables the search for non-traditional biosignatures. “PAL” is 

present atmospheric level. Credit: G. Arney (NASA GSFC)
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3-6 The LUVOIR Interim Report

properties of Earthlike worlds. These spectra 

provide context and motivation for our search 

strategies for a diversity of habitable worlds.

The environmental conditions of the 

Hadean eon (before 4 billion years ago) 

are poorly constrained due to an extremely 

sparse geological record, but life on Earth 

may have arisen during this eon (Bell et al. 

2015; Nutman et al. 2016). Hadean Earth 

may have experienced significantly higher 

rates of meteoric infall compared to today. 

Similar young exoplanets may likewise be 

enshrouded by thicker interplanetary dust 

disks produced by planetesimal destruction 

(exozodiacal dust).

At the onset of the Archean (4–2.5 billion 

years ago), the Earth likely had a flourishing 

anaerobic biosphere. The Sun was only 

about 75–80% as luminous as today, so a 

robust inventory of greenhouse gases—

which likely included carbon dioxide (CO2) 

and methane (CH4)—would have been 

required to keep Earth hospitable. There is 

disagreement on the level of atmospheric 

Archean CO2: estimates have ranged from 

approximately modern Earth-like levels to 

orders of magnitude more than modern Earth 

(e.g., Kanzaki & Murakami 2015; Rosing et 

al. 2010). Methane levels may have ranged 

between 2–3 orders of magnitude higher 

than today (Pavlov et al. 2000), produced 

by biological and geological processes. 

These high, biologically-mediated Archean 

methane levels may have occasionally 

triggered formation of an atmospheric 

organic haze (e.g., Arney et al. 2016; 

Zerkle et al. 2012), which could itself be 

considered a biosignature (Arney et al. 

2018). Spectral features from CH4, CO2, and 

haze are strongly apparent in the Archean 

Earth spectrum (Figure 3.2). Note high 

amounts of CH4 generate spectral features 

that overlap with the strongest H2O features 

in the near-infrared (NIR), so disentangling 

these gases will likely require measurements 

of multiple bands and/or high signal-tonoise measurements. A dramatic difference 

between modern and Archean Earth is the 

lack of O2 in the Archean atmosphere (and in 

its spectrum).

In the Proterozoic (2.5 billion years 

ago–541 million years ago), significant 

atmospheric oxygen accumulation occurred, 

irreversibly altering the oxidation state of the 

atmosphere and generating a powerful UV 

shield in the form of the ozone layer (O3 can 

be seen in the UV part of the spectrum). It has 

been suggested that mid-Proterozoic oxygen 

levels may have been as low as 0.1% of the 

present atmospheric level, precluding directly 

detectable O2 spectral features (Planavsky et 

al. 2014), although O2 might be inferred by 

detecting O3 in the UV. Thus, the Proterozoic 

teaches us that O2, while present, may still 

be spectrally invisible. Biologically modulated 

season variations in O3 may be detectable at 

UV wavelengths for the lower oxygen levels 

of the Proterozoic (Olson et al. 2018), but 

not for modern Earth whose higher levels of 

O3 mean its UV spectral feature is saturated. 

Also, the presence of O2 and oceanic sulfate 

may have limited mid-Proterozoic CH4 to 

low (i.e., approximately modern) levels, 

diminishing methane’s role as a greenhouse 

gas and its spectral features (Olson et al. 

2016). However, other biogenic greenhouse 

gases that could act as biosignatures, such 

as N2O, may have been enhanced in the 

mid-Proterozoic due to anoxic and sulfidic 

waters limiting availability of metals required 

for the enzymatic conversion of N2O back to 

N2 (Buick 2007).

Phanerozoic or modern Earth (541 

million years ago–present) has been heavily 

studied in the context of remote sensing and 

mission development studies (e.g., Sagan 

et al. 1993). Key gases in modern Earth’s 

atmosphere include O2, O3, CH4, and CO2. 
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Modern Earth can be used to validate and 

constrain models of the remote observable 

properties of Earth as an exoplanet (e.g., 

Robinson et al. 2010, 2011), and its 

biosignatures and chemical disequilibria 

inform the search for life (Hitchcock & 

Lovelock 1967; Krissansen-Totton et al. 

2016a).

The co-evolution of life and its 

environment. Over Earth’s history, life has 

co-evolved with its environment, and it is 

often impossible to separate the history of 

Why LUVOIR?

[Why direct imaging? / Why from space? / Why NUV-Optical-NIR? / Why so big?]

The most powerful way to characterize potentially habitable worlds is a space-based 

telescope with a large aperture, which can conduct direct spectroscopy of habitable 

planets over a broad wavelength range. 

Understanding the surface and near-surface environment of a potentially habitable planet 

is the key to discovering whether that planet can support a liquid water ocean. Direct 

imaging provides a direct, and short, line of sight down through the planet’s atmosphere to 

probe the planetary surface. Transmission spectroscopy will observe a much longer path 

through the planetary stratosphere and is therefore sensitive to atmospheric trace gases. 

However, it cannot see the planetary surface, and due to the effects of aerosols and 

refraction, it will have difficulty sampling the near-surface atmosphere. Transit observations 

are also not sensitive to planets around Sun-like stars (long orbital periods, shallow transit 

depths), so they do not allow us to characterize planets more similar to our own. 

LUVOIR’s direct imaging spectroscopy will probe deep into planetary atmospheres, 

potentially all the way to the surface. This will provide crucial information on near-surface 

physical conditions, water vapor abundance, and biosignature gases, including those that 

are not abundant at higher altitudes. Because LUVOIR will be observing from space, it will 

not have to stare through Earth’s atmosphere, which contains many of the gases LUVOIR 

will search for. Those gases include molecular markers of habitable surface conditions 

(e.g., water vapor, CO2, CH4, liquid surface water) and biomarkers (e.g., O2, O3, CH4, 

biological surface features). Absorption features of all these molecules lie in the near-UV, 

optical, and near-IR wavelength ranges.

Of the space-based telescopes being considered for the next decadal survey, LUVOIR has 

the largest collecting area and the smallest inner working angle. This will allow LUVOIR to 

detect and spectrally characterize a greater number of planets, and characterize a given 

planet in a shorter amount of observing time. The combination of tighter IWA and large 

collecting area will permit spectral characterization from the near-UV into the near-IR for 

most targets. 

Observations over multiple, short time intervals will allow for time-dependent observations 

that can map planetary surface inhomogeneity and rotational periods. This combination 

of advantages means LUVOIR will provide mapping and in-depth spectroscopic 

characterization of a large number of rocky planets around other stars, increasing the 

probability that we will discover habitability and life, and improving the robustness of the 

interpretation of any biosignatures that we may find.
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3-8 The LUVOIR Interim Report

the two. As a result, the signs of habitability 

and life are occasionally the same signatures. 

Life has impacted its global environment 

in many ways. For example, oxygenic 

photosynthesis caused major changes 

to the chemical state of the planetary 

atmosphere and oceans, which caused the 

eventual rise of atmospheric oxygen (Lyons 

et al. 2014). This rise of oxygen created an 

ozone UV shield, which may have allowed 

life to spread from lakes and oceans (where 

water is an effective UV shield) onto land 

surfaces where it might produce detectable 

surface reflectance biosignatures. Earlier 

in Earth history, the evolution of methaneproducing microbes (methanogens) may 

have generated large atmospheric quantities 

of CH4, providing an important Archean 

greenhouse gas, in addition to possibly 

triggering periods of global organic haze 

that would have dramatically impacted the 

climate and spectral properties of Earth 

(Arney et al. 2016). If LUVOIR finds signs of 

life, it will advance our understanding of how 

life interacts with its host planets.

Because these interactions between 

life and the atmosphere are complex, and 

can occur on diurnal, seasonal, and much 

longer timescales, studying them will require 

sampling a large number of potentially 

habitable exoplanets, and time-resolved 

high-fidelity observations over the longest 

possible baseline for any planets for which 

LUVOIR has found signs of life. LUVOIR is 

uniquely suited to pursue these goals. The 

large aperture of LUVOIR enables a survey 

of potentially dozens of HZ exoplanets 

that offers the unique and unprecedented 

opportunity to witness worlds with a variety 

of properties. This includes habitable worlds 

at different ages and stages of evolution, 

allowing us to compare nascent worlds to 

ones much older. LUVOIR will also allow us 

to compare planets around a variety of stellar 

spectral types, informing our understanding 

of how star-planet interactions shape 

biospheres. LUVOIR’s large survey has the 

potential to deliver profound insights into 

how life emerges and survives over time, 

and the longer-term impact it can have on 

its environment. LUVOIR can search for 

planetary variability on diurnal and seasonal 

timescales, and serviceability allows for 

longer baseline observations (e.g., decadal 

timescales). This may allow LUVOIR to test 

hypotheses related to the response of the 

biosphere to time-dependent phenomena 

such stellar activity or insolation variations 

due to the planetary orbit. LUVOIR will be 

capable of unlocking the story of life in the 

cosmos, and in so doing will improve our 

understanding of the history of life on Earth.

We emphasize that a wide wavelength 

range, spanning from the near-UV (NUV) to 

the NIR, is needed to characterize Earth and 

detect signs of life during different epochs 

during Earth history. One of the goals of 

LUVOIR is to be able to detect life on Earthanalog worlds for as much of Earth history 

as possible. Furthermore, this is a specific 

example of a more generic goal, which is to 

detect biospheres different from Earth. This 

is especially important when considering the 

history of oxygen in our planet’s atmosphere 

(Figure 3.3). O2 is regarded as the canonical 

biosignature of Earth, and today it can be 

detected at visible and NIR wavelengths. 

However, it was undetectable in the 

Archean eon, so alternative biosignatures 

like methane would have to be relied upon 

for similar anoxic exoplanets. Methane 

in the Archean likely produced stronger 

absorption bands than today and may have 

been detectable even at visible wavelengths 

(Figure 3.2); abundant methane is a much 

stronger biosignature if it can be detected 

together with CO2, which absorbs in the 

NIR and would indicate an atmosphere 

in chemical disequilibrium most easily 

explained by biology (Krissansen-Totton et 
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The LUVOIR Interim Report 3-9

al. 2018). During the mid-Proterozoic, which 

encompasses roughly 30% of Earth’s history, 

the NUV offers the only wavelengths (from 

the UV through the thermal IR) where oxygen 

can be detected via its photochemical 

byproduct O3 (Schwieterman et al. 2018). 

This is especially significant given that CH4 

levels may have been particularly low during 

this same time period (Olson et al. 2016; 

Reinhard et al. 2017), removing CH4 as 

a significant biosignature at this time and 

highlighting the need for NUV observations 

to detect biosignatures over a significant 

fraction of Earth history.

In the following sections, we describe two 

signature science cases in detail, covering 

the science rationale, target selection, 

measurement requirements, and planet 

observations.

3.2 Signature science case #1: 

Which worlds are habitable?

Planetary habitability is the outcome of the 

complex interplay of processes between 

the planet, planetary system, and host star. 

Understanding which planets can be, and 

are, habitable, will require multiple pieces 

of information including the nature of the 

host star, a census of greenhouse gases, 

observed or modeled estimates of planetary 

surface temperature and pressure, and—

most importantly—evidence of surface 

liquid water. LUVOIR can provide this 

information for multiple exoplanets. Because 

it is observing in reflected light, it can probe 

a planet’s deep atmosphere and surface. 

LUVOIR’s direct imaging observations 

can therefore provide better sensitivity to 

surface habitable conditions than is possible 

with upper-troposphere and stratosphereskimming transmission observations that 

cannot observe below even thin cloud layers.

We now know that small exoplanets with 

R < 1.6 REarth are most likely to indicate a 

rocky world (e.g., Rogers 2015) that is able 

to support a surface liquid water ocean. 

Compelling Kepler statistics indicate these 

small planets are common (e.g., Dressing & 

Charbonneau 2013). However, a planet that 

exists within the HZ may not support surface 

liquid water due to lack of initial volatile 

delivery (e.g., Lissauer 2007; Raymond et 

al. 2006) or subsequent loss of atmosphere 

and volatiles. This may be either due to 

its star (Luger & Barnes 2015; Ramirez & 

Kaltenegger 2014; Tian 2015) or its small 

size (e.g., Mars). Consequently, although HZ 

planets are common, the frequency of small, 

temperate planets with habitable surface 

conditions remains unconstrained, and 

can only be determined via observations of 

exoplanetary environmental characteristics. 

LUVOIR will search for planets orbiting 

in or near the habitable zones of their 

Figure 3.3. O2, the canonical biosignature of 

modern Earth, has not always been detectable 

during Earth’s history (bottom row). Therefore, 

a wide wavelength range from the NUV to 

the NIR is insurance against false negative 

detections of life. During the Archean eon, 

other biosignatures such as methane must 

be relied on because oxygen is undetectable. 

During the mid-Proterozoic, oxygen can only 

be detected via its photochemical byproduct 

ozone at NUV wavelengths (top row). Credit: 

E. Schwieterman (UCR) / S. Olson (UCR) / C. 

Reinhard (GA Tech) / T. Lyons (UCR)
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3-10 The LUVOIR Interim Report

host stars (Kopparapu et al. 2013) and 

then conduct detailed investigations of 

their atmospheric compositions, energy 

budgets, surface properties, and seasonal 

variability to assess whether any of those 

potentially habitable worlds can support a 

surface liquid ocean. This study will assess 

the frequency of habitable worlds in our 

local solar neighborhood and measure the 

diversity of habitable planetary environments. 

No currently planned mission—including 

TESS, JWST, and WFIRST—can carry out 

this census of habitable environments, or 

characterize planets orbiting Sun-like stars.

In the solar system, the discovery of 

subsurface liquid water oceans on several 

icy moons has sparked a hope of finding 

habitable conditions in the icy moons of 

our cosmic backyard. LUVOIR can monitor 

plumes and surface features discovered on 

Enceladus and Europa over long temporal 

baselines, providing valuable information on 

the timing and locations of eruption activities. 

LUVOIR could also measure reflected sunlight 

in Titan’s near-infrared spectral windows, 

which provide access to the surface and 

sub-haze environment. These studies will 

provide much needed remote sensing data 

on nearby worlds that can be used in tandem 

with spacecraft measurements to gain a 

holistic picture of these environments.

3.2.1 Habitable planetary 

environments

The search for habitable exoplanets (Figure 

3.2) is a search for worlds with liquid surface 

water. Water is central to the search for 

life beyond Earth, and while subsurface 

liquid water may also provide a habitable 

environment (e.g., on Europa and Enceladus 

within the solar system), the focus on 

surface water for exoplanets is driven by 

detectability concerns. A biosphere that can 

readily exchange gases with the atmosphere 

can produce habitability markers and 

biosignatures that are more easily detected 

over interstellar distances compared to a 

subsurface biosphere. In addition to being 

a proven solvent for life for a vast diversity 

of metabolisms on Earth, water has many 

fundamental chemically favorable attributes 

for life (Pohorille & Pratt 2012), and it is the 

most common polyatomic molecule and 

solvent in the universe. Earth, with its surface 

water ocean, is the only planet known to 

support life. Elsewhere in the solar system, 

habitable environments may also exist 

where liquid water flows over rock in the 

subsurface of Mars (Ehlmann et al. 2011), 

and in the subsurface oceans of the moons 

of Jupiter and Saturn (Carr et al. 1998; 

Roberts & Nimmo 2008). Clues to habitable 

sub-surface environments may be revealed 

by detailed studies of these bodies’ surfaces 

and any outgassing, atmosphere, or plumes 

of ejecta.

LUVOIR’s survey of exoplanets will 

produce the first empirical constraints on 

the limits of the HZ for a variety of stellar 

types and broaden our understanding of the 

processes that shape habitability. In general, 

Earth-like atmospheres are considered 

those composed of a background of N2, and 

greenhouse gases such as H2O and CO2. The 

failure of these principle greenhouse gases 

(and others, like CH4, N2O, and O3 cf. Section 

3.1) to provide a clement temperature range 

as a function of incident stellar radiation 

defines the limits of the HZ. While LUVOIR 

may test this classic habitable zone definition, 

it will also explore and test alternate modes of 

habitability that may be possible such as: a) 

worlds where dense CO2 atmospheres may 

be formed by the early extreme luminosity of 

the host star for planets orbiting M dwarfs 

(e.g., Meadows et al. 2018); b) planets 

with low water abundance (so-called Dune 

worlds; Abe et al. 2011); c) planets with 

dense H2-dominated atmospheres (Gaidos 

& PIerrehumbert, 2013).
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LUVOIR will observe planets in the HZs of 

F, G, K, and M dwarfs, potentially allowing a 

comparison of the evolution of habitability on 

planets orbiting stars with different luminosity 

evolution (Baraffe et al. 1998) and activity 

levels. The differing extreme UV (XUV; l=10–

124 nm) and UV (l < 400 nm) spectra of host 

stars will lead to different atmospheric loss 

and photochemical processes (Figure 3.4), 

both of which affect planetary atmospheric 

composition. LUVOIR will characterize how 

planet atmospheric composition varies with 

and responds to the host star spectrum.

FGK dwarfs. LUVOIR will primarily 

target F, G, and K dwarf stars, which offer 

the best chance of discovering planets 

with evolutionary histories similar to Earth’s 

(compared to planets orbiting M dwarfs 

which may undergo substantially different 

evolutionary paths as described below). 

Figure 3.4. LUVOIR can interpret the atmospheric compositions of planets orbiting a variety of 

stars. Top: UV spectra of the Sun, Proxima Centauri, a K1V dwarf, and a F2V dwarf. Bottom: UV 

absorption cross-sections for key gases. Atmospheric photochemistry is effectively controlled 

by the product of the stellar UV and the molecular absorption cross-sections. Thus, different 

UV spectra from different stars, which could be obtained with LUMOS, will lead to different 

photochemical outcomes. Credit: G. Arney (NASA GSFC) / Proxima Cen: Meadows et al. (2018); 

HD97658: France et al. (2016), Youngblood et al. (2016), Loyd et al. (2016); Sigma Bootis: Segura 

et al. (2003).
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3-12 The LUVOIR Interim Report

About 90% of the LUVOIR stellar targets 

will be FGK stars, which may represent 

something more akin to the Earth-Sun 

system, with moderate stellar evolution. 

These systems represent our best chance of 

finding true Earth analogs to set our home 

world in context. FGK dwarfs, compared to 

M dwarfs, host HZ planets at large enough 

orbital separations that they can be seen 

outside the coronagraph inner working angle 

(IWA) in direct imaging for a large number 

of targets. The IWA measures the smallest 

planet-star angle at which a planet can be 

resolved, and it scales with l/D where D is 

the telescope diameter. Figure 3.5 shows 

how the IWA truncates the observable 

spectrum for planets orbiting a variety of 

stellar types. HZ planets orbiting FGK stars 

are less accessible to transit observations 

due to the long orbital periods of these 

worlds and small transit depths. If transits 

can be observed, refraction will likely inhibit 

observations of their lower atmospheres 

where water vapor is most abundant on a 

habitable planet (Misra et al. 2014a). Initial 

ground-based characterizations of HZ 

terrestrials with high-resolution spectroscopy 

will likely focus their observations on M 

dwarf targets, which offer the best planetstar contrast and best prospects for transit 

observations. Space-based direct imaging 

observatories, therefore, provide the first and 

best opportunity to image terrestrial planets 

Figure 3.5. Larger telescopes can observe more of an exoEarth’s spectrum. The colors in each 

panel show spectral coverage for stars at different distances from the Sun as a function of 

telescope diameter for the visible (515–1030 nm) and NIR (1–2 mm) channels. Spectral coverage 

of 100% means the entire wavelength range in each channel can be observed for an Earth-like 

exoplanet orbiting at the inner edge of the conservative habitable zone (0.95 AU for a G2V star). 

We assume IWAVIS = 3.5l/D, IWANIR = 2l/D, and OWA = 64 l/D. A larger fraction of the spectrum 

can be observed for hotter stars, whose HZ planets orbit further away and are less prone to being 

cut off by the IWA (although the contrast ratio will be less favorable for these stars). Although 

the cool M dwarfs have low spectral coverage because of their compact HZs, they have a higher 

occurrence rate than hotter stars. The vertical dashed line indicates LUVOIR-A (15-m). Credit: R. 

Juanola-Parramon (NASA GSFC)
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the super-luminous pre-main sequence 

stellar phase (Luger & Barnes 2015), tidal 

locking (Heath et al. 1999), and extreme M 

dwarf activity (Tarter et al. 2007). However, 

there are mechanisms that could allow for 

habitable conditions even in the face of 

these challenges: a) later migration into the 

habitable zone or late delivery of volatiles can 

mitigate the damage of the super-luminous 

pre-main sequence phase (Morbidelli et al. 

2000); b) atmospheric circulation on tidally 

locked worlds can prevent atmospheric 

collapse on the perpetual night side and 

generate thick substellar cloud decks that 

extend the habitable zone closer to the star 

(Kopparapu et al. 2016; Yang et al. 2013); 

and c) oceans can shield marine life from the 

damaging radiation produced by energetic 

flaring (Segura et al. 2010)—provided, 

of course, that the planet can retain its 

atmosphere (e.g., Garcia-Sage et al. 2017).

LUVOIR could directly observe nontransiting exoEarth planets around the 

closest M dwarfs (e.g., Proxima Centauri b), 

and it could also observe the same transiting 

M dwarf planets targeted by JWST or large 

ground-based telescopes in a shorter 

complementary wavelength range for 

transit transmission spectroscopy. Uniquely, 

LUVOIR’s access to the UV could test for 

atmospheric escape processes by detecting 

escaping H in transit (Bourrier et al. 2017; 

Ehrenreich et al. 2015). Figure 3.6 shows 

possible transit transmission spectra of 

TRAPPIST-1 b and e for a variety of scenarios, 

including habitable and non-habitable states 

(Lincowski et al, in prep). The “false positive 

O2” planet (green line) represents a world with 

abiotic photochemical O2 (Section 3.3.2) 

that can be can be distinguished from truly 

a habitable planet by its lack of H2O features 

and through the appearance of strong (O2)

2

dimer features. These are pressure-induced 

features that appear in atmospheres with 

extreme amounts of O2 characteristic of 

in reflected light and characterize Earthlike 

worlds orbiting truly Sun-like stars.

M dwarfs. M dwarfs comprise 75% of 

the main sequence stars in our galaxy, so 

the question of whether their planets can 

be habitable is critical to understanding 

the distribution of life in our galaxy. While 

M dwarf habitable planets are on close-in 

orbits around their dim stars, LUVOIR’s large 

aperture will provide an IWA that allows direct 

observations of planets orbiting nearby M 

dwarfs, and they will comprise about 10% 

of the stars LUVOIR will survey. LUVOIR 

can also conduct transit transmission 

observations of M dwarfs in a wavelength 

range complementary to JWST, but direct 

imaging of M dwarf HZ planets will be the 

most effective way of detecting water vapor 

in their deep atmospheres. For HZ terrestrial 

planets, measurement of deep atmosphere 

water vapor will likely elude transmission 

spectroscopy measurements of the handful 

of M dwarf targets that JWST will observe, 

as transmission probes higher stratospheric 

altitudes with much lower water abundances 

(Meadows et al. 2018; Cowan et al. 2015). 

However, recent work suggests that 

“habitable moist greenhouse” states with wet 

stratospheres are possible for slowly rotating 

planets with thick substellar cloud decks that 

cool the surface environment. High altitude 

water can still be lost to space, but if loss 

occurs slowly, this may result in a novel type of 

habitable state with observable stratospheric 

water vapor in transit observations (Fujii et al. 

2017; Kopparapu et al. 2017).

Although F, G, and K stars may harbor 

habitable planets with histories and 

characteristics more like Earth’s, M dwarf 

planets provide a fascinating complement 

from a habitability standpoint as they likely 

undergo a significantly different evolutionary 

sequence compared to planets orbiting F, 

G and K stars. This different evolution can 

include possible extreme water loss during 
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3-14 The LUVOIR Interim Report

certain abiotic photochemical processes. 

Also included are a Venus-analog world (red 

line) that exhibits a relatively flat spectrum 

broken only by a few CO2 bands in the NIR 

and SO2 absorption in the UV. The modern 

Earth-analog planet shows spectral 

indicators of H2O, O2, and O3, although the 

H2O bands are weak since most of the water 

is trapped in the lowest atmospheric scale 

height not sensed in transit.

Atmospheric composition of habitable 

worlds. To comprehensively characterize 

the atmosphere of a detected planet in 

reflected light, the presence or absence of as 

many gases of interest as possible must be 

constrained. It is also important to measure 

the continuum flux level and slope set by 

Rayleigh and cloud/haze scattering. Table 

3.1 summarizes several gases of interest, 

and the wavelengths at which they absorb.

In practice, observations out to only ~1.8 

mm will be obtainable for most Earth-like 

targets, because thermal radiation from the 

warm telescope mirror (270 K) swamps the 

planet signal at longer wavelengths; LUVOIR 

is not a cryogenic telescope to prevent UVabsorbent contamination from condensing 

on the mirrors (see Appendix D). For a 

modern Earth-like or early-Earth-like HZ 

terrestrial planet, at wavelengths from 0.2–

1.8 mm, trace gas absorption will be primarily 

from important greenhouse gases H2O and 

CH4. Absorption from CO2, NH3, O3, O2, CO, 

and O4 also occurs in this wavelength range 

(Table 3.1) and can be used to characterize 

diverse terrestrial planet atmospheres, and 

Figure 3.6. LUVOIR could observe planets transiting M dwarfs in the UV-Visible-NIR, 

complementing longer wavelength observations of these same worlds with JWST and groundbased telescopes. Shown here are possible transmission spectra of TRAPPIST-1 b and 

TRAPPIST-1 e for several atmospheric scenarios, including habitable and non-habitable states. 

Non-habitable states include a planet with “false positive” O2 produced by abiotic photochemical 

processes and a Venus-analog. Also included is an Earth analog for TRAPPIST-1 e. Credit: A. 

Lincowski (University of Washington). 
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The LUVOIR Interim Report 3-15

also provide key information to discriminate 

terrestrial planets from H2-dominated subNeptune planets. The longer the wavelength 

range and the larger number of absorption 

bands that can be detected, the greater the 

fidelity with which atmospheric composition 

can be discerned. Detecting multiple 

features improves our confidence in the 

unique detection of a specific gas by ruling 

out similar, overlapping absorption features 

produced by different gases. For example, 

CH4 and H2O have several overlapping 

absorption features at visible and nearinfrared wavelengths, including near 1.1 and 

1.4 mm, which could be confused in spectra 

of low to moderate resolution. LUVOIR’s NIR 

channel can obtain spectra at R=200 to help 

break this degeneracy (it can also obtain 

spectra at R=70). However, in this example, 

water can be identified cleanly near 0.94 

mm, and CH4 cleanly near 1.7 mm (also near 

1 mm at Archean-like abundances), allowing 

the degeneracy to be broken, and improving 

model fitting and retrieval for both gases. 

The smaller 8-m aperture (LUVOIR-B) will 

reduce the number of planets that this full 

wavelength range can be obtained for, but 

the study of the 8-m architecture, and its final 

design, are not completed. We will ensure 

that our major science questions can still be 

addressed with LUVOIR-B and will discuss 

this in detail in our Final Report.

Atmospheric composition and pressure 

affects habitability, as the stability of surface 

liquid water depends on both surface 

temperature and pressure from the overlying 

atmosphere. Both will be challenging to 

constrain. A Rayleigh scattering slope, or the 

detection of dimer (i.e., a molecular complex 

consisting of two identical molecules linked 

together) or pressure-induced absorption 

features (Misra et al. 2014b), could allow 

measurements of atmospheric pressure, at 

least to the lowest observable atmospheric 

layer. The lowest observable layer may not 

be the surface due to cloud cover, scattering 

aerosols, or molecular opacity. Detection of 

Rayleigh scattering requires observations at 

visible wavelengths and does not place strict 

requirements on spectral resolution. It will 

not be possible to measure temperatures 

directly in the absence of planetary thermal 

radiation (although thermal detection of 

exoplanets orbiting Sun-like stars using 

ground-based observations with ELTs could 

provide excellent complementary information 

on planetary temperature; e.g., Quanz et 

al. 2015). However, it may be possible to 

constrain temperature, and also habitability, 

by retrieving a water vapor profile consistent 

with condensation (i.e., a condensation 

Table 3.1. Desired spectral features for habitability assessment. 

Habitability Markers

Molecules/Feature 0.2–1.0 mm 1.0–2.0 mm

H2O 0.65, 0.72, 0.82, 0.94 1.12, 1.4, 1.85

H2 0.64 – 0.66, 0.8 – 0.85

CO2 1.05, 1.21, 1.6

CH4 0.6, 0.79, 0.89, 1.0 1.1, 1.4, 1.7

S8 0.2-0.5

H2S < 0.3

SO2 < 0.3

Ocean glint 0.8 – 0.9 1.0 – 1.05, 1.3

Rayleigh scattering ≲ 0.5 
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3-16 The LUVOIR Interim Report

curve). Constraints on the water vapor 

profile could be obtained by observing water 

bands at different wavelengths, which sense 

different depths in the atmospheric column. 

Water bands between 0.6 and 1 mm probe 

the near-surface environment, while NIR 

water bands at 1.15 and 1.4 μm sense the 

middle and upper troposphere.

Other atmospheric features. A 

wide spectral range permits discovery of 

unexpected atmospheric absorbers and 

provides a longer lever arm to constrain cloud 

particle sizes and composition through Mie 

scattering effects. Blue and UV wavelengths 

are particularly important for observing these 

effects. Observations of exoplanets may 

reveal the presence of hydrocarbon, sulfuric 

acid or water vapor hazes in terrestrial 

atmospheres, especially at UV-visible 

wavelengths (e.g., Arney et al. 2016; Hu et al. 

2013; Figure 3.2). Spectral estimates of the 

composition, particle size, and optical depth 

of the planetary aerosols could also constrain 

the albedo of the planet (whether it is more 

likely to be reflective, or dark), helping to 

improve the inherent size-albedo degeneracy 

for directly imaged, non-transiting planets. 

Spectral features or scattering slopes of 

volcanically-generated species such as CO2, 

H2S, SO2, or H2SO4 aerosols, especially if 

these vary on monthly or yearly timescales, 

could indicate active volcanism (Hu et al. 

2013; Kaltenegger & Sasselov 2010; Misra 

et al. 2015). Geological activity is important 

for maintaining planetary habitability on Earth 

over long timescales (Walker et al. 1981) 

through plate tectonics, the carbonate silicate 

cycle, and seafloor weathering (KrissansenTotton & Catling 2017). Elemental sulfur 

(S8) particles, produced by photochemical 

reactions involving volcanic gases such as 

H2S, can produce broad absorption features 

at l < 0.5 mm (Hu et al. 2013). H2S and SO2

could be detected in the UV at wavelengths 

< 0.3 mm. Some gases, which are highly 

soluble in water like SO2, could also serve 

as desiccation markers, hinting at a lack of a 

significant surface ocean.

Direct detection of liquid surface water. 

While gas phase water may be detected 

through its atmospheric spectral features, 

direct detection of surface liquid water may 

also be possible for a subset of the most 

observable planets through detection of 

specular reflectance from liquid water oceans 

(the “glint” effect). This phase-dependent 

phenomenon can be sought in either 

reflection or polarization. These observations 

are best made in the near-infrared where the 

relative lack of Rayleigh scattering enhances 

the detectability of the planetary surface 

and the glint phenomenon (Robinson et al. 

2010; Zugger et al. 2011). Ocean glint could 

be detected with photometric observations 

at continuum wavelengths (i.e., between 

strong atmospheric absorption bands), and 

especially those near 0.8–0.9, 1.0–1.05, and 

1.3 mm; Robinson et al. 2010), at multiple 

planetary phases between quadrature and 

crescent. Near-infrared observations also 

minimize glint false positive signatures from 

polar ice (Cowan et al. 2012) which is less 

reflective at longer wavelengths. For an 

Earth-twin, clouds and hazes may make the 

polarization signature from surface oceans 

challenging to detect (Zugger et al. 2011), 

and the reflective glint signal may prove 

the best option for the direct detection of 

surface liquid water. For Earth, glint is most 

pronounced near a star-planet-observer 

(phase) angle of 150°, where a glinting 

planet would be nearly twice as bright as a 

non-glinting planet. The ability to measure 

planetary phase functions at such close 

planet-star separations will depend on the 

IWA of the high-contrast field-of-view and 

cannot be achieved for planetary systems 

with inclinations below about 60 degrees. An 

IWA of 2 l/D in the NIR channel allows glint 

detection in the 1.33 mm continuum out to 
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The LUVOIR Interim Report 3-17

Figure 3.7. Icy moons of our solar system like Europa (left) and Enceladus (right) offer an incredible 

opportunities to find habitable conditions and life in our cosmic backyard. The relative paucity 

of craters on Europa testifies to its young surface; the dark linear features point to cracks in the 

ice shell leading to the deep ocean. On Enceladus, plumes erupt from the “Tiger Stripes” region 

visible around the south pole. Credit: NASA

almost 10 pc for habitable planets around G 

dwarfs, and nearly 5 pc for K dwarfs.

3.2.2 Ocean/icy worlds of the solar 

system

Within the solar system, the icy moons of 

the giant planets remain some of the most 

intriguing observational targets, motivated 

strongly by their potential to harbor life in 

subsurface oceans. While dozens of moons 

have or have had oceans, a few special 

cases are considered the best targets in 

the search for life and prebiotic conditions: 

Europa, Enceladus, and Titan. These moons 

represent end members of a different 

kind of habitable world from the terrestrial 

planet: worlds whose icy surfaces, deep 

oceans, and interactions of endogenic and 

exogenic energy sources give rise to the 

physical and geochemical processes that 

might create and maintain life. LUVOIR will 

conduct game-changing science for the 

solar system’s ocean worlds, via multi-epoch 

imaging at high spatial resolution (~25 km 

resolution at 500 nm for Jupiter at opposition 

for LUVOIR-A, comparable to imaging with 

the Juno spacecraft) and sensitive spatially 

resolved spectroscopy at UV through NIR 

wavelengths.

For solar system icy ocean worlds, the 

question is not whether these oceans exist, 

but rather: “how may we study them?” and 

“how do these potentially habitable systems 

operate?” Europa orbits Jupiter between Io 

and Ganymede in a Laplace resonance, and 

it has maintained geologic activity over the 

lifetime of the solar system as a result of early 

accretional heat, radiogenic activity, and for 

at least the past 2.5 billion years, immense 

tidal heating from Jupiter. Europa’s surface 

is churned over by tectonic and resurfacing 

processes and displays so few craters that 

it can only be constrained to an age of 

40–90 Myr (Figure 3.7). The young age of 

the surface implies ongoing surface-ocean 

communication, and materials in the cracks 
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3-18 The LUVOIR Interim Report

composition does not vary across the moon, 

suggesting upwelling from a globally mixed 

ocean. LUVOIR could monitor for variations 

in the surface distribution of these species 

that could indicate active upwelling of fresh 

material from the underlying ocean.

Plumes have been reported for both 

of the ice-covered ocean moons Europa 

and Enceladus, which may allow direct 

access to the chemistry and composition 

of the deep ocean. Two different groups 

have employed the HST STIS instrument 

to find evidence for transient water plumes 

emanating from its surface via observations 

of H and O emission (Figure 3.8; Roth et al. 

2014) and in UV continuum absorption while 

Europa transited the bright surface of Jupiter 

(Sparks et al. 2016, 2017). In both cases, 

the detections were at the limit of the HST 

instrumentation, yielding detections at 3–6 s

levels of significance (for a water column of 

~2 x 1020 m-2 to 2 x 1021 m-2). While some 

and fissures on the surface may represent 

compounds that have upwelled from the 

deep sea and/or could be transferred down 

into the sea.

Spectroscopic measurements of the surface of Europa have revealed the presence 

of a variety of surface materials. Oxidants 

such as hydrogen peroxide (H2O2) and solid state O2

 are generated by charged particle bombardment from the environment 

(Delitsky & Lane 1998). Such oxidants may 

be important for fueling metabolisms in the 

Europan ocean if they can be transported 

through the icy shell (Chyba 2000). Hydrated 

salts and other materials have been detected and absorb between 1 and 2 mm, accessible to LUVOIR. These include magnesium 

sulfate hydrates (MgSO4•nH2O), bloedite 

(Na2Mg(SO4)

2

•4H2O) (McCord et al. 1999), 

and sulfuric acid (H2SO4•nH2O) (Carlson 

1999). Typically, these materials occur along 

ridges, in chaos terrain, and in craters. Their 

Figure 3.8. Spectroscopic imaging of Europa and its water jets. The left panel shows an aurora 

on Europa observed with HST (Roth et al. 2014). This UV hydrogen emission (Lyman-alpha) 

comes from dissociation of water vapor in jets emanating from the surface. The right panel shows 

a simulation of how this hydrogen emission from Europa would look to a 15-m UV telescope. 

The moon’s surface is bright due to reflected solar Ly-alpha emission. With LUVOIR, one could 

monitor the ocean worlds of the outer solar system for such activity and image the individual jets. 

Credit: G. Ballester (LPL)
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The LUVOIR Interim Report 3-19

of the observations were repeated, no 

definitive cyclic timing could be discerned; 

thus, the frequency and dynamics of these 

sporadic plumes is unclear. With the planned 

spatial resolution and collecting power of 

LUVOIR-A, emission from individual plumes 

could be resolved in FUV spectroscopic 

imaging observations with the LUMOS 

instrument (Figure 3.8). Such observations 

would constrain plume frequency, dynamics, 

and plume-magnetosphere interactions. ELT 

ground-based observatories will be able to 

perform complementary NIR studies, yet 

strong attenuation of the water signatures 

in the infrared due to telluric opacities and 

not having access to the UV will severely 

hinder the sensitivities obtained using these 

facilities.

On Enceladus, plumes are known to erupt 

near the south pole from the “Tiger Stripes” 

region (Figure 3.7). The mass of the ejected material varies following a diurnal cycle 

(Hedman et al. 2013), responding to openings and closings of the fissures (Nimmo et 

al. 2014). The plumes themselves operate 

stochastically, possibly caused by variations 

in tidal stresses (Teolis et al. 2017). Cassini’s 

Visual and Infrared Mapping Spectrometer 

(VIMS) has revealed that the Tiger Stripes 

region is characterized by water, organics, 

carbon dioxide, and amorphous and crystalline water ice (absorbing at 1.5, 2.0 mm; 

Brown et al. 2006). While the plumes of 

Enceladus have so far only been detected 

with the Cassini spacecraft, the reported column densities of plume material (~1x1020 m2

; 

Hansen et al. 2006) are not much less than 

those inferred for Europa, making it highly likely that the more frequent Enceladus 

plumes will be observable with LUVOIR-A 

in a similar fashion to those of Europa. The 

possible detection of other materials within 

the plumes with LUVOIR—for example water 

vapor, ice particles, or other more intriguing 

trace species from the deep ocean—will be 

investigated.

Titan, in addition to its sub-ice internal 

ocean, harbors a thick methane- and 

haze-rich atmosphere as well as surface 

reservoirs of liquid ethane-methane mixtures 

(Figure 3.9). Titan’s hazes are transparent 

in the NIR and allow access to the surface 

and lower atmosphere for monitoring of 

surface features. These “spectral windows,” 

Figure 3.9. The surface of Titan (false color) observed in NIR spectral windows as seen by a 

2.4-m telescope versus a 15-m LUVOIR telescope. Credit: NASA Cassini / R. Juanola-Parramon 

(NASA GSFC)
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3-20 The LUVOIR Interim Report

accessible to LUVOIR, have previously been 

used to discover methane clouds in the 

lower atmosphere (Griffith et al. 1998) and to 

find evidence of hydrocarbon lakes (Stephan 

et al. 2010). Meanwhile, the UV provides 

key diagnostics of the hazes and the 

atmosphere. With LUVOIR’s spectroscopy of 

the complete observable disk accessible over 

a long baseline, we will better understand 

the processes acting on the organics / water 

reservoirs of Titan.

3.2.3 Strategy for identifying 

habitable exoplanets

LUVOIR will revolutionize exoplanet science 

by searching hundreds of stars for potentially Earth-like exoplanets and discovering 

hundreds of diverse exoplanets in the process. Precisely estimating the expected exoplanet science yield necessitates modeling 

the execution of such a mission, which in 

turn requires constraints on several key astrophysical parameters as well as a high-fidelity simulator of exoplanet direct observation missions. A decade ago, such modeling 

was not possible. Now, the Kepler Mission 

has constrained the frequency of Earth-sized 

potentially habitable planets around Sun-like 

stars, the Keck Interferometer Nuller and the 

Large Binocular Telescope Interferometer 

have constrained the presence of warm dust 

around nearby stars, and mission simulators 

have advanced dramatically.

Here we describe an observational 

procedure to find habitable exoplanet 

candidates, then establish whether they 

may in fact be habitable (i.e., have signs of 

water). For planets that remain promising 

for habitability and life, the steps for 

characterization continue in Section 3.4. 

Figure 3.10 shows a graphical summary 

of the key questions and the observational 

means of answering them. Blue steps shown 

in Figure 3.10 are discussed here for planet 

detection, while green steps are discussed in 

Section 3.4 for planet characterization.

1. Establish target list. LUVOIR’s list 

of target stars is already known. Using a 

benefit-to-cost metric, we will optimize this 

list of stars to maximize the probability of 

detecting an exoEarth based on existing 

information available in the 2030s—such as 

the amount of exozodiacal dust around the 

star, stellar multiplicity, and the presence of 

other known planets.

2. Determine which point sources are 

planets. Once LUVOIR obtains an image 

of a potential exoplanet system, metrics 

are needed to distinguish planets from 

background stars, galaxies, brown dwarfs, 

and other confusion sources (Figure 3.11). 

A combination of methods could be used to 

counter background confusion. Multi-epoch 

observations will reveal targets with the same 

proper motion as the parent star, an excellent 

metric for source confusion discrimination. 

Program at a Glance – Potentially Habitable Solar System Worlds

Goal: Monitor potential habitable solar system worlds.

Program details: Observations of solar system icy moons to monitor surface morphological 

and compositional changes, study plume activity, and measure atmospheric properties 

(Titan) 

Instrument(s) + Configuration: HDI multi-band imaging; LUMOS FUV spectroscopy; 

ECLIPS open mask IFS spectroscopy

Key observation requirements: Long temporal baseline observations of icy moons for 

monitoring surface changes and plume activity. Spatially resolved FUV (R~30,000) and 

NIR (R~70, 200) spectroscopy.
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The LUVOIR Interim Report 3-21

During these same observations, LUVOIR’s 

coronagraph for Architecture A has a camera 

mode that collects photometric data across 

a given channel for the whole field-of-view. 

Thus, multi-color point-source photometry 

will be obtained during these multi-epoch 

observations, providing additional information 

to discriminate planets from background 

objects.

3. Constrain orbits. Multi-epoch 

observations will reveal targets with the same 

proper motion as the host star and place 

constraints on orbits, with observations likely 

in the V band. About four detections of the 

planet spaced out over the orbit are needed 

to constrain the orbit. Some observations will 

miss the planet, so on average of 6 visits per 

star may be needed for orbital determination.

Figure 3.10. Science questions and observational strategy in the search for habitable planets 

and life. Blue steps at the top of the figure refer to identifying habitable exoplanets; green steps 

at the bottom of the figure refer to characterizing habitable exoplanets. Credit: T. B. Griswold 

(NASA GSFC)
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3-22 The LUVOIR Interim Report

4. Search for water. A useful wavelength 

range for initial habitability assessment (i.e., 

the search for atmospheric water vapor) is 

0.87–1.05 mm for two reasons. First, this 

channel is less likely to be cut off by inner 

working angle constraints compared to 

longer wavelength water features. Second, 

the 0.94 mm H2O feature is accessible, as is 

the 1.0 mm CH4 feature, which could provide 

evidence of an atmosphere rich in CH4 (e.g., 

an Archean Earth analog or a Jupiter analog). 

The H2O and CH4 features do not overlap 

significantly in this wavelength range, which 

will remove possible degeneracies for the 

type of planet under observation. The H2O 

band at 1.12 mm, although stronger, overlaps 

closely with the 1.1 mm CH4 feature, leading 

to potential misinterpretation of planets with 

abundant CH4 but no water.

3.2.4 Exoplanet yields for the 

baseline LUVOIR-A concept

Using new exoplanet yield estimation 

methods (e.g., Stark et al. 2014), we 

have estimated the quantity and quality of 

exoplanet science that the LUVOIR mission 

concept could produce. Here we summarize 

the exoplanet yield that would result from 

a habitable planet search with the baseline 

15-m LUVOIR-A concept. This initial 2-year 

campaign includes a survey of hundreds 

of stars that would take us through Step 4 

in Figure 3.10, establishing a target list for 

more detailed characterization to be carried 

out subsequently (Steps 5–10 in Figure 

3.10), following the strategy outlined in 

Section 3.4. Note that although this report 

discusses extending the LUVOIR mission 

lifetime via servicing, the whole strategy 

shown in Figure 3.10 is designed to fit 

within a 5-year program. In practice, planets 

to receive more detailed characterization 

would be selected through competed 

proposals because LUVOIR is planned as a 

guest observer facility. A detailed description 

of the exoplanet science yield calculations, 

the techniques used, assumptions made, 

and justification for the adopted observing 

strategy appears in Appendix B.

ExoEarth candidates are defined to 

be on circular orbits and reside within the 

Figure 3.11. Is that a planet? A high resolution simulated view of the solar system at a distance 

of 10 pc including expected noise and confusion sources: scattered light from interplanetary dust 

(aka. exozodiacal dust), background galaxies, and background stars. Credit: Roberge et al. (2017)
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The LUVOIR Interim Report 3-23

conservative HZ, spanning 0.95–1.67 AU for 

a solar-twin star (Kopparapu et al. 2013). We 

only consider planets with radii smaller than 

1.4 REarth and radii larger than or equal to 

0.8a–0.5 REarth, where a is semi-major axis for a 

solar-twin. The lower limit on our definition of 

the radius of exoEarth candidate comes from 

an empirical atmospheric loss relationship 

derived from solar system bodies (Zahnle 

& Catling 2017). The upper limit on planet 

radius is a conservative interpretation of an 

empirically measured transition between 

rocky and gaseous planets at smaller semimajor axes (Rogers 2015).

We estimate the yield of detected 

exoEarth candidates for the baseline 

LUVOIR 15-m mission to be 51+75

-33, where 

the range is set by uncertainties on hEarth and 

finite sampling uncertainties (see Appendix 

B). If the frequency of habitable conditions 

is 10%, then 29 exoEarth candidates would 

be required to guarantee seeing one true 

exoEarth at 95% confidence. Thus, LUVOIR 

should be able to detect at least a small 

number of true “Earths”—or discover that 

habitable conditions on planets in their 

stars’ habitable zones are rare. Following 

the detection of habitable planet candidates 

(i.e., a positive water vapor detection), we 

will continue to characterize the rest of the 

spectrum following the strategy in Section 

3.4.

While searching for and characterizing 

exoEarth candidates, LUVOIR will detect 

dozens of other planets, from hot rocky 

worlds to cold gaseous planets. These other 

planets are essentially detected “for free” 

during the exoEarth search. Figure 3.12

shows the expected exoplanet yields of all 

planet types when optimizing the observation 

plan for the detection of exoEarth candidates 

(green bar). Following the planet classification 

scheme in Kopparapu et al. (2018), the other 

planet class types in this figure are: rocky 

planets (radii of 0.5–1 REarth), super-Earths 

(radii of 1–1.75 REarth), sub-Neptunes (radii 

of 1.75–3.5 REarth), Neptunes (radii of 3.5–6 

REarth), and Jupiters (radii of 6–14.3 REarth). 

ExoEarth candidates are a subset of the 

rocky and super-Earth planets within their 

stars’ habitable zones. “Hot” planets (red 

bars) receive 182 to 1x Earth’s insolation; 

“warm” plants (blue bars) receive 1 to 

0.28x Earth’s insolation; “cold” planets (ice 

blue bars) receive 0.28 to 0.0035x Earth’s 

insolation.

Figure 3.13 summarizes the expected 

target list LUVOIR’s search program. LUVOIR 

will observe ~260 stars covering a wide 

variety of spectral types, primarily FGK stars 

with some M dwarfs and a few A dwarfs. 

Target stars will be within 25 pc; most will 

be brighter than 7th magnitude. LUVOIR will 

achieve a high average HZ completeness, 

and the majority of its time would be spent 

observing planets around high completeness 

stars.

In reality, yields may vary from the 

expected values shown in Figure 3.12

due to astrophysical uncertainties and 

the actual distribution of planets around 

nearby stars. The yield uncertainties shown 

in Figure 3.12 are estimated as the rootmean-square combination of the NASA 

Exoplanet Exploration Program Analysis 

Group SAG131 1-σ exoplanet occurrence 

rate uncertainties and the uncertainty due 

to the random distribution of planets around 

individual stars. The latter was estimated 

by assuming that planets are randomly 

assigned to stars, such that multiplicity is 

governed by a Poisson distribution, and that 

each observation represents an independent 

event with probability of success given by 

that observation’s completeness. Error 

bars do not include uncertainty in the 

median exozodi level, any uncertainties 

in mission performance parameters, or 

uncertainty in observational efficiency. 

1 https://exoplanets.nasa.gov/exep/exopag/sag/
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The uncertainties for the cold planet yields 

are likely underestimated, as the SAG13 

occurrence rates are purely extrapolations in 

this temperature regime.

The role of precursor/follow-up 

knowledge for exoplanet direct imaging. 

Counter-intuitively, precursor knowledge 

of the host stars and orbits of exoEarth 

candidates does not greatly increase the 

total number of such planets discovered and 

characterized, in the case where a direct 

observation mission is target-limited rather 

than time-limited. This is the situation for 

both LUVOIR observatory concepts, A and 

B. However, such precursor knowledge is 

of great value for increasing the observing 

efficiency of exoplanet direct observation 

missions. A full explanation of this issue will 

be provided in Stark et al., in preparation. 

Here we summarize the main conclusions 

about exoplanet precursor and supplemental 

studies for future missions like LUVOIR and 

HabEx.

1) Orbit determination is required to quantify 

the energy fluxes incident on an exoplanet 

from its host star. This is particularly 

necessary to show that an exoplanet is 

located in its star's HZ. If not done in 

Figure 3.12. Exoplanet detection yields for different classes of planets from an initial 2-year 

habitable planet survey with the LUVOIR-A concept. Red, blue, and ice blue bars indicate hot, 

warm, and cold planets, respectively. The green bar shows the expected yield of exoEarth 

candidates, which are a subset of the warm rocky and super-Earth planets. Planet class types 

from left to right are: exoEarth candidates, rocky planets, super-Earths, sub-Neptunes, Neptunes, 

and Jupiters. Planet types other than exoEarth candidates are detected “for free” during the 

2-year search campaign. Color photometry and orbits are obtained for all planets. Two partial 

spectra are obtained for all planets in systems with exoEarth candidates. Credit: C. Stark (STScI)
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The LUVOIR Interim Report 3-25

advance of direct observations, then it 

should be done concurrently. 

2) Mass measurements are highly desirable 

in order to understand an exoplanet's 

bulk properties and to constrain the 

effects of surface gravity on the observed 

atmospheric spectrum. This information 

could be obtained either through 

precursor, concurrent, or follow-up work. 

Figure 3.13. Summary of targets selected for observation in the exoEarth search campaign for 

the LUVOIR 15-m concept. The bottom right panels show which coronagraph was used for each 

star: 1=Small-IWA APLC, 2=Medium-IWA APLC, 3=Large-IWA APLC, 4=Vector Vortex. Credit: 

C. Stark (STScI) 
LUVOIR The Large UV Optical Infrared Surveyor

3-26 The LUVOIR Interim Report

3) Precursor observations that identify stars 

with detectable exoplanets (especially 

those in the HZ), and which develop 

orbital ephemerides sufficient to predict 

when the targets could be best observed, 

are desirable for mission planning. This 

information could reduce the mission 

resources (e.g., integration time) devoted 

to exoplanet searches and thus save 

them for other uses. 

4) The number of HZs (or ice lines) accessible 

to a given imaging mission architecture is 

a 1:1 function of the inner working angle 

of that architecture. When the mission has 

sufficient observing resources to study 

all the exoplanets of interest outside its 

design IWA, it is target-limited and the 

effect of prior knowledge on mission yield 

is small. When there are more accessible 

targets outside the IWA than mission 

resources to study them, the effect of 

prior knowledge on mission yield may be 

substantial. 

5) The primary targets of direct observation 

missions are stars with HZs outside 

the design IWA. Precursor knowledge 

showing the absence of a HZ planet 

may be used to de-prioritize certain 

target stars. However, a target's value 

to comparative planetology and planet 

formation studies should be taken 

into account before excluding it from 

observation.

The scope of the observing program(s) 

required to address Items 1–3 depends on 

whether these measurements are done prior 

to a mission, concurrently, or afterwards. 

Precursor observations would need to 

survey all primary targets accessible to 

the mission design, whereas concurrent 

or follow-up observations would only need 

to study the subset of stars with planets of 

interest. Exoplanet orbits may be determined 

in advance with ground-based radial velocity 

instruments or concurrently from a series of 

exoplanet direct imaging observations with 

LUVOIR/HabEx. Either ground-based radial 

velocity instruments or a dedicated space 

astrometry mission could make exoplanet 

mass measurements. It may also be possible 

to make the necessary mass measurements 

by equipping the direct imaging mission with 

the appropriate astrometric or radial velocity 

instrument. A dedicated astrometry mission 

would likely be the most expensive option to 

fulfill Items 1–3, while ground-based radial 

velocity would likely be the least expensive 

(with other options falling in between). 

However, a factor of 5–10 improvement in 

radial velocity measurement accuracy on 

Sun-like stars will be needed before this 

method, either from ground or space, could 

measure the masses of rocky exoplanets.

3.3 Signature science case #2: 

Which worlds are inhabited?

LUVOIR will provide an unprecedented 

capability to search for life beyond the solar 

Program at a Glance – Detecting Habitable Exoplanet Candidates

Goal: Detect habitable exoplanet candidates around a range of FGKM stars. 

Program details: Observations of nearby stars in target list to search for water-bearing 

habitable zone rocky exoplanets

Instrument(s) + Configuration: ECLIPS coronagraphic imaging and spectroscopy

Key observation requirements: Contrast < 10-10; photometry near 500 nm to detect 

planets; spectroscopy at R=70, SNR=5 to detect water near 900 nm
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 3-27

system by characterizing the surfaces and 

atmospheres of a large sample of terrestrial 

planets in the HZ and searching for signs of 

life (Figure 3.14). A planet-wide biosphere 

can modify its environment to leave 

characteristic indications of its presence 

such as specific gases (Des Marais et al. 

2002; Sagan et al. 1993; Schwieterman et al. 

2018), atmospheric chemical disequilibrium 

(Hitchcock & Lovelock 1967; KrissansenTotton et al. 2018, 2016a), surface reflectance 

features (Gates et al. 1965; Hegde et al. 

2015; Schwieterman et al. 2015; Seager et 

al. 2005) and time-dependent modification 

of environmental characteristics caused by 

biological processes (Meadows 2008, Olson 

et al. 2018). Not all features produced by life 

will be detectable; LUVOIR will only be able 

to detect the subset of biosignatures that 

produce absorption features in its wavelength 

range. In addition to detecting potential 

biosignatures, it is important to gather 

additional information so that biosignatures 

can be interpreted in the context of the whole 

planetary and stellar environment, a process 

that can help rule out abiotic processes called 

“false positives” that can mimic biosignatures. 

Biosignature assessment is then completed 

with a search for other corroborating 

contextual signs of habitability and life. 

Detection and interpretation of biosignatures 

and environmental features requires highcontrast, high sensitivity, moderate resolution 

spectroscopy, and a broad wavelength range 

to detect multiple spectral features from 

both potential biosignatures and telltale false 

positive indicators. A large mirror enables a 

wider wavelength range to be obtained for 

any given IWA, and the IWA (not the OWA) 

is what limits the wavelength range that 

can be observed for most habitable zone 

planets at distances accessible to LUVOIR. 

LUVOIR’s survey of possibly dozens of 

nearby exoEarths offers the best chance in 

the coming decades to answer at long last 

whether we are alone in the universe, and to 

place Earth’s biosphere in context.

3.3.1 The search for biosignatures

There are several target biosignatures to 

be sought, and also signs of disequilibrium 

chemistry that may allow us to constrain 

surface fluxes of gases that are unlikely to 

be produced by abiotic planetary processes. 

Absorption features from biosignatures and 

biosignature false positive discriminators are 

provided in Table 3.2. Key environmental 

characteristics to be sought for interpretation 

of biosignatures include planetary mass/size/

density, atmospheric mass and composition, 

the nature of the planetary surface, and 

stellar characteristics including spectral type, 

UV activity, and age.

While the specific target gases identified 

for Earth (Table 3.2) will be searched for, a 

more generic search for biosignatures is to 

determine fluxes (i.e., flow rates) of gases into 

the environment and constrain sources and 

sinks for those gases. This can be done by 

searching for environmental characteristics 

that are out-of-equilibrium, implying a constant 

replenishment of a gas against chemical or 

Figure 3.14. LUVOIR will provide a statistical 

search for life on nearby exoplanets. In the 

2040s, LUVOIR will offer the best chance 

of answering the question “Are we alone 

in the universe?” Exotic photosynthetic life 

may generate novel surface reflectance 

biosignatures, as suggested in this image, and 

LUVOIR’s direct observing capabilities can 

search for these types of novel photosynthetic 

pigments. Credit: NASA
LUVOIR The Large UV Optical Infrared Surveyor

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photochemical destruction. For instance, 

O2 is out of kinetic equilibrium with CH4 on 

Earth, implying significant surface fluxes for 

these gases that cannot readily be explained 

by abiotic processes (Etiope & Sherwood 

Lollar 2013; Hitchcock & Lovelock 1967). 

If no other (abiotic) plausible explanation 

can be found, the gases in question could 

represent unexpected biological processes 

and should be explored further. Below we 

describe specific target biosignature gases.

Oxygen (O2). The highest priority 

biosignature gas is molecular oxygen 

(Meadows 2017). O2 is the byproduct of 

oxygenic photosynthesis, currently the 

dominant metabolism on our planet and 

possibly the most productive metabolism 

for any planet (Kiang et al. 2007b, 2007a). 

Oxygenic photosynthesis uses cosmically 

abundant water, atmospheric CO2, and 

sunlight to create biomass and power life. 

Oxygen is unusual for biogenic products 

in having its strongest features in the 

visible and near-infrared. Because its high 

abundance results in even mixing throughout 

the atmospheric column, it produces strong 

spectral features even above a planet-wide 

cloud deck. Oxygen (or its photochemical 

byproduct ozone) may have been detectable 

in Earth’s spectrum since the Proterozoic 

period (2.5 billion years ago–541 million 

years ago; Segura et al. 2003; Kaltenegger 

et al. 2007), although recent geochemical 

results may indicate a significant rise in O2 as 

recently as 0.8 Gyr (Planavsky et al. 2014). 

Although the photochemical destruction 

of H2O and CO2 may generate abiotic O2, 

along with telltale spectral signatures of 

these processes (Meadows 2017; Section 

3.3.2), discrimination between abiotic and 

biological sources of O2 can also hinge on its 

production rate and the nature of O2 sinks in 

the environment. Consequently, constraints 

on atmospheric and surface composition are 

required to determine if the biosignature gas 

has the measured concentration because 

there is a large surface flux of the gas in an 

environment that has strong sinks for it, or if 

there is a weaker, possibly abiotic source of 

the O2 in an environment with fewer sinks.

Ozone (O3). Ozone is a photochemical 

byproduct of O2. Because it produces UV 

spectral features that are detectable at low O2 

levels that render O2 itself spectrally invisible 

(e.g., Domagal-Goldman et al. 2014), it can 

be used to infer atmospheres with low (e.g., 

low Proterozoic-like) oxygen concentrations 

(Figure 3.2). As a photochemically produced 

gas, its production rate also depends on 

the UV spectrum of the host star in addition 

to the amount of O2 in the atmosphere. 

The very strong O3 Hartley-Huggins band 

Table 3.2. Desired spectral features for biosignature assessment. Note some gases relevant to 

habitability are also biosignatures and so also appear in Table 3.1 (e.g., methane is a greenhouse 

gas important to habitability that can also be produced by life).

Biosignatures & False Positive Discriminants (indicated with *)

Molecules/Feature 0.2–1.0 mm 1.0–2.0 mm

O2 0.2, 0.63, 0.69, 0.76 (strong) 1.27

O3 0.2–0.3 (strong), 0.4–0.5

O4 (O2-O2)* 0.345, 0.36, 0.38, 0.45, 0.48, 0.53, 0.57, 0.63 1.06, 1.27 (strong)

CO* 1.6

CH4 0.6, 0.79, 0.89, 1.0 1.1, 1.4, 1.7

N2O 1.5, 1.7, 1.78, 2.0

Organic haze < 0.5

Vegetation Red Edge 0.6 (halophile), 0.7 (photosynthesis)
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The LUVOIR Interim Report 3-29

can be detected between 0.2–0.35 mm. 

In atmospheres with low oxygen levels, 

O3 concentrations are highly responsive 

to changes in O2 levels. Over the course 

of a planet’s orbit, changes in seasonallydependent biological O2

 production would 

induce changes to strength of the UV 

ozone spectral band as a type of temporal 

biosignature (Olson et al. 2018). The weaker 

Chappuis band extends between 0.4–0.7 mm 

and is prominent in the spectrum of modern 

Earth that has abundant O3 (KrissansenTotton et al. 2016b).

Methane (CH4 ). Methane can be 

produced by biological and geological 

processes, with biological processes 

dominating on Earth. The ratio of biological to 

abiotic production rates on Earth is possibly 

close to 65:1 (Etiope & Sherwood-Lollar 

2013; IPCC 2007). Methanogenesis is a 

simple anaerobic metabolism that produces 

CH4 from CO2 and H2, and is primordial on 

Earth (Ueno et al. 2006; Woese & Fox 1977). 

Almost all abiotic mechanisms that produce 

CH4 on an Earthlike planet involve waterrock reactions (Etiope & Sherwood-Lollar 

2013), and so methane in an atmosphere 

can also indicate habitability by implying 

the existence of liquid water. Methane’s 1.4 

and 1.7 mm bands are weakly detectable at 

modern CH4 concentrations, and it has other 

shorter wavelength bands that are possibly 

detectable at higher Archean-like (e.g., 

~0.1% of the atmosphere; 4–2.5 billion years 

ago) concentrations (Figure 3.2). CH4 also 

shows strong bands between 0.8–1.4 mm for 

a modern Earth-like planet orbiting M dwarf 

stars like Proxima Centauri b (Meadows et al. 

2018), and possibly also for planets orbiting 

later K dwarfs (Arney et al. in prep.) due to 

the longer photochemical lifetime of methane 

around these types of stars. Interpretation 

of methane as a biosignature, even more 

so than oxygen, hinges on understanding 

its sources and sinks, which will require 

thorough characterization of the environment 

coupled to modeling. Large methane fluxes 

may imply biology (Guzmán-Marmolejo et al. 

2013). Simultaneous detection of CH4 and 

O2 will strengthen the biosignature kinetic 

disequilibrium argument. For anoxic planets, 

simultaneous detection of abundant CH4 and 

CO2 (i.e., CH4 mixing ratios > 10-3) indicates 

an atmosphere in chemical disequilibrium, 

and therefore a potential biosignature 

(Krissansen-Totton et al. 2018).

Nitrous Oxide (N2O). Nitrous oxide is 

produced by life, and it may have been 

present in large quantities during parts of 

the Proterozoic eon (Buick 2007; Roberson 

et al. 2011). Known abiotic sources are 

minor, and include lightning and reactions 

involving dissolved nitrates in hypersaline 

ponds (Samarkin et al. 2010). Nitrous oxide’s 

strongest bands occur longward of the 

LUVOIR wavelength range; other bands exist 

between 1.4 and 2 mm, but these bands are 

weak and tend to overlap with other gases 

that are likely to be present (such as H2O and 

CO2) and would therefore require either very 

high abundances, or high signal-to-noise 

data to recover.

Other potential biosignatures. To 

increase the chances of detecting life, 

especially given the likely diversity of terrestrial 

planetary environments and considering the 

varied dominant life forms throughout our 

own planet’s history, other biological impacts 

on planetary environments should be sought 

in the UV-visible-NIR wavelength range. 

These may include atmospheric disequilibria, 

hazes, surface reflectivity, and temporal 

variations in atmospheric composition or 

surface albedo due to life processes.

Hazes. Hazes are photochemically 

produced particles. Organic haze produces 

a strong, broadband UV-blue absorption 

feature. Organic haze is generated primarily 

via the photolysis of CH4, but can only form 

in the presence of Earth-like CO2 amounts 
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3-30 The LUVOIR Interim Report

if CH4 source fluxes are at vigorous levels 

consistent with biological production rates 

(Arney et al. 2018). Additionally, in the 

presence of other methyl-bearing biological 

gases such as dimethyl sulfide (C2H6S) 

and methanethiol (CH3SH), organic haze 

can form at a lower CH4/CO2 ratio than for 

atmospheres where the methyl groups are 

solely sourced from CH4, strengthening 

the interpretation of biological involvement. 

Similarly, the formation of sulfur aerosols such 

as S8 (which produces a blue-UV absorption 

feature akin to organic haze) is impacted by 

the H2S/SO2 ratio (Hu et al. 2013), and H2S 

is produced by biological and geological 

processes. Constraints on the H2S flux 

required to produce an S8 haze in a given 

atmosphere may suggest the involvement of 

biology if high production levels are inferred.

Surface reflectivity. Leaf structure 

produces a steep increase in reflectivity near 

0.8 mm known as the “red edge” (Gates et al. 

1965; Seager et al. 2005), which produces a 

<10% modulation in Earth’s disk-integrated 

brightness at quadrature. Other biological 

pigments can also produce strong reflectivity 

signatures (Hegde et al. 2015; Schwieterman 

et al. 2015). Reflectance edges, which are 

discontinuities in planetary reflectance as a 

function of wavelength, could occur through 

the visible and NIR, depending on the 

pigmented and/or photosynthetic organism 

producing the signature. Unexpected 

reflectance signatures that do not match 

abiotic compounds could therefore suggest 

life.

Temporal changes. On Earth, life 

produces seasonal changes in vegetation 

coverage and albedo, and periodic changes 

in gas abundances. CO2 concentrations 

change seasonally (~10 ppm) due to changes 

in temperature and sunlight intensity, which 

modulate photosynthetic drawdown of CO2

in the spring and summer, and its release 

back to the atmosphere with vegetation 

decomposition in autumn and winter 

(Keeling et al. 1976). Methane also changes 

seasonally by ~10–20 ppm. At low O2 levels, 

seasonal O2 variations might be discerned 

from variability in O3 (Olson et al. 2018). 

Seasonal variations like these could also 

be sought in exoplanet spectra (Meadows 

2008; Schweiterman et al. 2017), but would 

require very high precision, high signal-tonoise spectral data to detect changes on 

this Earth-like scale, as well as observations 

at multiple points during the planet’s orbit to 

capture any seasonal variability.

3.3.2 False positives for life

Any claims of evidence of life beyond Earth 

will be met with appropriate scrutiny from 

the scientific community. Attempts will 

be made to determine ways in which the 

claimed evidence for life could be produced 

without invoking extraterrestrial biology. 

The exoplanet astrobiology community has 

spent considerable effort simulating ways 

in which non-biological processes could 

mimic proposed exoplanet biosignatures, 

in particular atmospheric O2 and O3. These 

efforts have identified multiple such “false 

positives” for biologically produced O2 and 

O3, as well as the observations required to 

discriminate between them and the “true 

positives” associated with global biological 

O2 production (see Meadows 2017 for a 

review).

All of the known abiotic O2 and O3 

generation mechanisms rely on photolysis of 

water vapor or CO2 and a dramatic shift in 

the redox state of the planet’s atmosphere. 

These changes to the bulk atmospheric 

composition could be driven by H loss 

caused by photolysis of high-altitude H2O 

on planets without a water “cold trap” 

(Wordsworth & Pierrehumbert 2014) or by 

massive H loss from the planet as a result of 

high stellar luminosity early in the star/planet 

history during the pre-main sequence phase 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 3-31

(Luger & Barnes 2015). This latter process 

can potentially build up atmospheres 

with hundreds of bars of abiotic O2, but is 

probably only relevant to planets orbiting M 

dwarf stars.

Preferential loss of H from the planet’s 

atmosphere raises the planet’s O/H ratio. 

This sets up favorable conditions for 

photochemical accumulation of abiotic O2 

and O3, as the lack of H atoms will decrease 

the rate at which O2 and O3 are chemically 

destroyed. These slow destruction rates 

allow O2 and O3 to accumulate despite 

the relatively slow production rates. In an 

extreme case, where the planet is devoid of 

H2O but relatively high in CO2, both O2 and 

O3 can build up in the atmosphere, as the 

recombination of photolyzed CO2 is slow (Hu 

et al. 2012; Tian et al. 2014; Harman et al. 

2015; Gao et al. 2015). Planets with H2O 

can also accumulate potentially detectable 

amounts of O3, if they have limited reservoirs 

of reduced (H-bearing) gases such as 

methane (CH4) in the atmosphere and are 

orbiting F- or active M-type stars (DomagalGoldman et al. 2014; Harman et al. 2015).

The most direct ways to rule out all of the 

false positives for biological O2 production 

within LUVOIR’s wavelength range are 

to 1) constrain the total abundance and 

atmospheric pressure of O2 to rule out the 

ocean loss scenario (Schwieterman et al. 

2016) and 2) constrain the atmospheric 

redox state by detecting the presence of 

H-containing species in the atmosphere, 

specifically H2O and CH4. The detection of 

significant quantities of H2O would indicate 

an atmosphere with at least some H atoms 

and would make many of the abiotic 

O2 accumulation scenarios unlikely. The 

detection of CH4 would indicate substantial 

atmospheric H, a significant flux of reduced 

gases to the atmosphere, rapid chemical 

destruction of O2 and O3, and therefore 

kinetic disequilibrium, which is thought to 

be one of the strongest remotely detectable 

biosignatures (Hitchcock & Lovelock, 1967).

In some circumstances, detection of CH4

together with O2 may be difficult. The detection 

of modern-day Earth’s CH4 concentrations 

will be challenging if it is not possible to 

access wavelengths longer than 1.5 mm due 

to IWA constraints. Other host stars, such as 

M dwarfs (Rugheimer & Kaltenegger 2017; 

Segura et al. 2005) and late K dwarfs (Arney 

et al. in prep), may increase detectability 

of planetary CH4 and O2 simultaneously at 

wavelengths shorter than 1.5 mm, due to the 

longer photochemical lifetime of CH4 around 

these stars. For situations where O2 and CH4

cannot be accessed simultaneously, the 

different mechanisms for generating O2 and 

O3 abiotically would each have to each be 

ruled out individually.

To rule out all known abiotic O2/O3

generation processes, LUVOIR will: 

1. Search for the presence of other 

O-bearing molecules in the atmosphere 

(e.g., CO2, SO2) to estimate the potential 

for these gases to be photolyzed and 

generate O3 abiotically (Figure 3.15).

2. Measure the spectral properties of the 

star in the ultraviolet to understand the 

energy available to photolyze O-bearing 

gases and drive H escape.

3. Search for the byproducts of the 

photolysis of O-bearing gases (e.g., CO).

4. Search for spectral features associated 

with water and water clouds, to determine 

if the planet has a cold trap that would 

prevent H-loss associated with high 

stratospheric water vapor content.

5. Constrain the potential for a cloud 

deck by estimating the pressure of the 

atmosphere. This could be done by 

accurately constraining the Rayleigh 

scattering slope and presence of 
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3-32 The LUVOIR Interim Report

aerosols, modeling the shapes of wellresolved absorption features, or detecting 

the presence of collisionally induced 

absorption (CIA) features (e.g., O2-O2; 

Misra et al. 2014b).

6. Look for O2-O2 CIA features near 1.06 and 

1.27 mm that would indicate a massive 

O2 atmosphere more likely to be due to 

ocean loss than a biosphere.

This combination of measurements will 

be able to identify or rule out all of the known 

false positive mechanisms by detecting 

observable features of these processes and 

estimating the efficiency with which they 

could lead to O2 and O3 accumulation.

The other way to strengthen a potential 

biosignature is to identify independent, 

secondary features from life. These will be 

particularly strong if they are associated 

with the same biological process as the first 

biosignature. For example, the O2 and O3 

in Earth’s atmosphere come from oxygenic 

photosynthesis. LUVOIR could also detect 

the seasonal drawdown and release of CO2, 

and/or surface reflectance features like the 

vegetation red edge and pigments, both of 

which are associated with organisms that 

utilize oxygenic photosynthesis. No abiotic 

spectral mimic is known to exist for Earth’s 

vegetation red edge (Schwieterman et al. 

2017). However, this “edge feature” may 

exist at other wavelengths on exoplanets, 

Figure 3.15. A planet with detectable ozone but no life. This spectrum shows a model of an 

Earth-size exoplanet orbiting a F2V star. The abundant O3 is photochemically produced from 

non-biological O2 (Domagal-Goldman et al. 2014). Such a world could be mistaken for an 

inhabited Proterozoic Earth-like planet (Figure 3.2). However, the strong CO2 features present 

in this spectrum suggest the abiotic nature of the oxygen, as a CO2-rich atmosphere has a high 

likelihood of generating oxygen through photochemistry around this type of star. Credit: LUVOIR 

tools / S. Domagal-Goldman (NASA GSFC)
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The LUVOIR Interim Report 3-33

based on the spectral properties of the stars 

(Kiang et al. 2007a), and mineral surfaces 

could mimic these features if they occur at 

other wavelengths. Cinnabar and sulfur, 

for example, produce steep reflectance 

features at 0.6 and 0.45 mm that may mimic 

shorter wavelength reflectance “edges” 

(Schwieterman et al. 2017). Any seasonal 

variations would also need to be distinguished 

from the effects of abiotic photochemistry.

LUVOIR’s large, in-depth survey of 

numerous potentially habitable exoplanets 

provides an additional unique pathway to 

increase the confidence of a detection of 

extraterrestrial life: the integrated confidence 

level that biosignatures have been detected 

on at least one of LUVOIR’s targets will rise 

if the same signals are detected on multiple 

worlds. For example, if LUVOIR detects 

the simultaneous presence of oxygen and 

methane on one rocky world in the habitable 

zone, this could be explained by a transient 

phenomenon or by a binary planet system, 

with one CH4-rich world and one O2-rich 

world (e.g., Rein et al. 2014). Because 

both of these scenarios are highly unlikely, 

detection of multiple planets showing the 

simultaneous presence of O2 and CH4 would 

provide significantly stronger evidence that 

we are not alone in the universe than the 

detection of O2 and CH4 in a single planet.

3.4 The strategy for habitability 

and biosignatures confirmation

LUVOIR will enable the comprehensive 

assessment of planetary habitability, and 

a search for signs of life, on dozens of 

potentially habitable exoplanets. However, 

many potentially habitable planets may be 

uninhabitable, and some habitable planets 

Figure 3.16. The spectrum that could be obtained with LUVOIR-A for an Earth twin orbiting 

a Sun-like star at 5 pc in 15 hours per coronagraphic bandpass, or roughly four days of total 

observing time, assuming that two channels can be observed concurrently. This integration time 

is sufficient to obtain SNR = 10 on the continuum in the V band. Although the planet is dim in 

the UV, low-resolution photometry in the NUV channel (as shown here) allows the drop off from 

O3 absorption to be detected. Credit: LUVOIR tools / G. Arney (NASA GSFC) / T. Robinson 

(Northern Arizona University) / J. Lustig-Yaeger (University of Washington).
LUVOIR The Large UV Optical Infrared Surveyor

3-34 The LUVOIR Interim Report

may be uninhabited. A high-fidelity spectrum, 

over a wide wavelength range is insurance 

against misinterpretation of planetary 

characteristics.

For planet characterization, we assume 

that SNR=10 on the planet continuum is 

required to constrain gas abundances, 

based on the results of spectral retrieval work 

using R=140 in the visible channel (Brandt & 

Spiegel 2014; Feng et al. 2018) and R=70 

in the NIR. The R=140 spectral resolution 

in the visible is driven by the requirement to 

accurately characterize the O2 A-band, and 

also to sufficiently retrieve H2O, O2, and O3

abundances (Feng et al. 2018). The NIR 

resolution is chosen to measure the broad 

NIR features like H2O and CH4. Note that 

LUVOIR-A also includes an option for R=200 

in the NIR to resolve the narrow CO2 feature 

near 1.5 mm. Figure 3.16 shows what an 

Earth-twin orbiting a solar analog at 5 pc 

might look like to LUVOIR-A in 15 hours of 

integration time per coronagraphic bandpass 

to obtain SNR > 10 on the continuum for 

most of the spectrum. Figure 3.17 shows 

the exposure times needed to obtain for 

SNR=10 with a coronagraph as a function of 

wavelength for an Earth-like exoplanet with 

different sized coronagraphic telescopes 

with performance like LUVOIR-A for planets 

orbiting solar-analog stars 5 pc away.

As a proof-of-concept for the types of 

observations LUVOIR will be able to perform, 

here we outline a series of steps (summarized 

in Table 3.3) that could be followed to 

characterize Earth analogs. The order of 

Figure 3.17. Integration times required to obtain SNR=10 on an exoEarth orbiting a Sun-like 

star at a distance of 5 pc for different-sized coronagraphic telescopes. The calculations assume 

total system throughput representative of LUVOIR-A with RVIS = 140 and RNIR = 70. Credit: G. 

Arney (NASA GSFC) / T. Robinson (Northern Arizona University) / J. Lustig-Yaeger (University of 

Washington) 
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The LUVOIR Interim Report 3-35

these observations may change depending 

on adjustments to the wavelength ranges of 

the coronagraph channels and according to 

prior spectral information that may lead to 

prioritization of specific gas detections. We 

continue our discussion from the numbered 

steps outlined in Section 3.2.3 for detecting 

potentially habitable exoplanets (Figure 

3.10), beginning at Step 5 to pick up where 

the previous discussion left off. These steps 

are shown in green text to match the color 

coding of Figure 3.10. Detailed spectral 

yield calculations are ongoing and will be 

presented in our Final Report. Good targets 

may allow for full spectral characterization in 

approximately several days of observing time 

(Figure 3.17), with additional time for followup, higher resolution observations, and/or 

searches for seasonal variability.

5. Characterize the star and determine 

planet masses. A well-characterized host 

star spectrum is important for understanding 

the characteristics of orbiting planets. In 

particular, the UV spectrum of the star and 

constraints on stellar activity will be important 

for understanding the potential for planetary 

atmospheric loss and photochemical 

processes. The LUMOS instrument on board 

LUVOIR could be used for these stellar 

observations.

It will be important to estimate the mass 

of the planets observed by LUVOIR. If not 

available via ground-based radial velocity 

observations, estimates of planet masses 

could be obtained with astrometry using 

the HDI instrument on board LUVOIR. We 

will require extremely precise astrometry 

(≲ 0.1 mas) to measure the masses of Earthlike exoplanets, and the design of HDI is 

baselined to meet this goal.

6. Search for O2 and abundant CH4 

and constrain water abundance. To 

constrain water abundance and search for 

O2 and CH4, observations can be obtained 

over 0.675–1 mm. The presence of H2O will 

already be known from initial observations, 

but higher SNR follow-up will be required to 

constrain abundance. Key features in this 

spectral range include: H2O (0.65, 0.72, 

0.82, 0.94 mm), O2 (0.69, 0.76 mm), and CH4

(0.79, 0.89, 1.0 mm, for high Archean-like 

levels). We consider this a critical wavelength 

range for spectral characterization due to the 

availability of habitability markers (water) and 

key biosignatures (oxygen, methane).

Table 3.3. This table presents a series of spectral observations that could be followed for habitable 

exoplanet characterization with LUVOIR-A, with increasing wavelength coverage for each step 

and cumulative additions to the science return. Values in the third column show the number of 

stars in our target list for which the entire wavelength range needed for a given step is accessible 

to LUVOIR-A given our IWA and OWA assumptions.

Wavelength 

Range

Purpose Number of stars in target list with 

complete wavelength coverage over 

this range for an orbiting exoEarth

Characterization Steps

0.675–1 mm 

(Step 6)

Constrain water, search for O2, CH4 201

0.565–1.1 mm 

(Step 7)

O3, false positive O2 indicators, additional 

H2O and CH4 features

191

0.4–1.32 mm 

(Step 8)

Organic haze, S8 particles, additional O3, O2, 

H2O, CH4, CO2 at high levels

190

0.2–1.7 mm 

(Step 9)

UV absorbers like O3, Earth-like CO2

concentrations, additional CH4, H2O features

184
LUVOIR The Large UV Optical Infrared Surveyor

3-36 The LUVOIR Interim Report

7. Search for abundant ozone and 

false positive O2 indicators. Next, the 

spectrum could be extended to bluer and 

redder wavelengths to include the 0.565–

0.685 mm bandpass and the 1.0–1.1 mm 

bandpass. The extreme case of massive O2

accumulation from high stellar activity in the 

system’s early history (Luger & Barnes 2015) 

could be ruled out by the absence of O2–O2 

CIA features, which become stronger at high 

O2 levels (e.g., > 10 bars O2). Such features 

can be present in this spectral range at 0.57, 

0.63, and 1.06 mm, in both our visible and NIR 

bandpasses. Very high CO2 levels that might 

indicate abiotic photochemical O2 generation 

may also be detectable through the CO2 

feature near 1.05 mm. The long wavelength 

side of the broad ozone Chappuis band 

(0.5–0.7mm) can also be seen in the visible 

bandpasses for planets with modern Earthlike oxygen amounts. Additional important 

features that could be detected in this 

wavelength range are H2O (1.12 mm), and 

CH4 at high Archean-like abundances (0.6, 

1.1 mm).

8. Search for blue wavelength 

absorbers, Rayleigh scattering, and 

other NIR absorbers. Next, the spectral 

coverage could be extended to include 0.4–

0.565 mm range in the visible, and also to 

include the 1.1–1.32 mm range in the NIR. 

The short wavelengths could show signs of 

blue absorbers including organic haze for 

methane-rich atmospheres (< 0.5 mm, and 

haze can be a biosignature), S8 particles 

for planets exhibiting high levels of volcanic 

outgassing (0.2–0.5 mm), and the full O3 

Chappuis band. In the NIR, an indicator 

of high CO2 atmospheres is its 1.21 mm 

band, which is critical for constraining CO2 

photolysis as a potential abiotic source of O2

and O3 (Schwieterman et al. 2016). Additional 

constraints on O2 could come from 1.27 mm 

band for modern O2 levels.

9. Observe the UV and search for 

Earth-like CO2 in the NIR. By extending 

the spectrum in the NIR to include the 1.31–

1.6 mm bandpass, additional H2O (1.4 mm) 

and CH4 (1.4 mm) features can be observed, 

and importantly, a CO2 band detectable 

at modern Earth-like concentrations and 

greater is present near 1.6 mm. This CO2

band can be used to compare to CH4

abundances to determine whether haze or 

CH4 can be interpreted as biosignatures 

(Arney et al. 2018; Krissansen-Totton et al. 

2018). Methane may be detected near 1.7 

mm at modern Earth-like abundances. While 

NIR observations are obtained, the NUV 

could also be observed. Planets will generally 

be dim in the UV, but the spectral falloff 

due to NUV absorbers could be detected; 

the low-resolution photometry LUVOIR will 

Program at a Glance – Confirming Habitability and Biosignatures

Goal: Detect and characterize inhabited exoplanets.

Program details: Obtain NUV/Visible/NIR spectra of potentially habitable planets and 

search for biosignatures. Measure abundances of biosignature gases and search for 

surface reflectance biosignatures. Characterize host stars.

Instrument(s) + Configuration: ECLIPS coronagraphic spectroscopy to characterize 

planets; LUMOS UV spectroscopy for host star characterization 

Key observation requirements: Contrast < 10-10; SNR=10 & R=140 to measure O2 at 

0.76 mm; R=70 in the NIR channel to measure water and CH4; R=200 in the NIR channel 

to detect CO2; NUV photometry to detect O3 and other UV absorbers (SNR=5)
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 3-37

obtain in the NUV improves signal-to-noise. 

For some atmospheres, like the Proterozoic 

Earth, weak O3 may be detectable and even 

quantifiable in the UV because the bottom 

of the band is not near zero flux and could 

therefore be measured (Figure 3.2, middle 

panel).

10. Revisit: Time-dependent and 

multi-epoch observations. Photometry 

of the brightest imaged planets at timeresolved intervals of hours during a single 

visit can be used to search for timedependent changes in planet brightness or 

color which could be indicative of surface 

or cloud inhomogeneity (e.g., Ford et al. 

2001; Cowan et al. 2009). These data could 

either be taken as multiple photometric 

images (Step 2 of our characterization 

strategy; Figure 3.10), or extracted from 

long spectral integrations obtained for other 

purposes, as long as discrete time intervals 

in the longer integration could be retrieved 

at sufficient S/N. Rotational mapping and 

unmixing models (e.g., Cowan and Strait 

2013) can infer the colors and geographic 

distribution of uniquely identifiable surfaces 

that contribute to the observed variability 

with 2–3% hourly multiband photometry 

spanning a single planetary rotation (see also 

Cowan et al. 2009). Once a rotational period 

is determined, variability that deviates from 

this periodic signal could indicate transient 

phenomena such as clouds (Pallé et al. 2008). 

Longer time baseline observations that span 

a substantial portion of the orbital period 

will enable searches for seasonal variability 

(e.g., Olson et al. 2018) and the construction 

of more precise (two-dimensional) surface 

maps (Kawahara and Fujii 2010; Fujii and 

Kawahara 2012).

Large aperture telescopes are essential 

for time-variability studies. To resolve the 

rotation period of the planet (Figure 3.18), 

the individual integrations that comprise 

a time-series observation must sample at 

significantly shorter time intervals than the 

rotation period. To that end, the rotation 

period of Earth can be determined to 

within 10% for a 15-m telescope with 5% 

throughput. Smaller aperture telescopes 

must sacrifice time-resolution for S/N, in 

effect, eliminating the ability to study the 

time-variability of rapidly rotating planets. 

Obliquity determination can be performed 

with excellent photometry at a few distinct 

orbital phases (Schwartz et al. 2016), or 

with nearly continuous low S/N monitoring 

(Kawahara 2016). Observations of this quality 

require a large space telescope like LUVOIR. 

As a final note, the combination of the 

planet’s orbit determined in Observational 

Strategy Step 3 (Figure 3.10) and the 

photometric/spectral observations of the 

planet throughout as much of its orbit as 

possible, will provide a comprehensive and 

powerful dataset to characterize the planetary 

environment. With these observations, one 

Figure 3.18. Larger telescopes enable more 

accurate planet rotation rate measurements. 

This chart shows retrieved rotation periods (left) 

and estimated errors on the period (right) for 

an Earth-Sun analog at 5 pc observed across 

a grid in telescope aperture diameter and total 

system throughput (Lustig-Yaeger et al. 2018, 

in prep). Credit: J. Lustig-Yaeger (University of 

Washington).
LUVOIR The Large UV Optical Infrared Surveyor

3-38 The LUVOIR Interim Report

can constrain planetary obliquity, determine 

atmospheric changes in response to longterm variations in incoming and outgoing 

radiation, and search for longer term 

variations in surface albedo, cloud patterns 

and gas abundances that may be indicative 

of planetary atmospheric dynamics, seasonal 

changes in climate, planetary volcanism, and 

other environmental processes.

3.5 Summary

LUVOIR is a history-making telescope 

and offers humanity’s best chance of 

answering the ancient question “Are we 

alone in the universe?” If there is a global 

biosphere on a planet orbiting a star in the 

Sun’s neighborhood, LUVOIR will find it by 

identifying atmospheric and surface features 

that can only be produced by life. If life exists 

Synergies During the LUVOIR Era

In the next 5–10 years, we may be able to obtain transmission spectra of habitable zone 

planets orbiting M dwarfs, which are observationally favored for transmission spectroscopy 

due to their relatively large transit depths and frequent transits. This will constitute our 

first search for potential biosignatures on potentially habitable exoplanets. This search will 

likely continue in the LUVOIR era, and this technique may be augmented with a second 

generation of adaptive optics systems and ELT instrumentation. These advances also 

may enable ELTs to conduct direct spectroscopy measurements, albeit limited to planets 

in orbit around M dwarfs. 

In addition to a search for signs of life, this research on M dwarf planets will provide 

intriguing insights into terrestrial planet evolution, as the high stellar activity and early high 

luminosity of their host stars create a different photochemical environment. However, this 

same energy source may also cause ocean loss for planets in the habitable zone (see 

Section 3.2.1). This presents a double-edged sword for a biosignature search, as this 

energy may accentuate the signals from some biosignature gases (such as CH4), but also 

make atmospheric and ocean retention less likely.

Regardless of the result of these future observations of M dwarf habitable planets, LUVOIR 

will provide complementary observations on these targets. If biosignatures are detected 

with ELTs, LUVOIR will confirm the presence of these gases, by detecting them from 

space and with a complementary shorter wavelength range tied to other molecular lines. 

LUVOIR can observe transits of the same M dwarf planets targeted by JWST and groundbased telescopes, and it can observe the nearest M dwarf habitable zone worlds in direct 

imaging. And if planets around M dwarfs are found to suffer catastrophic volatile loss, 

LUVOIR’s UV transit spectroscopy capabilities will probe the atmospheric loss process 

(Chapter 4). 

Additionally, LUVOIR will conduct a biosignature search on a complementary set of targets, 

by observing terrestrial planets orbiting stars like our own Sun. Should prior searches for 

life prove unsuccessful, LUVOIR will expand our search for life to planets in more familiar 

stellar environments. And if prior searches for biosignatures prove successful, LUVOIR 

will expand the diversity of stellar environment for which we can find biosignatures. Either 

way, LUVOIR will represent a dramatic leap forward in our ability to understand how life is 

a function of its stellar environment.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 3-39

within our own solar system, LUVOIR will help 

other missions search for it. In both cases, 

LUVOIR will place the search for life inside 

a global context, and within an expanded 

understanding of planetary processes.

For worlds in our solar system, LUVOIR’s 

imaging and spectroscopic capabilities will 

allow global mapping of the atmospheric and 

mineral composition for worlds large and small. 

For exoplanets, LUVOIR’s large aperture and 

wide spectral range will provide a rigorous 

assessment of exoplanet biosignatures as 

well as their global environmental contexts. 

This environmental context will include a 

search for key markers of global habitability, 

including signs of a hydrological cycle, and 

the presence of clouds and oceans. LUVOIR 

will conduct this search for signs of life and 

habitability on dozens of worlds orbiting a 

diversity of Sun-like (FGK) stars, representing 

an expansion on prior and concurrent 

searches for biosignatures on M-dwarf stars. 

Finally, LUVOIR will also complement these 

prior observations of potentially habitable 

planets around M-dwarfs with space-based 

observations that will expand the wavelength 

coverage for transit spectroscopy of these 

worlds.

Overall, LUVOIR will conduct observations 

of dozens of potentially habitable worlds 

around a wide variety of stars. These 

observations will complement many other 

facilities, including ground-based telescopes 

and orbiters/landers to solar system targets. 

And each of these investigations will provide 

scientific context for the others; what we 

learn from LUVOIR’s observations of the 

solar system will influence our interpretation 

Table 3.4. Summary science traceability matrix for Chapter 3

Scientific Measurement Requirements Instrument Requirements

Measurement Observations Instrument Property Value Value

Find habitable 

exoplanets

Direct detection of 

habitable planet 

candidates and 

search for water 

vapor 

Direct imaging 

and spectroscopy 

of rocky 

exoplanets in 

habitable zones 

to S/N=5

ECLIPS High contrast 

imaging near 

500 nm, 

spectroscopy 

near 900 nm

Contrast < 10-10); 

Rvis~ 70 (H2O)

Explore 

habitability of 

solar system 

moons

Long temporal 

baseline monitoring 

of icy moons to 

detect surface 

changes, 

study plumes, 

and measure 

atmospheric 

properties

Direct imaging 

and spatially 

resolved 

spectroscopy 

of moons and 

plumes

HDI NUV to NIR 

imaging

NUV, UBVRI (UVIS 

channel); JH (NIR 

channel)

LUMOS FUV multiobject 

spectroscopy

FUV G120M, 

G150M, G180M; 

R ~ 30,000

ECLIPS Open mask 

NIR IFS 

spectroscopy

RNIR~ 70 or 200 

Detect and 

characterize 

inhabited 

exoplanets

Abundances of 

biosignature gases, 

surface reflectance 

biosignatures

Direct imaging 

(S/N=5) and 

spectroscopy 

(S/N>10) of 

exoplanet 

atmospheres

ECLIPS High-contrast 

NUV/VIS/NIR 

imaging and 

spectroscopy

Contrast < 10-10; 

Rvis~ 140 (O2); 

RNIR~ 70 (H2O/

CH4); RNIR~ 70 

(CO2); NUV 

photometry (O3)

Characterize 

exoplanet host 

stars

Measure UV 

spectrum and 

activity of host stars

Point-source 

spectroscopy of 

exoplanet host 

stars

LUMOS UV 

spectroscopy

LUMOS G120M, 

G150M, G180M, 

G155M, G300M; 

R ~ 30,000
LUVOIR The Large UV Optical Infrared Surveyor

3-40 The LUVOIR Interim Report

of LUVOIR’s observations of exoplanets, and 

vice versa. As such, LUVOIR will serve as the 

flagship observatory for a new area of scientific 

research: comparative astrobiology.

References

Abe, Y., Abe-Ouchi, A., Sleep, N. H., & 

Zahnle, K. J. (2011). Astrobiology, 11, 

443

Arney, G., Domagal-Goldman, S. D., 

Meadows, V. S., et al. (2016). 

Astrobiology, 16, 2015.1422

Arney, G. N., Domagal-Goldman, S. D., & 

Meadows, V. S. (2018). Astrobiology, 

18, 1

Baraffe, I., Chabrier, G., Allard, F., & 

Hauschildt, P. (1998). A&A, 337, 403

Bell, E. A., Boehnke, P., Harrison, T. M., & 

Mao, W. L. (2015). Proc Natl Acad Sci,

112, 14518

Bourrier, V., Ehrenreich, D., Wheatley, P. J., 

et al. (2017). A&A, 599, L3

Brandt, T. D., & Spiegel, D. S. (2014). Proc 

Natl Acad Sci, 111, 13278

Brown, R., Clark, R., Buratti, B., et al. 

(2006) Science, 311, 1425

Buick, R. (2007). Geobiology, 5, 97

Carlson, R. W. (1999). Science, 286, 97

Carr, M. H., Belton, M. J. S., Chapman, C. 

R., et al. (1998). Nature, 391, 363

Chyba, C. F. (2000). Nature, 403, 381

Cowan, N. B., Abbot, D. S., & Voigt, A. 

(2012). ApJ, 752, L1

Cowan, N. B., Greene, T., Angerhausen, D., 

et al. (2015). PASP, 127, 311 

Delitsky, M. L., & Lane, A. L. (1998). J 

Geophys Res, 103, 31391

Domagal-Goldman, S. D., Segura, A., 

Claire, M. W., Robinson, T. D., & 

Meadows, V. S. (2014). ApJ, 792, 90

Dressing, C. D., & Charbonneau, D. (2013). 

ApJ, 767, 95

Ehlmann, B. L., Mustard, J. F., Murchie, S. 

L., et al. (2011). Nature, 479, 53

Ehrenreich, D., Bourrier, V., Wheatley, P. J., 

et al. (2015). Nature, 522, 459

Etiope, G., & Sherwood Lollar, B. (2013). 

Rev Geophys, 51, 276

Feng, Y. K., et al. (2018). ApJ, 155, 200.

Fujii, Y., Del Genio, A. D., & Amundsen, D. 

S. (2017). ApJ, 848, 100

Gao, P., Hu, R., Robinson, T. D., Li, C., & 

Yung, Y. L. (2015). ApJ, 806, 249

Garcia-Sage, K., Glocer, A., Drake, J. J., 

Gronoff, G., & Cohen, O. (2017). ApJ, 

844, L13

Gates, D. M., Keegan, H. J., Schleter, J. C., 

& Weidner, V. R. (1965). Appl Opt, 4, 11

Griffith, C. A., Owen, T., Miller, G. A., & 

Geballe, T. (1998). Nature, 395, 575

Guzmán-Marmolejo, A., Segura, A., 

& Escobar-Briones, E. (2013). 

Astrobiology, 13, 550

Harman, C. E., Schwieterman, E. W., 

Schottelkotte, J. C., & Kasting, J. F. 

(2015). ApJ, 812, 137

Heath, M. J., Doyle, L. R., Joshi, M. M., & 

Haberle, R. M. (1999). Orig Life Evol 

Biosph, 29, 405

Hedman, M. M., Gosmeyer, C. M., 

Nicholson, P. D., et al. (2013). Nature, 

500, 182

Hegde, S., Paulino-Lima, I. G., Kent, R., 

Kaltenegger, L., & Rothschild, L. (2015). 

Proc Natl Acad Sci, 112, 3886

Hitchcock, D. R., & Lovelock, J. E. (1967). 

Icarus, 7, 149

Hu, R., Seager, S., & Bains, W. (2012). ApJ, 

761, 166

Hu, R., Seager, S., & Bains, W. (2013). ApJ, 

769, 6

IPCC. 2007, Climate Change 2007: 

Synthesis Report. Contribution of 

Workings Groups I, II, and III to the 

Fourth Assessment Report of the 

Intergovernmental Panel on Climate 

Change (Geneva)

Kaltenegger, L., & Sasselov, D. (2010). ApJ, 

708, 1162
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 3-41

Kaltenegger, L., Traub, W. A., & Jucks, K. 

W. (2007). ApJ, 658, 598

Kanzaki, Y., & Murakami, T. (2015). 

Geochim Cosmochim Acta, 159, 190

Keeling, C. D., Bacastow, R. B., Bainbridge, 

A. E., et al. (1976). Tellus, 28, 538

Kiang, N. Y., Segura, A., Tinetti, G., et al. 

(2007a). Astrobiology, 7, 252

Kiang, N. Y., Siefert, J., Govindjee, 

& Blankenship, R. E. (2007b). 

Astrobiology, 7, 222

Kopparapu, R. K., Ramirez, R., Kasting, J. 

F., et al. (2013). ApJ, 765, 131

Kopparapu, R. kumar, Wolf, E. T., Arney, G., 

et al. (2017). ApJ, 845, 5

Kopparapu, R. kumar, Wolf, E. T., HaqqMisra, J., et al. (2016). ApJ, 819, 84

Krissansen-Totton, J., Bergsman, D. S., & 

Catling, D. C. (2016a). Astrobiology, 16, 

39 

Krissansen-Totton, J., & Catling, D. C. 

(2017). Nature Comm, 8, 15423

Krissansen-Totton, J., Olson, S., & Catling, 

D. C. (2018). Sci Adv, 4, eaao5747

Krissansen-Totton, J., Schwieterman, E. W., 

Charnay, B., et al. (2016b). ApJ, 817, 

31

Lissauer, J. J. (2007). ApJ, 660, L149

Luger, R., & Barnes, R. (2015). Astrobiology, 

15, 119

Lyons, T. W., Reinhard, C. T., & Planavsky, 

N. J. (2014). Nature, 506, 307

Des Marais, D. J., Harwit, M. O., Jucks, K. 

W., et al. (2002). Astrobiology, 2, 153

McCord, T. B., Hansen, G. B., Shirley, J. 

H., & Carlson, R. W. (1999). J Geophys 

Res, 104, 157

Meadows, V. S. (2008). in Exoplanets: 

detection, formation, properties, habitability, ed. J. Mason (Springer Science 

& Business)

Meadows, V. S. (2017). Astrobiology, 17, 

1022

Misra, A., Krissansen-Totton, J., Koehler, M. 

C., & Sholes, S. (2015). Astrobiology, 

15, 462

Misra, A., Meadows, V., Claire, M., & Crisp, 

D. (2013). Astrobiology, 14, 67

Morbidelli, A., Chambers, J., Lunine, J. I., et 

al. (2000). Meteorit Planet Sci, 35, 1309

Nimmo, F., Porco, C., & Mitchell, C. (2014). 

AJ, 148, 46

Nutman, A. P., Bennett, V. C., Friend, C. R. 

L., et al. (2016). Nature, 537, 535

Olson, S. L., Reinhard, C. T., & Lyons, T. W. 

(2016). Proc Natl Acad Sci, 113, 11447

Olson, S. L., Schwieterman, E. W., 

Reinhard, C. T., et al. (2018). ApJ, 858, 

L14

Palle, E., Ford, E. B., Seager, S., MontanesRodriguez, P., & Vazquez, M. (2008). 

ApJ, 676, 1319

Pavlov, A., Kasting, F., Brown, L. L., et al. 

(2000). J Geophys Res, 105, 11981

Planavsky, N. J., Reinhard, C. T., Wang, X., 

et al. (2014). Science, 346, 635

Pohorille, A., & Pratt, L. R. (2012). Orig Life 

Evol Biosph, 42, 405

Quanz, S. P., Crossfield, I., Meyer, M. R., et 

al. (2015). Int J Astrobiol, 14, 279

Ramirez, R. M., & Kaltenegger, L. (2014). 

ApJ, 797, L25

Raymond, S. N., Quinn, T., & Lunine, J. I. 

(2006). Icarus, 183, 265

Rein, H., Fujii, Y., & Spiegel, D. S. (2014). 

Proc Natl Acad Sci, 111, 6871

Reinhard, C. T., Olson, S. L., Schwieterman, 

E. W., & Lyons, T. W. (2017). 

Astrobiology, 17, 287

Roberson, A. L., Roadt, J., Halevy, I., & 

Kasting, J. F. (2011). Geobiology, 9, 

313

Roberts, J. H., & Nimmo, F. (2008). Icarus, 

194, 675

Robinson, T. D., Meadows, V. S., & Crisp, 

D. (2010). ApJ, 721, L67

Robinson, T. D., Meadows, V. S., Crisp, D., 

et al. (2011). Astrobiology, 11, 393
LUVOIR The Large UV Optical Infrared Surveyor

3-42 The LUVOIR Interim Report

Rogers, L. A. (2015). ApJ, 801, 41

Rosing, M. T., Bird, D. K., Sleep, N. H., & 

Bjerrum, C. J. (2010). Nature, 464, 744

Roth, L., Saur, J., Retherford, K. D., et al. 

(2014). Science, 343, 171

Rugheimer, S., & Kaltenegger, L. (2017). 

ApJ, 854, 19

Sagan, C., Thompson, W. R., Carlson, R., 

et al. (1993). Nature, 365, 715

Samarkin, V. A., Madigan, M. T., Bowles, M. 

W., et al. (2010). Nature Geosci, 3, 341

Schwieterman, E. W., Cockell, C. S., & 

Meadows, V. S. (2015). Astrobiology, 

15, 341

Schwieterman, E. W., Meadows, V. S., 

Domagal-Goldman, S. D., et al. (2016). 

ApJ, 819, L13

Schwieterman, E., Reinhard, C., Olson, S., 

& Lyons, T. (2018a). arXiv:1801.02744

Schwieterman, E. W., Kiang, N. Y., 

Parenteau, M. N., et al. (2018b). 

Astrobiology, 18, 663

Seager, S., Bains, W., & Hu, R. (2013). ApJ, 

777, 95

Seager, S., Turner, E. L., Schafer, J., & Ford, 

E. B. (2005). 5, 372

Segura, A., Kasting, J. F., Meadows, V., et 

al. (2005). Astrobiology, 5, 706

Segura, A., Walkowicz, L. M., Meadows, V., 

et al. (2010). Astrobiology, 10, 751

Sparks, W. B., Hand, K. P., McGrath, M. A., 

et al. (2016). ApJ, 829, 121

Sparks, W. B., Schmidt, B. E., McGrath, M. 

A., et al. (2017). ApJ, 839, L18

Stephan, K., Jaumann, R., Brown, R. H., 

et al. (2010). Geophys Res Lett, 37, 

L07104

Tarter, J. C., Backus, P. R., Mancinelli, R. L., 

et al. (2007). Astrobiology, 7, 30

Teolis, B. D., Perry, M. E., Hansen, C. J., et 

al. (2017). Astrobiology, 17, 926

Tian, F. (2015). Earth Planet Sci Lett, 432, 

126

Ueno, Y., Yamada, K., Yoshida, N., et al. 

(2006). Nature, 440, 516

Walker, J. C. G., Hays, P. B., & Kasting, J. 

F. (1981). J Geophys Res Ocean, 86, 

9776

Woese, C. R., & Fox, G. E. (1977). Proc Natl 

Acad Sci, 74, 5088

Wordsworth, R., & Pierrehumbert, R. (2014). 

ApJ, 785, L2

Yang, J., Cowan, N. B., & Abbot, D. S. 

(2013). ApJ, 771, L45

Zerkle, A. L., Claire, M. W., DomagalGoldman, S. D., et al. (2012). Nature

Geosci, 5, 359

Zugger, M. E., Kasting, J. F., Williams, D. 

M., et al. (2011). ApJ, 739, 12
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 4-1

Chapter 4
LUVOIR The Large UV Optical Infrared Surveyor

4-2 The LUVOIR Interim Report

4 How do we fit in? Planetary systems and comparative planetary 

science

We learn about planets by comparing them 

to each other. The atmospheres of Venus, 

Earth, Mars, and Titan, for example, feature 

varying mixtures of atmospheric gases, 

clouds, and hazes. While only one is habitable, 

our understanding of how planetary surface 

temperature and climate are influenced by 

the interaction of atmospheric constituents 

with incident solar flux and gross planetary 

properties has been profoundly informed 

by comparative studies of all these worlds. 

Atmospheric escape processes, exemplified 

by the loss of water from Venus, sculpt 

planetary and exoplanetary atmospheres 

alike. Recognizing such synergies illuminates 

our understanding of individual solar system 

worlds and informs our interpretation of all 

types of exoplanets, thereby revealing how 

the processes we understand well from 

studies of the Earth connect with those of 

other planets. By studying the processes 

at work among the stunning diversity 

of exoplanets (Figure 4.1) LUVOIR will 

exponentially increase the opportunities for 

comparative exoplanet science.

We also learn by watching as planets 

are formed and observing the outcomes, 

including the detritus, of this process. 

The architecture of planetary systems, 

how planets are arranged by mass and 

composition, and the characteristics of 

debris disks and remnant planetesimals all 

Figure 4.1. Artist’s conceptions of some of the 3,706 confirmed exoplanets (as of March 16, 

2018). Credit: M. Vargic
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The LUVOIR Interim Report 4-3

provide clues to the formation processes that 

produce the variety of observed planets and 

their atmospheres. Within the solar system, 

the bodies of the Kuiper belt contain a frozen 

record of the formation of the planetary 

system we understand most thoroughly, our 

State of the Field in the 2030s

Radial velocity: We expect a factor of ten improvement in ground-based radial velocity 

measurement precision, to ~10 cm/s. This is size of the signal that the Earth imparts on 

the Sun when the solar system is observed edge on. Depending on details regarding the 

stellar jitter and instrumental noise, precise radial velocities provide a possible, but not 

assured, path to finding and measuring the masses of terrestrial planets in the habitable 

zones of FGK dwarfs in the LUVOIR era.

Astrometry: The ESA Gaia astrometry mission is expected to discover ~21,000 planets 

between 1–15 Jupiter masses in long period orbits (Perryman et al. 2014). The nearest 

discovered giant planets, with good mass and orbit constraints, will be excellent targets 

for LUVOIR comparative planetary studies.

Millimeter/submillimeter observations: The Atacama Large Millimeter/Submillimeter 

Array (ALMA) will study all phases of planet formation, with spectroscopy of molecular and 

atomic gas and high spatial-resolution (but still 2 to 3 times less than LUVOIR) mapping of 

cold dust in protoplanetary and debris disks. 

Transit photometry: Starting in 2018, NASA’s all-sky Transiting Exoplanet Survey 

Satellite (TESS) is expected to detect thousands of relatively nearby transiting planets. 

The ESA Characterizing ExOPlanets Satellite (CHEOPS) will follow up previously detected 

exoplanets. The ESA PLAnetary Transits and Oscillations (PLATO) mission, planned for a 

2026 launch, has the goal of detecting transiting terrestrial planets. 

Transit spectroscopy: NASA’s James Webb Space Telescope (JWST) will yield detailed 

characterization of 10–100 close-in exoplanets ranging from super-hot gas giants around 

all types of stars to temperate terrestrials around low-mass stars. Both NASA and ESA 

are considering dedicated transit spectroscopy missions for the 2020s (FINESSE and 

ARIEL, respectively). These missions would build on JWST by enabling a homogeneous 

and statistical census of the atmospheres of hundreds of close-in giant planets. However, 

a large sea of Earth- to Neptune-mass planets around solar-type stars will not yet be 

characterized by the 2030s.

Microlensing: The Wide-Field InfraRed Space Telescope (WFIRST) microlensing survey 

will detect thousands of exoplanets further from their host stars, leading to a statistical 

census of planets with masses > 0.1 MEarth from the outer habitable zone out to free 

floating planets. 

High-contrast imaging: WFIRST will also develop high-performance coronagraphic 

imaging and spectroscopic technology in space but will only be able to detect a few 

large exoplanets (gas and ice giants). Second generation instruments on the planned 

thirty-meter class ground-based telescopes (ELTs) will image thermal emission from young 

giant planets at much smaller spatial separations and around fainter stars than currently 

possible, will image and characterize some giants in reflected light in the near-IR, and will 

detect terrestrial planets in the habitable zones of a handful of nearby low-mass stars.
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own. By characterizing all types of planets, 

discovering and observing Kuiper belt 

objects, and observing circumstellar debris 

disks, the users of LUVOIR will dramatically 

enhance our understanding of the process 

of planet formation. 

This chapter explores some of what we 

can learn from the exoplanetary and planetary 

science LUVOIR will enable beyond the 

search for life. We discuss and present a few 

exemplary LUVOIR science cases relating to 

the atmospheres of non-habitable planets, 

planet formation, atmospheric escape, and 

solar system science, focusing on the value 

of science enabled by large sample sizes. 

There are of course countless additional 

opportunities for exoplanet science with 

LUVOIR. Many such additional investigations 

are comprehensively documented in the 

ExoPAG Science Analysis Group 15 report 

“Science Questions for Direct Imaging 

Missions” (Apai et al. 2017), which is an 

excellent reference for understanding the 

exciting diversity of exoplanet science 

beyond the search for habitable worlds. 

Some additional exoplanet science cases, 

including transit observations beyond the 

UV, are described in Appendix A of this 

document.

The “Signature Science” cases discussed 

in this chapter represent some of the most 

compelling types of observing programs on 

a wide range of exoplanets that scientists 

might do with LUVOIR. As compelling as they 

are, they should not be taken as a complete 

specification LUVOIR’s future potential in 

these areas. We have developed concrete 

examples to ensure that the nominal design 

can do this compelling science. We fully 

expect that the creativity of the community, 

empowered by the revolutionary capabilities 

of the observatory, will ask questions, 

acquire data, and solve problems beyond 

those discussed here—including those that 

we cannot envision today.

4.1 Signature science case #1: 

Comparative atmospheres

The most compelling exoplanetary characterization discoveries so far have arisen from 

comparative studies of planets. By revealing 

the diversity of exoplanet radii, including the 

stunning ubiquity of sub-Neptune sized planets, as well as unexpected trends in planetary system architectures, the Kepler mission 

inaugurated era of systematic comparative 

exoplanetology. LUVOIR will build on these 

results by thoroughly characterizing the atmospheres of hundreds of directly imaged 

and transiting exoplanets, over a much wider range of conditions than exist in the solar 

system, vastly improving our understanding 

of the fundamental atmospheric processes 

affecting habitable and inhospitable planets 

alike.

There will be plenty of opportunities 

for such studies. The search for exoEarth 

candidates will uncover hundreds of other 

types of planets (Stark et al. 2014). Figure 

4.2 illustrates the yield of detected planets 

of various sizes and temperatures expected 

to be found in the LUVOIR Architecture A 

habitable planet search. These additional 

exoplanets are ice and gas giants at a range 

of orbital distances from a variety of stellar 

primaries as well as non-habitable terrestrial 

and super-Earth sized planets. About 150 

sub-Neptune sized planets around a variety 

of stellar types and at various equilibrium 

temperatures are expected. For some 

planets, follow up characterization could be 

done simultaneously with long exposures to 

obtain spectra of habitable zone terrestrial 

planets. In other cases, alternate coronagraph 

masks and additional exposure time might be 

needed. For all of these worlds, LUVOIR will 

enable a host of science programs, ranging 

from studies of atmospheric dynamics in gas 

giants to searches for evidence of volcanic 

gases in the atmospheres of terrestrial 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 4-5

planets. The SAG15 report (Apai et al. 2017) 

discusses such programs and many others.

The most promising avenue among 

the wealth of science made possible by 

this diversity of worlds will be systematic, 

comparative studies of planetary 

atmospheres. Many of the more massive 

planets will have been detected by RV 

surveys or GAIA, thereby providing masses, 

while astrometry with HDI will measure or 

constrain the masses of smaller planets. 

Planets with known masses, incident fluxes, 

and measured atmospheric composition 

will provide a laboratory for testing theories 

for planet formation, atmospheric evolution, 

photochemistry, and cloud processes. In 

this section we briefly summarize some of 

the highlights of such investigations made 

possible by LUVOIR.

4.1.1 Diversity of composition

LUVOIR will characterize atmospheric diversity among the planets within each system 

that it observes, systematically measuring 

atmospheric composition of gas and ice giants, sub-Neptunes, super-Earths, and small 

rocky planets. Atmospheric composition is 

of special interest as the particular species 

present depend on atmospheric temperature and pressure regulated by the incident 

stellar flux and, for giant planets, emergent 

planetary flux from the deep interior. The atmospheres of all solar system giants are enhanced over solar abundance, a fingerprint 

that is generally attributed to the details of 

planetary formation process (Figure 4.3; 

Mordasini et al. 2016). For terrestrial planets, 

composition is a signature of initial atmospheric formation as modulated by escape, 

crustal recycling, and other endogenic and 

exogenic processes (such as impacts and 

volcanos).

The visible atmospheres of the relatively 

cool giant planets probed by direct imaging 

are directly connected to their deep interiors 

by a continuous convection zone and the bulk 

composition of their atmospheres will reflect 

that of the planet as a whole (Fortney et al. 

Figure 4.2. Planets of all types detected by LUVOIR Architecture A during a 2-year habitable 

planet survey. The histograms show the number of planets detected in six size bins: exoEarth 

candidates (defined in Chapter 3; green bar), rocky planets (0.5–1 REarth), super-Earths (1–1.75 

REarth), sub-Neptunes (1.75–3.5 REarth), Neptunes (3.5–6 REarth), and Jupiters (6–14.3 REarth). Red, 

blue, and ice blue bars indicate hot, warm, and cold planets, respectively. Right panels show the 

breakdown by stellar type. A detailed description of the exoplanet science yield calculations can 

be found in Appendix B.
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4-6 The LUVOIR Interim Report

2007). This is different from the atmospheres 

of the hot transiting giant planets, which 

are disconnected from their deep interiors 

by a radiative zone that extends to 100 

to 1000 bar or more. Thus, LUVOIR will 

characterize a distinct population of planets 

with notably different atmospheric structure 

and composition than most of the transiting 

planets expected to be observed with JWST, 

although note LUVOIR can also observe 

planetary transits in a wavelength range 

complementary to JWST.

Understanding how atmospheric composition varies with planet mass and stellar 

properties is vital, since planet formation 

models predict a wide range of enrichments 

in elements compared to stellar abundances (e.g., Oberg et al. 2011; Fortney et al. 

2013; Madhusudhan et al. 2016; Mordasini 

et al. 2016; Thorngren et al. 2016; Espinoza 

et al. 2017). Observations of transiting giant 

planets suggest a trend of decreasing atmospheric heavy element enrichment with increasing mass that mirrors the one seen in 

the solar system (Figure 4.3). LUVOIR users will test whether such trends hold for the 

cooler, more distant population of planets 

that more closely resemble solar system gas 

and ice giants. LUVOIR will also constrain 

the atmospheric composition of planets at 

the sub-Neptune/super-Earth boundary to 

understand if these are distinct planet types 

or simply a continuum of composition.

LUVOIR will obtain reflected light spectra of dozens of gas and ice giants, down 

to sub-Neptune/super-Earth sizes, at a variety of orbital distances around stars of varying mass and metallicity, thus balancing the 

population of planets expected to be characterized with JWST transit studies. Optical 

photometry and spectra of detected planets 

will readily constrain the presence or absence of atmospheres from the distinctive 

appearance of Rayleigh and cloud particle 

scattering. Atmospheric composition can be 

derived from optical and near-IR spectroscopy, as the wavelength ranges and spectral 

resolutions required for detecting gases in 

terrestrial atmospheres (Chapter 3) will also 

permit measurement of H2O, NH3, CH4, Na, 

and K in gas giants and additional species 

in the atmospheres of lower mass planets. 

Atmospheric metallicity can be determined 

using CH4, H2O, and NH3 bands in optical 

(Figure 4.4) and near-IR spectra. For planets warm enough that atmospheric water is 

not condensed into clouds, such spectra will 

also constrain carbon-to-oxygen ratios, a key 

indicator of formation and migration history. 

For systems with potentially habitable planets, atmospheric characterization of any giant planets will also provide valuable insights 

into the formation history of the system.

Figure 4.3. Metallicity of Solar System and 

extrasolar giant planets. Transiting planet 

metallicity derived from retrievals of H2O 

abundance inferred from transit spectra. Solar 

system abundances are from remote and in 

situ measurement of CH4. The dashed line 

shows the notional trend of decreasing [M/H] 

with increasing planet mass, a prediction of the 

core accretion gas giant formation mechanism. 

The red error bar shows expected precision of 

LUVOIR retrievals of gas giant CH4 and H2O 

atmospheric abundances based on retrieval 

studies for WFIRST (Lupu et al. 2016). Credit: 

J. Bean (U of Chicago).
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While atmospheric chemistry among 

the hydrogen-helium dominated giant 

planets can likely be grossly predicted, the 

atmospheric diversity of terrestrial planets 

discovered by LUVOIR will doubtless 

exceed our imagination. By carrying versatile 

integral field spectrographs covering a wide 

wavelength range, LUVOIR is well equipped 

to characterize the atmospheres of all types 

of terrestrial planets. A few examples of the 

diversity of worlds LUVOIR will be able to 

search for and characterize include:

a) Hot exoEarths / exoVenuses are predicted to be the end-state of planetary evolution for rocky worlds with high O2

 or CO2

atmospheres and rocky planets interior 

to the habitable zone of the star (BertaThompson et al., 2016; Luger & Barnes, 

2015; Schaefer et al., 2016). Worlds 

undergoing catastrophic ocean and atmospheric loss may exist in the habitable zone (Meadows et al., 2016) and 

their characteristics will vary with stellar 

spectral type (Segura et al., 2003, 2005, 

2007; Meadows et al., 2016). Optical and 

near-IR spectra will constrain H2O, O2, 

and CO2 abundances, providing insight 

to this critical class of planets that define 

the inner edge of the habitable zone. 

b) Water-rich atmospheres may also exist 

in the usual habitable zone, either on the 

path to the “exoVenus” end-state, or as 

a long-term stable configuration of the 

planet’s atmosphere (Goldblatt et al., 

2016).

Figure 4.4. Model albedo spectra of gas giant planets with atmospheric metallicity (log(m)) 

varying from solar to about thirty times solar. The most distinctive absorption features are those 

of CH4. The shaded regions show the influence of gaseous H2O opacity, which, counterintuitively, 

is stronger at lower metallicity because of the feedback in this model between cloud height and 

water vapor abundance. Observations by LUVOIR users will inform our understanding of such 

water vapor/cloud feedback effects. Credit: R. Macdonald (Cambridge University).
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4-8 The LUVOIR Interim Report

c) Extreme water loss can lead to “Dune-like” 

worlds with little surface water reservoirs 

but temperate climates (Abe et al., 2011). 

Such worlds would be identified by their 

Rayleigh scattering slope and lack of 

bright clouds or water vapor.

d) Terrestrial planets of different ages and 

atmospheric redox states may mimic a 

similar evolution that occurred throughout 

Earth’s history (Arney et al., 2017; Lyons 

et al., 2014); this evolution may occur on 

worlds with or without life (Catling et al., 

2018). Such evolution can be driven by 

atmospheric loss, which when scaledup can lead to evaporated core rocky 

worlds that are the remnants of massive 

atmospheric loss from larger planets 

(Gillon et al., 2017; Luger et al., 2017).

e) Terrestrial-sized planets with dense 

envelopes that were not lost can exist 

beyond the habitable zone, and due 

to the greenhouse effect from their H2

envelopes, lead to planets with stable 

oceans well beyond the “traditional” 

habitable zone (Owen & Mohanty, 2016). 

Pressure-induced absorption from 

high-pressure H2 atmospheres will be 

detectable in the optical and near-IR by 

ECLIPS.

Comparing the nature of all such 

terrestrial planets to each other and to any 

habitable worlds discovered by LUVOIR 

will place these planets in context. For 

example, if exoVenus-type planets turn out 

to be ubiquitous, the relative importance 

of the runaway greenhouse mechanism 

will be apparent. However, the diversity of 

such worlds (do they always sport bright 

sulfuric acid clouds?) will only emerge 

through comparative studies of multiple 

planets. Since we cannot readily predict 

likely atmospheric compositions for all of the 

terrestrial planets LUVOIR will discover, the 

most important capability for characterizing 

these worlds will be flexibility. As discussed 

more fully in Chapter 3, LUVOIR’s spectral 

range for coronagraphic spectroscopy 

provides the ability to detect and measure 

the abundances of a large array of possible 

gases, including H2O, CH4, O2, O3, and CO2. 

Figure 4.5 shows simulated ECLIPS spectra, 

of both giant and terrestrial planets, that give 

a flavor of LUVOIR’s powerful capabilities for 

atmospheric characterization.

4.1.2 Photochemistry and hazes

Photochemical processes play key roles 

in shaping the atmospheres of all solar 

system planets. In the solar system giant 

planet stratospheres, CH4 photochemistry 

generates hydrocarbons such as C2H6 and 

C2H4; these molecules can polymerize into 

more complex hydrocarbon species, some 

of which condense into aerosols. These 

hydrocarbons strongly absorb UV and blue 

light on Jupiter and Saturn. On Venus, 

complex sulfur chemistry transforms SO2 and 

H2O into H2SO4 that condenses into the thick 

Program at a Glance – Comparative Atmospheres

Goal: Measure atmospheric composition for a wide range of non-habitable exoplanets.

Program details: Optical and near-IR spectra of giant through terrestrial planets at multiple 

separations around variety of stellar types. Acquired via a mix of dedicated observations 

and concurrently during long integrations on habitable zone planets.

Instrument(s) + Configuration: High contrast spectroscopy with ECLIPS IFS

Key observation requirements: Spectral bandpass from 400 nm to 1400 nm; R ~ 100; 

Continuum SNR > 10
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The LUVOIR Interim Report 4-9

cloud deck (Yung and Demore 1982). 

On Earth, the UV-shielding ozone layer is 

the result of oxygen photochemistry, and 

sulfur aerosols like H2SO4 can be produced 

through volcanic outgassing; such aerosols 

can produce detectable spectral signatures 

that may be remotely identified as a sign 

of vigorous volcanism on terrestrial planets 

Figure 4.5. Simulated LUVOIR spectra of planets orbiting a Sun-twin star at 10 pc. The simulated 

data plotted with grey bars were generated using the online LUVOIR coronagraphic spectroscopy 

tool, assuming the LUVOIR-A architecture (15-m telescope), the current ECLIPS-A instrument 

parameters, and exposure times of 96 hours per coronagraph band. Input model spectra are 

over-plotted with colored lines. The Venus model spectrum was provided by the Virtual Planet 

Laboratory (VPL), generated using the Spectral Mapping Atmospheric Radiative Transfer (SMART) 

model (Meadows & Crisp 1996; Crisp 1997). The super-Mars model was created by scaling a 

Mars model spectrum from VPL to a radius of 1 REarth and an orbital separation of 1 AU. The 

super-Mars data were rebinned to increase S/N. The warm Jupiter model spectra are from Cahoy 

et al. (2010) and the warm Neptune model spectrum is from Hu & Seager (2014). Image credit: 

LUVOIR Tools.
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4-10 The LUVOIR Interim Report

(Misra et al 2015; Hu et al 2013). The hazy 

disk of Titan, a consequence of CH4 transport 

and photochemistry is another distinctive 

example of the importance of hazes. 

Likewise, no characterization of an exoplanet 

atmosphere—potentially habitable or not—

will be complete without an understanding 

of the role photochemical hazes play in its 

atmosphere.

Evidence of hazes has already been 

inferred from the featureless transit 

transmission spectra of several exoplanets 

(Bean et al. 2010, Kreidberg et al. 2014, 

Knutson et al. 2014a, 2014b, Sing et al. 

2014). Exoplanet atmospheres observed 

by LUVOIR will sample compositional and 

incident UV flux conditions not found in the 

current solar system, yielding a diversity of 

haze types. Examples of the potential impact 

of two plausible types of hazes, sulfur hazes 

on a warm giant planet and hydrocarbon 

hazes on an Archean Earth-like terrestrial 

planet, are shown in Figure 4.6.

Understanding the diversity of photochemical outcomes and how they relate to 

atmospheric and, for terrestrial planets, surface composition, temperature, and pressure will be a complex task, but one that will 

greatly inform our understanding of such processes as a whole. For multi-planet systems 

Figure 4.6. Two examples of the impact of photochemical hazes on planet spectra. (Left) Model 

albedo spectra for a cloud-free 0.8 AU Jupiter-like planet (grey) along with spectra for various 

column abundances of sulfur (S8) haze particles. In addition to markedly raising the red continuum 

flux level, note the strong NUV absorption of these hazes. Credit: Gao et al. (2017). Right panel: 

Model spectra of clear and hazy Archean Earth atmospheres. Credit: G. Arney (NASA GSFC)

Program at a Glance – Atmospheric Hazes

Goal: Constrain presence of atmospheric hazes all planets detectable in each system 

observed.

Program details: UV and optical low-resolution spectroscopy and/or photometry of 

a wide range of planet classes (giant through terrestrial) at multiple separations around 

variety of stellar types. Measure UV spectra of host stars.

Instrument(s) + Configuration: ECLIPS high contrast low-resolution spectroscopy and/

or multi-color photometry. LUMOS UV spectroscopy.

Key observation requirements: Spectral bandpass from 250 nm to 1000 nm; R ~ 10; 

Continuum SNR > 10
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The LUVOIR Interim Report 4-11

understanding photochemistry in one planet’s atmosphere will inform our understanding of the other planets, including potentially habitable ones. Measurements of the UV 

spectrum of the host star will also be critical 

for understanding these processes and interpreting exoplanet atmospheres. LUVOIR 

observations will illuminate how haze opacity 

varies with planetary and stellar characteristics, clarify the role hazes play in potentially 

habitable planets, and improve our ability to 

model photochemical processes in all types 

of planetary atmospheres

The UV capability of ECLIPS will be 

crucial for constraining photochemical hazes 

as they often absorb strongly in the blue 

and UV. S8 particles, expected in warm giant 

planet atmospheres as a consequence of 

sulfur photochemistry, and organic hazes, 

which form from methane photochemistry 

and cause Titan’s characteristic orange 

color, both produce strong UV-blue 

absorption features. The organic hazes 

could be detected at low spectral resolution 

at wavelengths shorter than 0.5 μm for hazy 

Archean Earth although thinner hazes will 

require extending to shorter wavelengths 

(< 0.4 μm; see blue spectrum in right panel 

of Figure 4.6).

Although WFIRST may image some 

exoplanets, the bluest WFIRST photometric 

band cuts off at about 0.55 μm, too red a 

wavelength to usefully characterize hazes. 

Studies have also showed that hazes 

and clouds that are opaque in transit 

transmission observations—thus obscuring 

other atmospheric features—can still allow 

observations of deep atmospheric features 

in direct imaging (Charnay et al 2015; 

Morley et al 2015). Thus, a UV-coronagraph 

equipped LUVOIR would provide an 

outstanding platform for detecting and 

characterizing photochemical hazes in a 

great variety of planetary atmospheres. The 

understanding LUVOIR observers will gain 

from understanding hazes in a diversity 

of atmospheres will doubtless aid the 

interpretation of habitable planet hazes as 

well.

4.1.3 Clouds

All Solar System planets with an atmosphere 

harbor condensate cloud layers. Clouds 

influence the atmospheric pressure-temperature structure of a planet, the reflected, 

emission, and transmission spectra, as well 

as planetary albedo and energy balance. 

Because clouds can play such a defining role 

in controlling the emitted and reflected flux, 

no characterization of an atmosphere—in 

particular abundance measurements—can 

be complete without an understanding of 

the nature of its cloud decks. To date clouds 

have also been studied in hot Jupiters via 

transmission and emission spectroscopy, 

spectral phase mapping, and in reflected light. Brown dwarfs and directly imaged 

planets are strongly impacted by their cloud 

layers and there have been robust studies of 

clouds in these objects.

Most of our current knowledge on cloud 

properties and compositions still comes 

from studies of Solar System planets (most 

importantly, Earth and Jupiter). Water vapor 

and water ice clouds in Earth can be studied 

in-situ and via remote sensing; models 

developed to explain their behavior and 

properties are often used as a starting point 

for models of extrasolar planet clouds (e.g., 

Ackerman & Marley 2001), although the full 

diversity of cloud processes in all types of 

exoplanets has not yet been explored.

Clouds may also be used as temperature 

indicators and tracers of atmospheric 

dynamics (circulation, mixing, turbulence). 

The atmospheres of giant planets sport a 

layered set of clouds, which respond to 

atmospheric temperature. Cold giants, like 

Jupiter, are characterized by the presence 

of ammonia clouds. In warmer giants the 
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evaporation of first the ammonia, and then 

the water cloud decks will alter the visible 

spectra, as a given mass planet is found 

progressively closer to its star (Figure 

4.5). Clouds may also mask the presence 

of specific atmospheric absorbers even if 

present at large abundances below a thick 

cloud deck.

A systematic survey of cloud properties 

across all types of planets will be 

transformative for building better models of 

cloud behavior under varying conditions. 

In particular the characterization of most 

habitable planets will likely ultimately rest on 

cloud models, so a broad, systematic survey 

of cloud properties in all types of planets 

with LUVOIR will directly impact habitable 

planet studies. An understanding of how 

global cloud properties (altitude, thickness, 

patchiness of various species) vary with 

observable planetary properties will be one 

of the most important legacies of LUVOIR 

exoplanet science.

LUVOIR users will aim to understand 

which particular cloud species are present 

in which atmospheres, the processes that 

control cloud particle sizes and vertical 

distributions, the dynamical and radiative 

feedbacks between clouds, atmospheric 

thermal structure, and global dynamics. For a 

few favorable planets, LUVOIR observations 

will shed light on how clouds respond to 

global planetary diurnal and seasonal cycles. 

Time-resolved high-precision observations 

(photometric and spectroscopic light curves) 

have been obtained with HST and groundbased telescopes for some young giant 

planets. Extending such studies to cooler 

atmospheres in reflected light will be possible 

for bright planets for which photometry can 

be obtained in an hour or so.

The capabilities of ECLIPS to obtain UV 

to near-IR reflected light spectra are well 

matched to illuminating cloud processes. 

Cloud particle sizes are best constrained 

through large wavelength coverage, permitting size identification through Mie scattering 

effects which vary with wavelength. Cloud 

vertical thickness and number density also 

strongly influence planetary reflection spectra in the optical. In addition, for a few wellplaced planets, the large aperture permits 

high cadence photometry, allowing searches 

for rotational modulation from clouds, tracing 

atmospheric dynamics and measuring rotation periods.

4.1.4 Atmospheric evolution and 

escape

Atmospheric escape is a fundamental physical process that leads atmospheric constituents to become unbound from a planet and 

alters the composition of the remaining atmosphere. Understanding the relative roles 

Program at a Glance – Clouds

Goal: Constrain presence and composition of the cloud layers all planets detectable in 

each system observed.

Program details: UV through low-resolution spectroscopy and/or near-IR photometry 

of a wide range of planet classes (giant through terrestrial) at multiple separations around 

variety of stellar types. 

Instrument(s) + Configuration: ECLIPS high contrast low-resolution spectroscopy and/

or multi-color photometry.

Key observation requirements: Spectral bandpass from 250 nm to 1000 nm; R ~ 10; 

Continuum SNR > 10
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The LUVOIR Interim Report 4-13

of escape, outgassing, and accretion in a 

variety of exoplanet atmospheres is critical 

to understanding the origin and evolution of 

planetary atmospheres. LUVOIR UV transit 

observations will both identify the escaping 

species and constrain the physics of escape.

The first observations of escape were 

obtained by Vidal-Madjar et al. (2003), who 

obtained STIS far-ultraviolet transmission 

spectra of the close-in giant planet 

HD209458b revealing that the planet 

possesses a highly extended hydrogen 

atmosphere due to heating by stellar 

X-ray and EUV photons. Subsequent HST 

observations have also detected various 

metals in the exospheres of giant planets 

including carbon, oxygen, and magnesium 

(Vidal-Madjar et al. 2004; Linsky et al. 2010), 

as well as escaping hydrogen from the warm 

Neptune-size planet GJ436b, which showed 

a large tail of escaped planetary material 

(Figure 4.7; Ehrenreich et al. 2015).

Atmospheric escape is a key factor 

shaping the evolution and distribution of 

low-mass close-in planets (e.g., Owen & 

Wu 2013) and their habitability (e.g., Cockell 

et al. 2016). Indeed, many highly irradiated 

rocky planets (e.g., CoRoT-7b, Kepler-10b) 

might be the remnant cores of evaporated 

Neptune-mass planets (e.g., Lopez et al. 

2012). As a consequence, atmospheric 

escape also has a major impact on our 

understanding of planet formation (e.g., Van 

Eylen et al. 2017).

Upcoming missions will provide us with 

a large number of transiting planets covering 

the whole parameter space necessary 

to thoroughly study escape from an 

observational perspective. For example, the 

TESS mission is expected to find about 1700 

planets orbiting nearby stars, some of which 

(~15) will be Earth-size planets in the habitable 

zone of M-dwarfs. The PLATO mission will 

greatly exceed these numbers, particularly 

Figure 4.7. Atmospheric escape. Left: Average Lyman alpha line profiles of the M-type star 

GJ436 that hosts a warm Neptune-mass planet. The solid lines correspond to out-of-transit 

(black), pre-transit (blue), in-transit (green), and post-transit (red) observations from individual 

spectra. The line core (hatched region) cannot be observed from Earth because of the interstellar 

medium absorption along the line of sight. Right: Polar view of a three-dimensional simulation 

representing a slice of the comet-like cloud coplanar with the line of sight of the GJ436 system. 

Hydrogen atom velocity and direction in the rest frame of the star are represented by arrows. 

Particles are color-coded as a function of their projected velocities on the line of sight (the dashed 

vertical line). Inset: zoom out of this image to the full spatial extent of the exospheric cloud (in 

blue). The planet orbit is shown to scale with the green ellipse and the star is represented with 

the yellow circle. Credit: Ehrenreich et al. (2015).
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4-14 The LUVOIR Interim Report

by extending the search to longer-period 

planets (Sullivan et al. 2015; Rauer et al. 

2014). LUVOIR is the only mission capable of 

characterizing the extended atmospheres of 

a statistical sample of these worlds, essential 

for understanding the global properties of 

atmospheric escape and understanding how 

the physics of UV-driven atmospheric mass 

loss determines the long-term stability of all 

types of planetary atmospheres.

The observational census of atmospheric 

escape requires the acquisition of UV 

transmission spectroscopy observations for 

about 100 transiting planets orbiting stars 

(later than spectral type A5V). The complete 

sample of planets is necessary to understand 

how atmospheric escape correlates with 

stellar and planetary system parameters. The 

observations should be carried out at both 

FUV and NUV wavelengths for the brightest/

nearest systems (for Lyman-alpha in 

particular it is necessary to observe systems 

within 60 pc due high ISM absorption). 

Moreover, because stellar UV emission from 

active stars can be highly variable from one 

transit to another, it is important to obtain 

high S/N detections within an individual 

transit (Bourrier et al. 2017).

LUVOIR observing program. The target 

planetary systems will be well distributed on 

the sky and the identification of the best 

targets will be possible only after the results 

of the CHEOPS, TESS, and PLATO missions 

are in hand (still before the launch of LUVOIR). 

The observations will need to cover UV 

wavelengths (100–400 nm) where a large 

number of strong resonance lines of various 

elements are present. These features, plus a 

large number of weaker resonance lines of 

several metals will be the primary target of 

the observations.

The observations will aim not only at 

measuring transit depths and shapes, but 

also at measuring the velocity profile of the 

absorbing material along the line of sight. 

In this way, it will be possible to provide a 

set of strong observational constraints on 

models of atmospheric escape. From the 

few relevant HST observations of transiting 

planets obtained so far, it appears that the 

typical velocity of the escaping material is 

of the order of 10–20 km/s. This requires 

instruments with a resolving power 40,000–

60,000 to resolve the velocities, readily 

achievable with LUMOS.

Obtaining these observations simultaneously for a range of species is particularly 

valuable since different species probe different regions in a planet’s photo-evaporative wind. For example, while Lyman-alpha 

can trace material beyond the planet’s Hill 

sphere as it interacts with the stellar wind, 

metal lines like CII trace the inner bound portions of a planet’s upper atmosphere, where 

the photo-evaporative wind is launched and 

Lyman-alpha is heavily extincted by the ISM. 

Additionally, detection of these metal lines 

provides a way to probe atmospheric compositions, which is not subject to the effects 

of clouds lower in the atmosphere.

The need to reach high-enough signalto-noise ratios (S/N) for multiple species 

in a single transit drives the specifications 

on the effective area, which in turn places 

constraints on the telescope diameter and 

component efficiencies. The exposure times 

are then driven by the transit durations 

and the need to get enough out-of-transit 

coverage to make sure transit asymmetries 

can be unambiguously detected. Additionally, 

high time resolution (~ 1 second) is required 

to be able to detect and characterize flaring 

events of the host stars that need to be 

taken into account in the data analysis and 

interpretation. The transit durations observed 

will cover a wide range of transit durations 

depending on stellar type and the planet’s 

irradiation, ranging from ~1 hour for planets 

close-in planets around very late M dwarfs 

like those in the TRAPPIST-1 system (Gillon 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 4-15

et al. 2017) up to 15 hours for planets in the 

habitable zones of Sun-like stars.

As an example of an observing program 

to characterize the mass-loss characteristics 

of low-mass planet, we reconsider GJ 436b, 

a 4.3 Earth radius warm Neptune on a 2.6-

day orbit around M2.5 dwarf with a V-band 

magnitude of 10.7. Although, this planet has 

been studied carefully in Lyman-alpha (e.g., 

Bourrier et al. 2016), it is not currently possible 

to detect other species with current facilities 

(e.g., Loyd et al. 2017). LUVOIR, using the 

LUMOS instrument’s G150M mode will 

achieve a 5-sigma detection of the velocityresolved absorption profile in the 133.5 

nm C II line in just 20 minutes. This would 

provide carbon detections in ~10 points as 

the planet’s Hill sphere transits, probing all 

the way into the inner bound portions of the 

upper atmosphere which are inaccessible 

to current observations. Moreover, while GJ 

436 b is an ideal example, upcoming surveys 

will uncover a large sample of planets around 

bright nearby stars that can be characterized 

to a similar level, enabling us to repeat these 

observations for dozens of planets spanning 

the full range of planetary parameter 

space. The Sullivan et al. (2015) simulated 

TESS catalog predicts that about 25 TESS 

planets will be amenable to transit maps of 

the densities, abundances, and velocities 

for multiple species in their escaping 

atmospheres.

4.2 Signature science case #2: 

Planetary system architectures

The history of a planetary system is 

encoded in its architecture, the systemwide configuration of planets and asteroid/

comet belts. Architectural information 

includes the number, masses, spacing, 

and orbits of the planets; as well as the 

structure, mass, and composition of any 

accompanying debris from asteroids and 

comets (aka. planetesimals). Our own solar 

system’s architecture—including the orbits 

and compositions of our asteroid and Kuiper 

belts (e.g., DeMeo and Carry 2014), the 

spacing and eccentricities of the terrestrial 

planets (e.g., Chambers & Wetherill 1998), 

and the orbits of the giant planets (e.g., 

Raymond et al. 2004)—informs us about the 

formation, early environment, and habitability 

of the Earth. Similarly, exoplanetary system 

architectures provide context for exoEarths; 

for example, an Earth-size exoplanet in a 

system with a nearby, highly eccentric Jupiter 

grew up in a very different environment from 

one tightly packed together with seven other 

rocky planets.

The large number of mature and young 

planetary systems LUVOIR will observe allows 

fundamental, systematic investigations of the 

environmental influences on planet formation 

and evolution. These studies are enabled 

Program at a Glance – Evaporating Exoplanets

Goal: Fully constrain the physics of atmospheric escape for planets across parameter 

space.

Program details: High S/N FUV and NUV spectra to detect outflows in ~100 transiting 

planets and fully map multiple escaping species for ~25 transiting planets.

Instrument(s) + Configuration: LUMOS G120M, G150M, G300M

Key observation requirements: Large aperture to obtain S/N ≥ 20 per R ~ 30,000 

resolution element spectra of the cores of stellar chromospheric emission lines in single 

transits in the NUV and FUV.
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by LUVOIR’s ability to simultaneously map 

each exoplanet system in detail across the 

field of view of ECLIPS. Such observations 

complement techniques that characterize a 

portion of each planetary system (e.g., transit 

spectroscopy) and surveys that provide 

planet occurrence rates (e.g., microlensing). 

Furthermore, LUVOIR will catalog the 

chemical outcomes of planet formation 

around a range of stars by measuring 

the bulk composition of young extrasolar 

comets and asteroids, measured through 

outgassing arising from collisions. LUVOIR 

also has a unique role to play in filling out our 

inventory of the smallest and coldest bodies 

in the solar system, which hold vital clues to 

its early history.

4.2.1 Mature exoplanet systems

The formation and evolution of even mature 

planetary systems (>1 Gyr) is imprinted in 

their residual architectures. By characterizing 

system architectures for multiple planetary 

systems, LUVOIR will open a window to 

understanding how planetary formation 

proceeds under a diversity of environments.

4.2.1.1 Orbital dynamics aid 

determination of planet masses and 

radii

The attributes of mass and radius are 

fundamental for characterizing exoplanets. 

The determination of bulk density allows 

us to distinguish rocky worlds from planets 

with substantial gas-envelopes or ice or 

water worlds. Masses and radii constrain 

surface gravity and are needed to interpret 

spectra of planetary atmospheres. Circa 

2018, most planet masses are measured 

via the radial velocity technique, with a 

small number determined from transit timing 

variations. Neither of these methods can 

currently provide mass measurements for 

true analogs of our Earth around solar-type 

stars. Therefore, the capability of measuring 

exoplanet masses with extremely precise 

astrometry (~0.1 μas) is a key attribute of the 

High Definition Imager (HDI). Such astrometric 

measurements will yield reliable orbits and 

masses for planets with M ≥ 1MEarth orbiting 

≥  1 AU from G2V stars within 10 parsecs 

(the mass measurement limit is smaller for 

lower mass stars). Additionally, LUVOIR’s 

immense light-gathering power, short 

integration times, large instantaneous field of 

regard, and rapid repointing will enable deep 

imaging of each exoplanet target star many 

times, ensuring both an accurate orbital 

solution as well as a high chance of planet 

discovery. The combination of direct imaging 

using ECLIPS and precision astrometry with 

HDI will robustly disentangle multi-planet 

systems (Guyon et al. 2012).

Since a planet’s brightness in reflected 

light is a function of the planet radius, 

albedo, and orbital phase, determining 

the radius is difficult. Repeat imaging in 

multiple filters at different orbital phases is 

key to untangling these degeneracies. As 

an example, for a planet with a substantial 

atmosphere observed at crescent phases, 

the forward lobe of Rayleigh scattering tends 

to dominate over atmospheric and surface 

absorption at blue wavelengths, reducing 

the albedo uncertainty. Exploratory retrieval 

studies show that at phase angles greater 

than ~100, degrees measurement of planet 

brightness in the blue offers improved 

constraints on planet radii compared to more 

gibbous phases (Nayak et al. 2016). Further 

retrieval studies of simulated directly imaged 

planets observed in reflected light will identify 

the optimum strategy for constraining radii, 

and hence bulk density and composition, of 

both terrestrial and giant planets.

4.2.1.2 Orbits, compositions, and 

formation

Linking information on planet orbits, 

compositions, and dynamical history of 
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The LUVOIR Interim Report 4-17

mature exoplanet systems can provide deep 

insights into their past formation and evolution. 

For example, giant planets can affect volatile 

delivery to the inner solar system, leading to 

potential differences in composition between 

terrestrial planets in systems with and without 

giant planets. Atmospheric composition of 

giant planets also provides clues to their 

formation and evolution. Gas giant planets 

formed by the core accretion mechanism are 

expected to have atmospheric heavy element 

abundances in excess of those of their parent 

stars. Available data on hot transiting planets 

already point to a trend of atmospheric heavy 

element enrichment with mass (Figure 4.3). 

LUVOIR will extend such comparisons to 

cooler, more solar system-like giant planets, 

and uncover systematic deviations from the 

trend that might indicate other formation 

mechanisms (like gravitational collapse) for 

subsets of planets.

Furthermore, differences in atmospheric 

bulk composition (recorded as C/O ratios) 

for planets at similar orbital distances may 

indicate migration has moved some of them 

from their birthplaces to the locations we see 

today. Planet migration can also establish 

planets in orbital resonances and alter their 

eccentricities. Thus, measurement of orbits, 

masses, and atmospheric composition for all 

the planets in any system together permit a 

much richer understanding than any of these 

measures alone. Integral field spectroscopy 

at R=70 will be able to measure precise 

volatile inventories for a hundreds of gas 

and ice giant planets, enabling comparisons 

of C/O ratios over a wide range of orbital 

separations and identifying relationships 

between composition, mass, and orbit.

4.2.1.3 Mature debris belts

Even in old planetary systems, asteroids, 

comets, and Kuiper Belt objects (KBOs) left 

over from the planet formation phase persist. 

Erosion of these planetesimals produces 

interplanetary dust, which can actually be the 

most easily observed feature of a planetary 

system, due to the large total surface area of 

the dust grains. Dust in the warm inner region 

of the solar system is called zodiacal dust, 

which largely comes from comets (Nesvorný 

et al. 2010). The spatial distribution of 

interplanetary dust in the solar system—with 

the signs of planets imprinted in it—is shown 

in Figure 4.8.

Although sufficiently high levels of exozodiacal dust can obscure exoplanets from 

view, planetesimal belts and the dusty debris 

they produce serve as records of the system’s early history and provide constraints on 

present day orbital properties. A belt located 

near a planet yields constraints on its mass 

and orbit: for example, a sufficiently massive, 

nearby, and/or elliptical planet would disturb 

the belt, which provides information on how 

the planet’s orbit has (or has not) changed 

Program at a Glance – Planetary System Architectures

Goal: Catalog the architectures of planetary systems around older main sequence stars

Program details: High contrast direct observations and astrometric measurements of 

nearby main sequence stars to measure bulk properties (masses, orbits, atmospheric 

composition) of all detectable planets and debris belts 

Instrument(s) + Configuration: ECLIPS broadband coronagraphic imaging and integral 

field spectroscopy, HDI high precision astrometry

Key observation requirements: Contrast < 10−8; OWA ≥ 48 λ/D; astrometric precision 

= 0.1 μas 
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4-18 The LUVOIR Interim Report

over time. Gravitational interactions between 

a planet and planetesimals can trap some of 

the latter into dynamical resonances, which 

can then be revealed through clumps in the 

interplanetary dust. Such trapping can serve 

as a record of the planet’s migration history (e.g., Malhotra 1993). Furthermore, if we 

assume a planetesimal belt is roughly coplanar with the planetary system, we obtain 

an immediate estimate of the inclination of 

the system and can constrain a planet’s orbit with fewer observations. High resolution, 

high contrast imaging of exoplanets will simultaneously reveal interplanetary dust, providing dynamical and contextual information 

without additional observational demands.

4.2.2 Young exoplanet systems

Young exoplanet systems offer valuable 

windows into the later stages of planet formation. Building rocky worlds and the final 

sculpting of planetary systems involve dynamic and sometimes violent processes. 

The young comets and asteroids that are the 

building blocks of larger planets are colliding and fragmenting, exposing dust and gas 

from their interiors. The orbital distribution 

of dusty debris from those collisions will tell 

us about the timescales for rocky planet formation and the properties of young planets. 

Spectroscopy of the gas provides a unique 

opportunity to measure the bulk composition 

of planetary material as a function of stellar 

type. These studies of terrestrial planet formation via debris in young systems will complement studies of giant planet formation 

through observations of younger protoplanetary disks (Chapter 7). LUVOIR’s small inner 

working angle will be needed to probe within the ice lines of young planetary systems, 

which in theory divide inner rocky planet 

Figure 4.8. The architecture of the modern Solar System, showing the interplay between planets 

and debris dust. The left panel shows a model of the entire system out to a radius of 50 AU from 

the Sun, while the inset panel zooms in on the inner system. For ease of viewing, the Sun and 

possible astrophysical background sources are not included. The bright region at the center of 

the image is emission from warm debris dust (aka. exozodiacal dust). Two circular gaps in the 

dust are visible, the inner one caused by Jupiter and the outer one marking the 3:2 mean motion 

resonance with Neptune. Neptune also creates a partial gap in its immediate vicinity. Credit: 

Roberge et al. (2017)
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The LUVOIR Interim Report 4-19

Figure 4.9. LUVOIR can peer into the unseen inner regions of young planetary systems. This 

image shows the famous Beta Pictoris debris disk, with the gas giant exoplanet Beta Pic b imaged 

at two epochs in its orbit (2003 and 2009). The semi-major axis of the planet’s orbit is ~ 10 AU 

(Lagrange et al. 2010). This planet is likely responsible for the long known “warp” in the inner 

portion of the dust disk (e.g., Heap et al. 2000). The inset panel shows a blow-up of the innermost 

region with the LUVOIR-A coronagraph inner working angle (red circle) and outer working angle 

(orange circle) overlaid. LUVOIR can search for warm dust and additional planets in the region 

between the two circles. Credit: ESA / A.-M. Lagrange

formation regions from the cold icy regions 

where giant planets form. 

4.2.2.1 Young debris belts: 

morphology and architecture

Young debris disks are a powerful probe of 

the formation and early dynamical evolution 

of planetary systems. On a massive 

scale, countless numbers of extrasolar 

planetesimals—the building blocks of both 

terrestrial and giant planets—are colliding 

with each other and producing disks of dust 

and gas debris that can be observed in detail 

(Figure 4.9). Since the ages of most debris 

disks span a plausible range of timescales for 

terrestrial planet formation (~ 10 to hundreds 

of Myr), the process can be observed in 

action, thereby constraining when and 

where it occurs, and shedding light on how 

processes vary from system to system and 

as functions of stellar properties.

Space- and ground-based instruments—

including HST, ALMA, GPI, and SPHERE—

have imaged debris disks containing planetsculpted dust structures that help constrain 

the orbital properties and dynamical history 

of giant planets in young systems (e.g., 

HR8799; Booth et al. 2016). However, current 

instruments lack the spatial resolution, 

sensitivity, and inner working angle to image 

warm material in the innermost regions 

of debris disks. The following features 

enumerate the types of formation signatures 

that LUVOIR can detect as a function of 

separation from the host star and stellar age, 

mass, and metallicity.
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Dust from sites of ongoing terrestrial planet formation. As terrestrial planets 

grow in a disk of planetesimals, they dynamically stir nearby bodies, generating collisional cascades that produce copious dust concentrated in rings. The dust is subsequently 

removed largely by radiation pressure. The 

dynamical simulation in Figure 4.10 shows 

that the region of active formation—marked 

by bright dust rings—moves outward with 

time. The growth timescale is expected to 

depend on radial distance from the star and 

the local solid disk mass, which may be correlated with the host star mass and metallicity.

These theories can be tested with high 

resolution, high contrast images of young 

debris disks with a range of ages. Current high 

contrast instruments, with their large inner 

working angles and relatively low contrast (> 

10−6), can only obtain such images for the 

outer regions of bright disks around earlytype stars. Millimeter/submillimeter facilities 

like ALMA are insensitive to warm dust in 

inner regions of disks. Figure 4.11 shows 

that ECLIPS can obtain high-resolution 

images reaching within the ice lines (~ 3 

AU for the solar system; e.g., Martin & Livio 

2012) of nearby young disks within about 

135 pc. LUVOIR also has the sensitivity to 

extend such imaging studies to fainter disks 

around lower mass stars than are currently 

possible.

Gravitationally sculpted features 

associated with terrestrial planets. 

Dust structures produced by dynamical 

interactions between planetesimals and 

planets can provide constraints on young 

planets’ masses and orbits. The Beta Pic dust 

Figure 4.11. Observing the terrestrial planet 

forming regions of young exoplanet systems 

with LUVOIR. The plot shows the distance out 

to which a coronagraph can observe the warm 

region inside the ice line of a Sun-like star (~ 3 

AU) as a function of telescope diameter. The 

coronagraph IWA = 3.5 λ/D and the assumed 

observation wavelength is 400 nm. In the blue 

region, the ice line is inside the IWA, so only 

cold material can also be observed. In the pink 

region, the ice line is outside the IWA, so warm 

material can be observed. The mean distances 

to a few nearby young (≤ 100 Myr) stellar 

associations are indicated with horizontal lines 

(BPMG = Beta Pic Moving Group, TW Hya = 

TW Hydrae Association, Tuc-Hor = TucanaHorologium Association, Pleiades = Pleiades 

Open Cluster). Observing inside the ice lines 

of Sun-like stars in the Pleiades requires a 

telescope with D ≥ 15 m. Credit: A. Roberge 

(NASA GSFC).

Figure 4.10. Signposts of terrestrial planet 

formation—dust rings moving outwards with 

time. Credit: Kenyon & Bromley (2004).
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The LUVOIR Interim Report 4-21

disk displays an example of a planet-induced 

warp (Figure 4.9). Other types of structures 

include gaps cleared of planetesimals by 

resonances with a planet (Figure 4.8) 

and dust clumps marking planetesimals 

captured into resonance during a planet’s 

migration (e.g., as invoked for Beta Pic in 

Dent et al. 2014). By comparing properties of 

planets within younger protoplanetary disks 

to those inferred within older debris disks, 

the evolution of planetary systems, planet 

migration, growth of planets through giant 

impacts after the dissipation of the primordial 

gas disk, and planet-planet gravitational 

interactions can be traced. However, as 

for the observations described immediately 

above, the smaller inner working angle and 

greater sensitivity of ECLIPS will be needed 

to see the subtler dust structures created by 

terrestrial planets.

4.2.2.2 Bulk composition of planetary 

material: extrasolar planetesimals

While the surfaces and atmospheres 

of exoplanets can be directly studied, 

their interiors remain hidden. Measuring 

planetesimal compositions offers the 

opportunity to infer the bulk composition of 

planetary material that formed around various 

types of stars. Compositional differences 

between relatively unprocessed gas in younger 

protoplanetary disks vs. gas produced 

from planetesimals in older debris disks set 

expectations for giant planet atmospheres 

accreted purely from relatively unprocessed 

primordial gas vs. those contaminated by 

planetesimal impacts. These measurements 

also inform the possible make-up of subNeptune atmospheres accreted during the 

protoplanetary phase vs. those outgassed 

from a planet’s interior. But even in the solar 

system, to probe the interior composition 

of planetesimals, one needs to break them 

open, as was done by NASA’s Deep Impact 

mission to comet Tempel 1. Fortunately, this 

breaking open is exactly what is happening 

to planetesimals in debris disks.

The goal is to determine the bulk 

composition of planetesimals around a 

variety of young stars by measuring elemental 

abundances of material in debris disks. 

Determining the composition of dust in debris 

disks is and will remain difficult, because the 

grains are relatively large and their spectral 

features are broad, weak, and ambiguous. 

Spitzer Space Telescope infrared spectra of 

many debris disks showed silicate emission 

features in very few cases, and little could 

be learned about the dust composition even 

when the emission was present (e.g., Chen 

et al. 2006). Instead it is the gas component 

that can provide abundances of all the major 

elements.

Gas in debris disks has been hard 

to detect, since the gas masses are low 

relative to those in younger protoplanetary 

disks (e.g., Dent et al. 2005). In contrast 

to those young gas-rich disks, debris gas 

appears to be primarily atomic rather than 

Program at a Glance – Terrestrial Planet Formation

Goal: Map the spatial distribution of dust coming from destruction of planetesimals during 

the late stages of planet formation

Program details: High resolution, high contrast direct imaging of dust in debris disks 

around solar-type and low-mass stars in nearby young moving groups with a range of 

ages

Instrument(s) + Configuration: ECLIPS broadband coronagraphic imaging

Key observation requirements: Contrast < 10-8; OWA > 48 λ/D
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4-22 The LUVOIR Interim Report

molecular, since the dust masses in debris 

disks are too low to shield molecular gas 

from rapid photodissociation. The advent of 

sensitive radio spectroscopy with ALMA has 

provided the opportunity to survey a limited 

number of gas species in many debris disks 

(specifically, CO and neutral carbon). But 

such observations cannot provide a full 

compositional inventory of gas in debris 

disks, as emission lines of other important 

species (water, atomic oxygen, and metals) 

are not accessible.

To access a wide range of atomic 

species in debris disks, astronomers have 

instead employed sensitive UV / optical 

absorption spectroscopy, using the host 

stars as the background light sources. An 

example appears in Figure 4.12, which also 

shows time-variable gas coming from stargrazing extrasolar planetesimals. Far-UV 

spectroscopy is vital for these studies, as the 

strong absorption lines of many atomic and 

molecular gases are only available in that 

bandpass. This is the technique that was 

used to inventory the gas in the Beta Pic 

and 49 Cet disks, both around intermediate 

mass A-type stars (Roberge et al. 2006, 

Roberge et al. 2014). In both cases, the gas 

elemental composition appears extremely 

and unexpectedly carbon-rich. The reasons 

for this are not fully understood at this time. 

For this technique to work, the disks must 

be edge-on to our line of sight to the central 

stars (like doing transit spectroscopy of an 

exoplanet, but all the time). But even more 

importantly, current UV spectroscopy with 

the Hubble Space Telescope is not sensitive 

enough to employ this technique on anything 

but a handful of UV-bright, nearby A-type 

stars.

The UV spectrographs on LUVOIR 

(LUMOS and/or Pollux) will extend this 

powerful observational technique to gas in 

disks around later types of stars. For the first 

time, we will be able to look for differences 

Program at a Glance – Composition of Planetesimals

Goal: Measuring bulk composition of gas coming from young planetesimals in edge-on 

debris disks.

Program details: Spectra of stars with a range of masses in nearby young moving groups.

Instrument(s) + Configuration: POLLUX and LUMOS UV / optical point-source 

spectroscopy

Key observation requirements: Spectral bandpass from 100 nm to 400 nm; R ~ 65,000 

to 120,000; Continuum SNR > 10

Figure 4.12. Ionized carbon gas coming 

from planetesimals in the 49 Ceti debris disk, 

observed via UV absorption spectroscopy using 

HST. The additional redshifted gas apparent in 

the second observation (Visit 2) comes from 

vaporization of star-grazing planetesimals 

transiting the central star. Credit: Miles et al. 

(2016).
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The LUVOIR Interim Report 4-23

in the volatile and refractory composition 

of planetesimals as functions of stellar 

mass and metallicity. To do this, we will use 

high-resolution point-source absorption 

spectroscopy at UV / optical wavelengths. 

In some cases, high spectral resolution 

(R > 100,000) will be needed to separate 

circumstellar absorption lines from interstellar 

ones. The instruments’ bandpasses cover 

many strong absorption lines of critical 

species, including volatiles (e.g., H, C, N, 

O, CO, and OH), lithophiles (e.g., Na, Mg, 

Al, Si), and siderophiles (e.g., Mn, Fe, Ni). 

Furthermore, access to absorption lines from 

more than one energy level and ionization 

state is important for determining the gas 

excitation temperature and the ionization 

balance of the gas. Both are needed to 

calculate the total elemental abundances. 

Similar observational techniques applied to 

metal polluted white dwarf stars can be used 

to measure the bulk composition of planet 

fragments after tidal disruption at the end of 

their lives (details in Appendix A).

4.2.3 Solar System comets, 

asteroids, and KBOs

The vast collection of icy bodies in the 

outer solar system contains an echo of the 

primordial processes that led to the formation 

and modern distribution of the giant planets. 

This population is extremely diverse, ranging 

from geologically active, atmosphere-bearing 

dwarf planets to small, porous undifferentiated 

planetesimals that are structurally reflective 

of primordial condensation and aggregation 

processes (Figure 4.13). Studies of the 

composition, size distribution, organization, 

and orbital dynamics of these objects 

offers our best window into the epoch of 

planet formation, from which we are able to 

constrain the structure of the solar system’s 

protoplanetary disk, the mechanisms of 

small body accretion, and the process of 

planetary migration. Furthermore, as the only 

remnant sub-planetary population available 

for direct study, the observed properties 

of these objects are broadly applicable 

to our understanding of planetary system 

development in other star systems.

It is only over the last 25 years (Jewitt 

and Luu, 1993) that we have begun to map 

the distribution of Trans-Neptunian Objects 

(TNOs) in the Kuiper-Edgeworth Belt (KEB), 

the inner of two icy body reservoirs. Over 

1800 TNOs have been directly observed, 

ranging in diameter from ~25 km (1999 

DA8, Gladman et al, 2001) to >2300 

km (e.g., Pluto, Eris). Prior to this, the 

existence of orbital reservoirs was inferred 

entirely from comets, icy bodies that are 

gravitationally perturbed into the inner solar 

system where they become visible by virtue 

volatile sublimation and proximity to Earth. 

Comet orbital properties reflect their original 

reservoir, with short period (P < 20 years) 

comets (SPCs) in the Jupiter and Encke 

families following prograde orbits centered 

on the ecliptic in a manner consistent with an 

origin in the KEB, while long (P > 200 years) 

period comets (LPCs) display an isotropic 

distribution of inclination and direction 

consistent with a more spherically distributed 

Oort cloud. Orbital simulations suggest that 

the evolution from the stable reservoirs to an 

active comet proceeds by slow accumulation 

Figure 4.13. At left, an HST image captures 

the nucleus and dust coma of C/2017 K2 at a 

heliocentric distance of 15.7 AU (Jewitt et al. 

2017). At right is the series of discovery images 

of MU69, the next New Horizons mission target, 

obtained with HST WFC3
LUVOIR The Large UV Optical Infrared Surveyor

4-24 The LUVOIR Interim Report

of gravitational perturbation by galactic tides 

(for LPCs) and successive orbit-crossing 

events (for SPCs) over time scales of up 

to 0.1 Gyr, with the majority being lost to 

ejection before entering the ‘activity zone’ 

inside 2.5 AU (e.g., Duncan, Levison, and 

Dones, 2005). With projected lifetimes of 

order 105–106 years, the typical active comet 

is therefore a short-lived remnant of a much 

larger population of evolving objects in the 

region beyond Jupiter.

The major factor limiting the study of 

icy planetesimals is detection efficiency for 

small bodies. These objects are only visible 

due to reflected sunlight that experiences 

inverse square dilution in both the outbound 

and reflected directions, resulting in a 

R-4 brightness function with heliocentric 

distance. This effect is magnified in the gas 

coma, because the rate of sublimation from a 

nucleus decreases rapidly with temperature, 

resulting in a lower scattering column density 

with increasing heliocentric distance. This 

combination of factors results in a severe 

observational bias against smaller and less 

active bodies that is reflected in the clear 

difference in the observed size distributions 

of TNOs and SPCs, where even the largest 

SPC nuclei (e.g., 109P/Swift-Tuttle) are 

smaller than every known TNO, and the 

paucity of gas production measurements 

from comets beyond 2.5 AU.

HST is currently the deepest telescope 

for detection of small bodies by virtue of the 

much lower sky-background and its use of 

diffraction limited imaging. WFC3 broadband 

(F350LP filter) imaging can currently identify 

(SNR~5) a 15 km diameter object in a circular 

orbit at 40 AU in ~600 s (the crossing-time 

for a 0.08" resel). The LUVOIR HDI will extend 

the HST advantage as a detection tool, even 

when compared with the next generation 

of 30-m class ground based telescopes. 

Over the same bandpass, LUVOIR-HDI 

would achieve SNR~5 detection of 3.5 km 

diameter objects over a ~70 s resel-crossing 

time, a 4-fold reduction in limiting diameter 

in 1/8th the time (Figure 4.14). In addition to 

point source detection, the HDI and LUMOS 

instruments would provide a 100x increase in 

sensitivity to the surface ice composition and 

low surface brightness UV emission from the 

gas comae of TNOs (Figure 4.15). LUVOIR 

is essential for the following types of studies.

Population statistics. The deepest 

wide-field surveys available are currently 

unable to detect objects <10 km in diameter 

beyond the orbit of Saturn, yet dynamical 

models identifying the KEB as the source of 

SPCs and Centaurs require that such bodies 

must be extremely common. LUVOIR-HDI is 

capable of extending the detection threshold 

down to 1–10 km sizes typical of SPCs. A 

campaign targeting size distributions in the 

classical KEB between 40–50 AU would 

identify any objects in this region ≥ 3 km in 

diameter at a rate, including time-sequenced 

returns to establish motion, of 1 sq-deg per 

day of integration. In addition to small KEBs, 

these observations would obtain trailed 

images of Centaurs between 6–30 AU to 

depths of 0.5–2 km and detect Sedna-sized 

Figure 4.14. The limiting diameter detectable 

at SNR ~ 5 is shown as a function of distance 

for a single image that is depth-optimized to the 

Classical Kuiper-Edgeworth Belt. The results 

from HST (in 670 sec) and LUVOIR (in 75 sec) 

are compared. Credit: W. Harris (Arizona LPL)
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 4-25

Dwarf planets at distances up to 1000 AU 

(Sedna’s aphelion).

Moons and rings. The cold classical 

Kuiper Belt’s binary population is thought 

to be a primordial remnant produced during 

the epoch of planetesimal formation. With 

negligible binding, binary pairs of 100 km 

objects have shown separations >105 that 

sharply constrain subsequent perturbations. 

Binary occurrence increases rapidly with 

decreasing separation to the limit of HST’s 

angular resolution. LUVOIR will be able to 

characterize the occurrence rate of Kuiper 

Belt binary systems down to the Roche limit 

for objects with diameters as small as 120 

km (Figure 4.15), approximately the size at 

which the Kuiper Belt size distribution breaks 

from a steep to shallow slope. Two of the 

largest Centaurs, Charliko and Chiron, are 

thought to support ring systems. Their origin 

Figure 4.15. Brightness of centaurs and trans-Neptunian objects compared to the capabilities of 

a LUVOIR-class observatory with sensitive spectrometers in the UV/Optical/NIR spectral range. 

LUVOIR will permit a unique characterization of the deepest regions (40–50 AU) of our solar 

system (objects with m<33), also permitting unprecedented studies of their atmospheres (m<20), 

surface composition / variability (m<22) and their origin and evolution via studies of their rings and 

binary configuration (m<24). Credit G. Villanueva (NASA-GSFC).
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4-26 The LUVOIR Interim Report

is a mystery, and determining the rate at which 

they occur, along with understanding the 

processes that create, maintain, and destroy 

them, has implications for the occurrence 

rate of small satellites, sub-surface volatile 

reservoirs, and collision rates in the outer 

solar system.

Surface composition. NASA’s New 

Horizons mission has revealed the complex 

cryogenic world Pluto (Figure 4.16), which, 

like Neptune’s moon Triton, has a spatially 

heterogeneous surface composition shaped 

by a variety of poorly understood processes 

including both geologic activity and 

atmospheric transport of CO, N2, and CH4 

ices. More remote (e.g., Eris) and smaller 

(e.g., Sedna, Makemake) TNOs have spectra 

dominated by CH4 absorption. Surface and 

seasonal variations, including sublimation/

condensation of atmospheres, on all of these 

worlds are poorly constrained and can be 

directly addressed by LUVOIR spectroscopic 

study in the 0.5–2.5 mm range. In contrast to 

the hydrostatically organized dwarf planets, 

Figure 4.16. LUVOIR high spatial resolution and spectroscopy in the 0.5–2.5 mm range can 

provide spectral characterization ranging from high fidelity spatial, diurnal, and seasonal coverage 

of larger Dwarf Planets to more complete identification of surface composition on smaller bodies. 

Credit: NASA/ New Horizons/R. Juanola-Parramon (NASA GSFC).

smaller TNOs are spectrally featureless or 

present surfaces with small amounts of nonvolatile water ice. This likely represents their 

primitive, more homogeneous structure. 

LUVOIR’s high sensitivity, spectroscopic 

capabilities and spatial resolution will permit 

a deep characterization of the surface 

composition on smaller bodies.

Volatility. Comets are the primary means 

through which we obtain volatile composition 

information from icy planetesimals. However, 

the sample is skewed by the strong 

temperature dependence of CO, CO2, 

and H2O sublimation, the R-4 reduction in 

scattered surface brightness, and a historical 

emphasis on wide-field observations of 

photochemical fragments (e.g., CN instead 

of HCN or HNC). These factors combine bias 

study toward the global properties of the most 

active nuclei at small heliocentric distances. 

This emphasis lacks perspective on factors 

including compositional heterogeneity, 

seasonal patterns in surface temperature, 

the characteristics of low-activity objects 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 4-27

(e.g., Main Belt Comets, the Jupiter Trojans), 

the characteristics of sublimation beyond 

the distance where H2O ice contributes, and 

the role of exotic processes such as thermal 

electron impact excitation (Feldman et al., 

2015). For objects beyond the orbit of Mars, 

the dominant volatile ice is either CO (e.g., 

Jewitt et al. 2017) or an exothermic reaction 

such as the amorphous to crystalline H2O 

ice conversion (Prialnik and Bar Nun, 1990). 

Particularly for distances beyond the orbit of 

Jupiter, the composition of a comet coma has 

been partially sampled for only a few objects, 

and, in several cases, is detectable only 

from an associated dust coma. The broad 

spectral coverage of LUVOIR will combine 

observations of atomic species (O, C, S, H, 

and D) in the UV with the classical visible 

band fragment molecules (OH, CN, C2, etc.) 

and their near-IR parents between 1–2.5 

microns. Moreover, it will have the sensitivity 

to detect, or place far more stringent limits 

on gas production from low activity, volatile 

rich bodies including some Trojan asteroids 

and the Main Belt Comets and to monitor the 

characteristics of ice sublimation for nuclei in 

the CO dominated region of comet orbits.

4.3 Summary

Before the Kepler mission flew we knew 

of only a few hundred exoplanets, most 

discovered by the radial velocity method 

and thus lacking information on planetary 

radii. Kepler added thousands of confirmed 

and candidate planets to the list of known 

exoplanets, but perhaps the greatest 

contribution to exoplanet science was the 

discovery of exceptional diversity among 

planets. Super-earth and sub-Neptune sized 

planets, not represented by any solar system 

objects, turned out to be exceptionally 

abundant. This diversity of planet types 

broadened our vision and led to a revolution 

in efforts to understand planet formation and 

evolution.

LUVOIR will enable an equivalent new 

era of comparative exoplanet science. 

Instead of comparisons of planet sizes and 

masses, LUVOIR will empower its users to 

conduct comparative studies of planetary 

atmospheres, including their composition, 

chemistry, clouds, dynamics, photochemistry, and escape processes for hundreds 

of planets. By improving our understanding 

of fundamental planetary processes on all 

types of planets our ability to interpret observations of and deeply understand habitable planets will only be magnified. Likewise, 

Program at a Glance

Goals: (1) Measure the size & spatial distributions and the rate & scale of binary/ring 

systems among the Centaurs and in the Kuiper belt. (2) Determine the orbital distribution 

of Trans Neptunian Objects. (3) Obtain the spectral properties of small (diameter < 100 km) 

bodies beyond Neptune. (4) Measure cometary volatiles.

Program details: Deep broadband optical/NIR imaging surveys with HDI; optical/NIR 

point-source spectroscopy; UV spatially resolved spectroscopy.

Instrument(s) + Configuration: HDI multi-band imaging; HDI grism spectroscopy or 

ECLIPS spectroscopy (without coronagraph masks); LUMOS multi-object spectroscopy.

Key observation requirements: Tiled imaging with repeats to obtain orbits; non-sidereal 

drift matched to orbital motion; spectral bandpasses 0.5 – 2.5 μm and 100 nm – 200 nm.
LUVOIR The Large UV Optical Infrared Surveyor

4-28 The LUVOIR Interim Report

Table 4.1. Summary science traceability matrix for Chapter 4

Scientific Measurement Requirements Instrument Requirements

Objectives Measurement Observations Instrument Property Value

Explore the 

diversity of 

planetary 

atmospheres

Direct 

measurement 

of exoplanet 

atmosphere 

compositions

Direct spectroscopy 

of exoplanets at 

multiple separations 

from a variety of 

stars; S/N>10

ECLIPS High-contrast 

VIS, NIR 

spectroscopy

Contrast < 

10-10; R ~ 100

Constrain the 

physics of 

atmospheric 

escape

Detection, 

mapping, and 

characterization 

of exoplanet 

outflows

Stellar point-source 

spectroscopy during 

transit for ~100 

planets

LUMOS FUV, NUV 

spectroscopy

G120M,

G150M,

G300M;

R ~ 30,000

Examine 

exoplanet 

atmospheric 

hazes and 

clouds

Haze/cloud 

detection and 

characterization

Direct spectroscopy 

and/or photometry 

of exoplanets at 

multiple separations 

from a variety of 

stars; UV stellar 

spectroscopy

ECLIPS High-contrast 

UV, VIS lowresolution 

spectroscopy 

and/or imaging

Contrast 

<10-10; R ~ 10

LUMOS UV 

spectroscopy

G120M, 

G150M, 

G180M 

G300M;

R ~ 30,000

Determine 

planetary 

system 

architectures

Masses, orbits, 

atmospheric 

composition 

of planets and 

debris belts

Direct imaging 

and spectroscopy; 

precision stellar 

astrometry

ECLIPS High-contrast 

VIS, NIR 

spectroscopy

Contrast 

< 10-8; 

OWA > 48 l/D;

R~100

HDI VIS astrometry < 0.1 mas 

precision

Determine 

morophology 

and 

architecture of 

debris disks

Spatial 

distribution of 

dust in debris 

disks

Direct imaging of 

debris disks around 

solar-type and lowmass stars over a 

range of ages

ECLIPS High-contrast 

VIS imaging

Contrast 

< 10-8;

OWA > 48 λ/D

Measure bulk 

composition 

of extrasolar 

planetesimals

Chemical 

composition 

of gas from 

planetesimals in 

debris disks

Point-source 

spectroscopy of 

stars with edge-on 

debris disks; S/N > 

10

POLLUX / 

LUMOS

UV 

spectroscopy 

from 100–400 

nm

R=65,000–

120,000

Characterize 

the full 

population of 

solar system 

minor bodies

Population 

statistics, 

morphology, 

and 

composition of 

minor bodies

Deep imaging for 

faint object detection 

at S/N > 5, time 

series imaging for 

orbits, spectroscopy 

of minor bodies

HDI Multi-band 

imaging, grism 

spectroscopy

Tiled imaging 

with repeat 

visits; moving 

object tracking 

ECLIPS Open mask 

VIS/NIR 

spectroscopy

R~70 to 200

LUMOS FUV multiobject 

spectroscopy
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 4-29

by providing a new view into the process 

and outcomes of planet formation LUVOIR 

has the potential to revolutionize our understanding of how planetary systems form and 

evolve. Equipped with a flexible, powerful 

observatory designed to characterize exoplanets of all stripes, the next generation of 

exoplanet astronomers will achieve a comprehensive view into the nature of the diversity of planets—particularly including their 

atmospheres—and planetary systems.

References

Agol, E., Steffan, J., Sari, R., Clarkson, W. 

(2005). MNRAS, 359, 567

Apai, D., Cowan, N., Kopparapu, R., et al. 

(2017). arXiv:1708.02821

Bean, J. L. & FINESSE Science Team 

(2017). AAS #229, 301.08

Beichman, C., Benneke, B., Knutson, H., et 

al. (2014). PASP, 126, 1134

Booth, M. Jordán, A., Casassus, S., et al. 

(2016). MNRAS, 460, L10

Borucki, W. J., Koch, D., Basri, G., et al. 

(2010). Science, 327, 977

Catling, D. et al. (2018). Astrobiology, 18, 

709. 

Charbonneau, D., Brown, T. M., Latham, D., 

& Mayor, M. (2000). ApJ, 529, 45

Cowan, N. B., Greene, T., Angerhausen, D., 

et al. (2015). PASP, 127, 311

Dent, W. R. F., Wyatt, M. C., Roberge, A., et 

al. (2014). Science, 343, 1490

Fischer, D. A., Anglada-Escude, G., 

Arriagada, P., et al. (2016). PASP, 

128f6001

Fortier, A., Beck, T., Benz, W., et al. (2014). 

SPIE, 9143, 91432J

Fulton, B. J., Petigura, E. A., Howard, A. W., 

et al. (2017). AJ, 154, 109

Heap, S. R., Lindler, D. J., Lanz, T. M., et al. 

(2000). ApJ, 539, 435

Henry, G. W., Marcy, G. W., Butler, R. P., & 

Vogt, S. S. (2000). ApJ, 529, 41

Howell, S. B., Sobeck, C., Haas, M., et al. 

(2014). PASP, 126, 398

Kenyon, S. J. & Bromley, B. C. (2004). AJ, 

127, 513

Lagrange, A.-M., Bonnefoy, M., Chauvin, 

C., et al. (2010). Science, 329, 57

Lyons, T., Reinhard, C., & Planavsky, N. 

(2014). Nature, 506, 307

Martin, R. G. & Livio, M. (2012). MNRAS, 

425, L6

Malhotra, R. (1993). Nature, 365, 819

Mayor, M. & Queloz, D. (1995). Nature, 378, 

355

Nesvorný, D., Jenniskens, P., Levison, H. F., 

et al. (2010). ApJ, 713, 816

Perryman, M., Hartman, J., Bakos, G. A., & 

Lindegren, L. (2014). ApJ, 797, 14 

Rauer, H., Catala, C., Aerts, C., et al. 

(2014). Exp. Astronomy, 38, 249

Ricker G. R., Winn, J. N., Vanderspek, R., 

et al. (2014). SPIE, 9143, 914320

Skidmore, W., TMT International Science 

Development Teams, & TMT Science 

Advisory Committee (2015). RAA, 15, 

1945

Snellen, I. A. G., deKok, R. J., de Mooij, E. 

J. W., & Albrect, S. (2010a). Nature, 

465, 1049 

Snellen, I. A. G., deMooij, E. J. W., Burrows, 

A. (2010b). A&A, 513, 76

Snellen, I. A. G., de Kok, R. J., le Poole, R., 

et al. (2013). ApJ, 764, 182

Spergel, D., Gehrels, N., Baltay, C., et al. 

(2015). arXiv:1503.03757

Sullivan, P. W., Winn, J. N., BertaThompson, Z. K., et al. (2015). ApJ, 

809, 77 

Winn, J. N. & Fabrycky, D. C. (2015). 

ARA&A, 53, 409
The Large UV Optical Infrared Surveyor LUVOIR

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Chapter 5
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5-2 The LUVOIR Interim Report

5 How do galaxies evolve?

This question is one of the oldest in 

astrophysics, dating from the very first 

realization a century ago that galaxies are 

“island Universes” unto themselves. The 

quest to answer it has motivated some of 

astronomy’s brightest minds to build its 

most ambitious telescopes. Over decades 

of discovery spanning the electromagnetic 

spectrum, astronomers have painted a rich 

picture of how galaxies grow. They begin 

as small seed fluctuations in the largescale structure to assemble “hierarchically” 

into self-bound dark matter halos. These 

potential wells draw baryons together, and 

this gas becomes galaxies and the stars, and 

in some places, life emerges. We know that 

supermassive black holes lurk in the hearts 

of nearly every galaxy, and for brief periods 

might govern their evolution. 

Yet our understanding of the physics 

behind these emergent patterns lags behind 

our ability to characterize them. We can 

model the tree-like hierarchical assembly of 

dark matter, but how baryons populate the 

branches of this tree is mapped out only 

for the largest pieces at late times, and we 

do not know what sets the minimum scale 

for galaxy formation. We do not know why 

some galaxies are dominated by their bulges 

while others have none: is it an effect of the 

hierarchy, gas accretion, or activity from their 

supermassive black holes? We do not know 

how galaxies sustain their star formation for 

much longer than their present gas supply 

allows. We do not know why the most 

massive galaxies cease forming stars at any 

significant level (called “quenching”), and 

stay that way. These questions, and many 

more, comprise the leading edge of galaxy 

formation studies today (Somerville & Davé 

2015; Madau & Dickinson 2014).

Building on today’s state-of-the-art, 

and anticipating developments in the next 

decade, this chapter will pose questions that 

we expect will remain unsolved even in the 

2030s, because they require observations 

at wavelengths, sensitivity, and spatial 

resolution that are well beyond those of 

other foreseeable facilities. We will focus 

on two broad domains of galaxy evolution 

as the “Signature Science” that motivates 

LUVOIR’s UV coverage, high-resolution 

imaging, and multiplexing capabilities, 

and in turn its aperture, wavelength range, 

and instrumentation. To understand “The 

Cycles of Matter,” astronomers using 

LUVOIR will be able to deploy multi-object 

spectroscopy in the ultraviolet, with up to 

50-fold sensitivity gains compared to Hubble 

and the multiplexing ability to observe more 

than a hundred objects at once. To map 

“The Multiscale Assembly of Galaxies,” 

astronomers using LUVOIR will employ 

extremely stable 10 mas imaging with an 

unprecedented combination of depth and 

wavelength coverage.

The Cycles of Matter: How do galaxies 

acquire the gas they use to form stars? How 

do they sustain star formation over billions of 

years when they appear to contain much less 

gas than this requires? How does feedback 

from star formation and active galactic nuclei 

(AGN) expel gas and metals, and to what 

extent is this feedback recycled into later star 

formation? What happens to a galaxy’s gas 

when it quenches? Is it used up, ejected, or 

hidden?

Much of galaxy formation is still hidden 

from view because it involves gas at 

temperatures that are too high and densities 

that are too diffuse to observe directly, or 

physical scales that are too small to resolve. 

All these scales, from < 100 pc to more 

than 1 Mpc, are relevant when we consider 

how gas flows drive galaxy evolution. With 

its uniquely powerful UV capability at high 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 5-3

spatial resolution, LUVOIR will directly 

observe these gas flows to trace the raw 

materials of galaxies, the ejected products 

of star formation, and the recycling between 

them over the latter half of cosmic time.

The Multiscale Assembly of Galaxies:

We know empirically that galaxy formation is 

a multiscale process spanning at least seven 

orders of magnitude in mass and three in size 

that unfolds over the 13 billion year sweep 

of cosmic time. The brightest galaxies we 

know about are 10 billion times brighter than 

the faintest, and the densest are 1 million 

times more crowded than the most diffuse. 

In between these extremes, galaxies follow 

orderly scaling relations between mass, 

size, star formation, and metal content. 

This remarkable regularity spanning a vast 

range of diversity challenges our current 

theories and even our imaginations to 

understand how Nature does it. Part of the 

story is that even the largest galaxies of 

today began as smaller seeds at the dawn 

of time, and gradually built up into massive 

giants by acquiring gas and merging with 

other galaxies. Along this “merger tree” 

path, galaxies grow by accreting gas and 

merging with their neighbors large and small. 

By reaching depths of 33–34th magnitude, 

LUVOIR will detect the early building blocks 

of galaxies like the Milky Way, and fill out 

the merger tree that leads to galaxies at the 

present time. It will also “see inside galaxies” 

to unprecedented limits, resolving their 

internal building blocks at < 100 pc scales 

to unravel the processes inside galaxies 

that drive their evolution. All these themes 

motivate and exploit LUVOIR’s unique spatial 

resolution and broad wavelength coverage. 

The “Signature Science” cases here 

represent some of the most compelling 

types of observations astronomers might do 

with LUVOIR at the limits of its performance. 

But, compelling as they are, they should 

not be taken as a complete specification of 

the LUVOIR program. We have developed 

concrete examples to ensure that the nominal 

design can do this compelling science, and 

so that the astronomical community has 

detailed examples they can adapt to their 

own interests. We expect that the creativity of 

the community, harnessing the revolutionary 

capabilities of the observatory, will ask 

questions, acquire data, and solve problems 

that we cannot envision today. That is as it 

should be, as a flagship should always reach 

far beyond current capabilities and break the 

current limits of our imagination. With this 

in mind, let us ask vital science questions 

that we believe will remain unsolved until 

LUVOIR, and consider how they drive the 

requirements for its aperture, resolution, and 

wavelength coverage. We hope these cases 

will stimulate the reader to imagine their own 

science cases and to begin applying these 

tools to design their own future.

5.1 Signature science case #1: 

The cycles of matter

Much of the still-unknown story of how 

galaxies evolve comes down to how they 

acquire, use, and recycle their gas. Inflows 

and outflows of gas likely shape the 

evolution of star formation within a galaxy. 

Inflows ultimately arise from the intergalactic 

medium (IGM) to provide fuel for continued 

star formation over the lifetime of a galaxy. 

Outflows from the interstellar medium, driven 

by stellar radiation and explosions or by 

AGN, or some combination of them, keep 

galaxies from forming too many stars in their 

central regions. The balance of these flows 

set the rate at which galaxies accumulate 

heavy elements.

All these flows meet in the circumgalactic 

medium (CGM), an extremely diffuse 

gaseous medium spanning roughly 30 times 

the radius and 10,000 times the volume of 
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5-4 The LUVOIR Interim Report

the visible stellar disk (Figure 5.1; Tumlinson, 

Peeples, & Werk 2017), made up of clouds, 

streams, and bubbles embodying galactic 

accretion, feedback, and recycling. We 

State of the Field in the 2030s

LUVOIR’s science aims must be considered in light of progress that is expected between 

now and its launch. What will the field of galaxy formation look like after the advances of 

the 2020s, when we will have operational JWST, ALMA, and one or more extremely large 

telescopes on the ground? Of course, we cannot know what surprises Nature will supply, 

but experience teaches that new facilities break new ground and are forced by competitive 

peer review to address the most demanding problems of their time. 

First, we can expect that essentially the entire sky will have been surveyed at seeinglimited resolution in the optical bands, e.g., Sloan Digital Sky Survey (SDSS), PanSTARRS, 

and the Large Synoptic Survey Telescope (LSST). LSST’s accumulated co-add will reach 

~28th magnitude in the optical over 10 years, with 0.8–1" resolution. These maps will 

be joined by the massive fiber-based spectroscopic surveys that started with SDSS and 

the 2dF survey and will expand deeper and wider with the Subaru PFS survey and its 

contemporaries. All-sky imaging provides accurate photometry, and spectroscopy provides 

additional physical diagnostics of stellar population age and metallicity, and galactic dust 

content. These vast datasets support rich multivariate analyses with statistical precision 

and excel at determining galaxy population statistics and galaxy/galaxy correlations in 

large-scale structure and halo substructure. After these surveys, there might be very little 

about the large-scale distribution of galaxies that remains to be learned. These massive 

spectroscopic surveys also detect millions of strong gas absorbers, such Mg II and Ca 

II lines, which probe dense interstellar medium (ISM) and CGM gas. However, as we will 

show, they cannot access the key UV physical diagnostics over most of cosmic time. 

The other major theme of the 2020s will be high-resolution imaging in the IR, sub-millimeter, 

and radio. JWST and WFIRST will be the prime space observatories, bringing 50–100 mas 

resolution to imaging and multi-object spectroscopy. WFIRST will provide Hubble-quality 

imaging to a large sky area, and JWST will detect galaxies to AB ~ 32. JWST is optimized 

to reach “the first galaxies,” seeing the first seeds of modern galaxies at z > 15. It will also 

be revolutionary in its mid-IR capability, able to observe the rise of dust, ice, and molecules 

in galaxies over most of cosmic time. Long-wavelength facilities such as ALMA and SKA 

will bring their powers to bear on the neutral and molecular gas content of galaxies at 

high resolution. Finally, 20–30 m telescopes on the ground will, if successful with adaptive 

optics, bring < 50 mas imaging and spectroscopy at IR wavelengths, in a comparable 

resolution and depth space as JWST. 

What observational parameter space remains after all this? Space observatories are the 

platform of choice for high-resolution optical imaging, UV imaging and spectroscopy, highly 

repeatable precision photometry, ultrastable astrometry, and high-performance optical 

coronagraphy. LUVOIR’s “Signature Science” motivates and uses these capabilities, while 

acknowledging the areas where ground-based telescopes or space facilities at other 

wavelengths do their best. And in the end, it is the complementarity and collaboration 

between these domains that drives the science forward.
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The LUVOIR Interim Report 5-5

now understand that the CGM has a total 

mass comparable to the stellar masses of 

galaxies and is richly structured in density, 

temperature, and chemical enrichment. No 

picture of galaxy formation and evolution is 

complete without understanding this major 

galactic component. 

To organize our discussion, we will 

proceed into galaxies and back out again, 

starting from the IGM and how it moves 

into the CGM as galactic fuel. From there 

we will consider the flows within the CGM 

and how gas from inside it and outside it 

are processed and recycled. Finally we will 

consider the disk/halo interface, where 

accretion becomes the ISM and the ISM 

becomes feedback. At each stage we will 

see how astronomers using LUVOIR could 

make definitive measurements of processes 

that have been hidden from view. First, we 

must stop to investigate why observing these 

processes makes LUVOIR’s UV sensitivity so 

essential to its design.

5.1.1 The essential ultraviolet

Life as we know it is acutely sensitive to 

energetic radiation from our Sun: UV photons 

cause rampant cellular damage and genetic 

mutations. It is possible that life would not 

exist were it not for the protective ozone layer 

that keeps these harmful photons out. We 

can have life on Earth, or UV astronomy from 

Figure 5.1. Galaxies are much more than they appear to be in starlight. Normal galaxies are 

surrounded by a massive reservoir of diffuse gas that acts as their fuel tank, waste dump, and 

recycling center: the circumgalactic medium. It is fed by accretion out of the cosmic web and 

by outflows from the galaxy. Recent evidence indicates that the CGM is a major factor in galaxy 

evolution. Unraveling the CGM and all its associated flows is major driving force for LUVOIR and 

its instruments. Adapted from Tumlinson, Peeples, & Werk (2017).
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5-6 The LUVOIR Interim Report

the ground, but it seems we cannot have 

both. In this state of affairs, we must place 

our telescopes in space if we are to make a 

full characterization of the gas in halos and 

the IGM over the last 10 Gyr of cosmic time.

Figure 5.2 shows that the ability to cover 

UV wavelengths is essential to explore the full 

range of temperature and density where we 

can find the CGM and IGM baryons—most 

of the normal matter in the universe. In the 

panel at lower left, the greyscale intensity map 

shows the distribution of gas in the halo of a 

simulated L* galaxy from the EAGLE project 

(Oppenheimer et al. 2016). This gas is all in 

the CGM of the simulated galaxy, yet it exists 

in multiple “phases” from 1 million K and 

densities near the cosmic mean (~10–5-10-6

cm-3) to denser gas close to the galaxy’s disk 

at 104 K and log nH > -1. In these conditions, 

the hydrogen and metals in the gas will exist 

in high states of ionization: 1% or less of the 

H may be neutral, while common metals 

like C, N, O, S, Mg, and Fe may be ionized 

multiple time to the stages indicated on the 

figure (e.g., O VI or O+5). In these conditions, 

the quantum mechanical rules of energy 

transfer dictate that the gas will emit and 

absorb energy primarily at UV wavelengths. 

Detailed calculations indicate that 60–80% 

of photons emitted and absorbed by diffuse 

CGM gas are UV photons (Bertone et al. 

2013). These lines appear in the upper right 

Figure 5.2. Diffuse gas surrounding galaxies requires UV capability for most of cosmic time. The 

greyscale image below shows that most CGM gas lies at T > 105 K, where atomic physics places 

its absorption and emission in the rest-UV. The colored squares mark diagnostic ions used to 

study this gas, placed at the temperature/density combination where they are strongest.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 5-7

panel where their observed wavelengths are 

plotted versus cosmic lookback time. We 

conclude that the UV species that best trace 

the temperature and density regime of diffuse 

galactic gas flows lie in the UV over the last 10 

Gyr of cosmic time. This inescapable piece 

of physics means that, if we are interested 

in resolving questions about how galaxies 

acquire, process, eject, and recycle their 

gas, over the last 10 Gyr of cosmic time, 

we need access to UV wavelengths that are 

observable only from space.

5.1.2 Gas flows in absorption: More 

sources, deeper limits, earlier times

The IGM and the CGM—the lowest density 

matter in the cosmos—are intrinsically difficult 

to study. Astronomers have spent decades 

trying to grasp the nature and impact of this 

material, with modest success. Absorptionline spectroscopy has been the primary tool 

of choice, because it excels at detecting 

diffuse gas down to the very lowest densities 

(even lower than the cosmic mean density), 

especially if those regions occupy large 

regions of space. Absorption lines can be 

detected in gas spanning a huge range of 

temperature and densities, providing a lever 

arm on physical diagnostics for almost all 

relevant phases of the gas.

The availability of background sources 

sets a critical limit on the use of absorption 

lines as probes of diffuse gas. The most 

throughput-optimized UV spectrograph 

at the moment is Hubble’s Cosmic Origins 

Spectrograph (COS), which can perform high 

S/N spectroscopy of sources only down to 

about 18th magnitude (AB mag in the FUV). 

There are only about 1000 such objects 

on the entire sky (Figure 5.3), making UV 

spectroscopy of intervening gas a game 

of choosing sources carefully to yield the 

desired population of foreground galaxies. 

Using this technique, Hubble users have 

explored modest samples of mainstream 

L* galaxies (N = 42; Tumlinson et al. 2013), 

dwarf galaxies (N = 40; Bordoloi et al. 2014), 

radio-selected star forming disks (N = 45; 

Borthakur et al. 2015), galaxies in dense 

environments (Burchett et al. 2017; Johnson 

et al. 2016) and galaxies hosting AGN 

(N = 13; Berg et al. 2018). As a result of the 

paucity of sources, the quasar absorption 

lines (QSOALS) method can make only crude 

Figure 5.3. Left: Distribution in redshift for SDSS quasars of various GALEX FUV magnitudes. 

For S/N = 20 spectroscopy, Hubble/COS is limited to the objects in the red wedge, which barely 

reaches past z ~ 0.7. With its 3 mag deeper grasp, LUVOIR/LUMOS users will have many more 

QSOs to choose from, with enough in the z = 1–2 interval to finally build precise maps of how gas 

feeds galaxies at that epoch. Right: LUVOIR+LUMOS S/N for a flat-spectrum FUV = 19 source in 

one hour of integration for each of the G120, G150, G180 and G300M gratings (R = 30–40,000). 

The LUVOIR limits were derived from the LUMOS ETC available at luvoir.stsci.edu .
LUVOIR The Large UV Optical Infrared Surveyor

5-8 The LUVOIR Interim Report

statistical maps by sampling many halos with 

one absorption-line path through each and 

then aggregating the results. The upshot is 

a set of essentially one-dimensional maps, 

with only a few forays beyond, and, with 

Hubble, limited mainly to galaxies at z < 0.5.

The LUVOIR Ultraviolet MultiObject 

Spectrograph (LUMOS) is designed for 

point-source spectroscopy 30–50 times 

more sensitive than Hubble/COS, at about 

double its highest resolution (R ~ 40,000). 

LUMOS users will be able to choose from 

any of the quasars counted in Figure 5.3, 

including thousands of choice objects at z 

> 1. This is a critical time in the history of 

the universe: star formation rates begin 

declining from their z ~ 2 peak, AGN have 

passed their epoch of maximum activity and 

are turning off, the “red sequence” of passive 

galaxies is beginning to emerge, and the 

first large concentrations that we call galaxy 

clusters are about to mature. To understand 

the almost totally unknown role of IGM and 

CGM gas at this epoch, we must be able 

to observe tens or even hundreds of QSO/

galaxy pairs. The areal density for such pairs 

is non-linear with magnitude (and so with 

aperture), given the QSO and galaxy UV 

luminosity functions. With a 15-m aperture, 

roughly 1% of all L* galaxies at z ~ 0.2 will 

have close-separation QSOs for which highS/N, high-resolution spectroscopy is feasible 

in an hour. For higher-redshift galaxies at z ~ 

0.7, the number is lower by a factor of 10, 

but that still gives access to a respectable 

1/1000 of all L*-type galaxies. With such 

large samples, LUMOS users will be able 

to address a wide range of critical open 

questions about the flows of matter.

Where are the missing baryons? 

Normal galaxies possess only a few percent 

of their expected budget of baryonic 

matter (Fukugita, Hogan, & Peebles 1998; 

McGaugh 2005). A large mass fraction exists 

in the CGM, but this measurement has so 

far been performed only near L* galaxies 

and only at z < 0.2 (Werk et al. 2014). The 

wavelengths accessible to LUMOS provide 

coverage to an extensive range of rest-frame 

extreme-UV ions that redshifts into the FUV 

for z > 0.5 (Tripp 2013). This includes nearly 

every ionization state of the most abundant 

heavy element, oxygen (O I though O VI). 

Performing a baryon census in which most 

of the gas is traced by a single element 

will eliminate some of the most serious 

systematic errors in ionization models that 

plague these measurements. The remaining 

oxygen ions—O VII and O VIII—reside in very 

low density, high temperature gas, generally 

accessed today through a small number of 

X-ray sightline in which megaseconds of 

Chandra or XMM time have been invested 

(the lines lie at ~20 Å in the rest-frame). 

Fortunately, other ions visible in the UV at z 

~ 1 probe that same regime. One example 

Program at a Glance – LUVOIR QSOALS Key Program (Figures 5.3 and 5.4) 

Program details: 100 QSOs at z = 1–1.5, approximately 190 hours of exposure time. 

Instrument(s) + Configuration: LUMOS FUV and NUV spectroscopy, R ~ 30–40,000, 

G120M, G150M, G180M, G300M. One hour per grating. 

Key observation requirements: S/N = 20 or higher from 1100 to 4000 Å (blue curve in 

Figure 5.3)

Synergies with other ground/space facilities: 30-m+ ground-based optical/IR 

telescopes obtain data on associated galaxies. ALMA and radio telescopes can constrain 

their ISM gas and metal content. 
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The LUVOIR Interim Report 5-9

is Ne VIII (λλ 775,780 Å), which to date has 

only been observed a handful of times with 

Hubble owing to the lack of bright sources at 

z > 0.5. As shown in Figure 5.3 and Figure 

5.4, LUVOIR can not only provide spectra 

with S/N = 10 or greater in an hour, it can 

do so for thousands of sources, with a total 

available Ne VIII redshift path, for single 

observations of one hour per sightline, more 

than a factor of 100 larger than that available 

to a 4-meter aperture with a LUMOS-like 

throughput. A program targeting Ne VIII 

would also search for T > 106 K gas via Mg 

IX, Si XII, S XIV, and Fe XVI, in effect making 

LUVOIR a powerful X-ray telescope, while 

retaining access to all the lower-ionization 

diagnostics in Figure 5.2. Every sightline at 

z ~ 1 thus offers an opportunity to observe 

baryons across the full spectrum of their 

phase space, from neutral, colder gas (e.g., 

O I, H I), through to warm, ionized gas (e.g., 

O VI), as shown in Figure 5.2. This Signature 

Science forms a synergy with ground-based 

telescopes, which are needed to measure 

the redshifts, masses, star formation rates, 

and metallicities of the associated galaxies. 

As Hubble and Keck have cooperated to 

advance this field in the 2020s, so can 

LUVOIR and the next generation of extremely 

large telescopes on the ground.

Follow the metals to map feedback. 

Heavy elements are “Nature’s tracer 

particles”—the equivalent of a message in 

a bottle that tells us where the products of 

star formation have been carried over time. 

These messages spread far and wide, clearly 

showing that some galaxies have transported 

their metals over vast distances—even 

megaparsecs—away from their birthplace. If 

we can piece together all the messages in 

all the bottles from all the island universes, 

we should come to understand how galactic 

feedback actually works.

Hubble’s UV capability has extensively 

probed the extent of metals around galaxies, 

culminating in the finding that only 20% 

of all metals produced are still retained in 

the galaxies that made them (Peeples et 

al. 2014). The rest are elsewhere, many in 

the CGM. It is striking how wide a range 

of metallicity is seen in normal galaxy halo 

gas at z < 1, with ~50% of the gas having 

metallicities around 5% Solar, while the other 

half is roughly Solar (Lehner et al. 2013; 

Wotta et al. 2016). The lowest-metallicity 

CGM gas may be associated with infalling 

material, either from the IGM or accreting 

from a hot corona, while the higher metallicity 

gas could be ongoing enriched feedback. 

Extensive work with numerical simulations 

indicates that the physical extent of metals 

around galaxies, and their relative mixtures 

in the phases of the gas, can provide 

critical tests of poorly understood feedback 

models. Understanding the origins of this 

gas, whether it will collapse onto the galaxy 

or will be subsumed back into the corona, 

is critical to understanding how and from 

where galaxies replenish their fuel supply for 

Figure 5.4. Redshift search path to Ne VIII 

absorption for various apertures assuming a 

LUMOS throughput. Observations of 1 hour 

each are assumed, with only those pixels at 

S/N > 10 contributing to the path. Sources are 

drawn from the GALEX catalog having FUV 

magnitudes AB < 22 mag, cross-matched with 

SDSS DR7 for redshifts. The total path available 

from this sample is shown in black. 
LUVOIR The Large UV Optical Infrared Surveyor

5-10 The LUVOIR Interim Report

forming stars. Or, indeed, whether they are 

able to do so at all.

LUVOIR will allow us to measure the 

prevalence of low-metallicity gas about 

galaxies of different masses, star-formation 

rates, and environments using UV absorption 

spectroscopy. This is currently not feasible 

now, because of the relative low density on 

the sky of UV-bright sources accessible to 

Hubble’s spectrographs. Moreover, LUVOIR 

will be able to do this over the critical z = 0.5–

1.5 epoch of galaxy formation. Performing a 

census of metals over this redshift range can 

exploit the simultaneous coverage of almost 

every UV ion of oxygen, sidestepping any 

systematic problems with using C, Si, O, and 

N variously, as is done at z < 0.2. This is only 

possible with UV coverage and redshift that 

places UV lines of O II, O III, and O IV into the 

space UV (Tripp 2013).

Using this capability, LUVOIR’s users 

can complete the low-z metals census, 

constrain feedback from galaxies, assess 

the importance of galactic recycling, and 

ultimately tell an important early part of 

the story in the rise of the elements and 

life. Another critical example is shown in 

Figure 5.5. AGN feedback is thought to 

play an important role in keeping hot halos 

from cooling in quiescent galaxies, though 

it is difficult to probe the jet and radiation 

interactions with the surrounding CGM gas. 

Increasing the density of sources to which 

we have access with UV spectroscopy will 

allow us to probe that interaction directly. 

The key needs for such science are high 

spectral resolution and sensitivity (aperture) 

to provide high enough source density that 

individual AGN hosts can be studied.

Precision cosmology with the IGM:

Though it lies outside galaxy halos, the IGM 

as traced by the Lyman alpha forest provides 

unique constraints on cosmological structure 

formation. As shown in Figure 5.3, the high 

Figure 5.5. A realization of the sky about the radio galaxy Cen A demonstrating the density of 

sources available for high-resolution, high-S/N spectroscopy against background QSOs behind 

low-redshift galaxies assuming different telescope apertures. Such observations are critical for 

understanding the driving of jets and their interaction with the CGM, which may dictate the 

rate at which CGM gas could cool to fuel further star formation. Each star represents a QSO 

down to FUV = 22 mag that could be observed to S/N > 10 in ~0.5–1 hour assuming a LUMOS 

spectrograph. A 9-m telescope has a source density smaller by ~5x than that of the 15-m. The 

nearest QSO for which Hubble spectroscopy is achievable in <20 orbits is at impact parameter 

R = 300 kpc, two times further from the core of this galaxy than the circle representing R = 150 

kpc. Background galaxies, which can also be used to map the gas, are ten times more numerous. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 5-11

throughput of the G300M grating on LUMOS 

provides S/N > 30 coverage of the forest in 

a single hour. To date, less than five quasars 

have been observed covering the forest at S/N 

~ 10 or greater at z ~ 1, at the cost of nearly 

100 Hubble orbits. The baryon census and 

feedback studies described above require 

access to redshifted Lyman alpha (HI 1215), 

so a program targeting metal absorption 

obtains unprecedented Lyman alpha forest 

“for free.” A LUVOIR program observing 100 

quasar sight lines at S/N > 30 would perform 

three major measurements: (1) constrain the 

matter power spectrum P(k) to better than 

5% accuracy on scales of 0.1 to 100 Mpc at 

z ~ 1, bringing precision cosmology to a new 

epoch in time; (2) measure the H I column 

density distribution function f(NHI), which at 

the low H I end measures the IGM, but at 

the intermediate and high HI end directly 

maps onto the CGM and ISM of galaxies; (3) 

measure the equation of state of the IGM, 

T=T0(ρ/ρavg)γ-1, to accuracies approaching 

10%. The thermal history of the universe 

bears the fingerprints of major heating events 

such as reionization of Hydrogen and Helium 

and the influence of the Hubble expansion, 

but also can be used to trace the influence of 

other hydrodynamic processes more closely 

linked to galaxy formation and evolution.

A comprehensive LUMOS QSOALS 

campaign: All of these studies of CGM 

missing baryons, metals, and the IGM can 

be carried out with a single, comprehensive 

QSO survey. For definiteness, and for scaling 

to other architectures, we scope this to use 

the 100 brightest QSOs identified in the SDSS 

DR7 QSO catalog, using matched GALEX 

FUV fluxes. We set the S/N goals such that 

1 hour in each of the LUMOS High resolution 

gratings will yield the blue curve in the upper 

right of Figure 5.3 for an FUV = 19 mag QSO. 

For brighter objects, we scale the exposures 

by FUV magnitude to maintain the same 

S/N. This survey of 100 QSOs will require 

just under 200 hours of integration time 

(and, perhaps ~300 hours with overheads). 

Spectroscopy for 100 background quasars 

at z~1 represents a fundamentally game 

changing prospect for the study of the 

formation and evolution of the baryons in, 

around, and in-between galaxies and the 

larger cosmological context galaxies live in. 

No mission current or planned other than 

LUVOIR could complete such a program in 

a treasury scale program allocation of time. 

Any claim of understanding the history of 

baryons in the universe demands a study of 

this epoch of transformation in cosmic time, 

and LUVOIR rises to the challenge.

5.1.3 Mapping the cycles of matter 

with the faintest light in the universe

Quasar absorption lines are optimal for 

detecting and characterizing the IGM and 

CGM without regard to gas density, but 

they are limited in what they can teach us 

because they reveal little about the 3D 

distribution of the absorbing gas. To improve 

this situation, we must attempt to “take 

a picture” of a galaxy’s gas flows in two 

dimensions, using emission from the gas 

itself that reveals its density, temperature, 

metallicity, and kinematics. With its millions of 

~0.1" apertures, customizable to nearly any 

source, LUMOS will be able to probe physical 

processes—shocks, accretion, ejecta, and 

recycling—at scales that even simulations 

today can barely reach. This capability will 

enable LUVOIR’s users to investigate smallscale structures within the CGM, especially 

in those vital regions where outflows meet 

inflows and gas is being recycled. We will 

consider two specific applications of this 

capability: large maps of small-scale gas 

dynamics in nearby galaxies, and detection 

of the extremely diffuse CGM gas that fills 

halos.

Ground-based IFU spectrographs such 

as MUSE at VLT and KCWI at Keck are 
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5-12 The LUVOIR Interim Report

pioneering the search for CGM gas emission 

at z > 2, where the relevant diagnostic lines 

pass into the visible bands (Figure 5.2). Gas 

reservoirs extending over hundreds of kpc 

appear to be illuminated by radiation from the 

stars and AGN in the galaxies (Cantalupo et 

al. 2014) and some structure can be resolved 

(Martin et al. 2016). These instruments can 

access some of the important UV diagnostic 

lines, most readily Lyα, but only at high 

redshift. They are also limited still to relatively 

low spatial resolution, which translates to 

physical scales of > 10 kpc. They are limited 

in sensitivity by bright and time-variable sky 

backgrounds that will make detection of the 

much weaker lines of key metals exceedingly 

difficult in individual halos.

LUMOS will bring at least three unique 

abilities—UV coverage, high spatial 

resolution, and extreme sensitivity—to bear 

on this important problem, in ways that 

ground-based telescopes cannot. With this 

instrument, astronomers will be able to map 

the density, temperature, and mass flow rates 

of the CGM, directly, using the UV radiation 

emitted by CGM gas as it cycles in and 

out of galaxies. Figure 5.6 shows how the 

LUMOS microshutters map to small-scale 

(< 1 kpc) clouds that carry gas and metals 

away from the prototypical starburst galaxy, 

Figure 5.6. Two examples of employing LUVOIR / LUMOS to examine the small-scale physics of 

galactic outflows. At left, X-ray and optical images of the M82 starburst galaxy are then zoomed 

in to the false color inset, where one microshutter of 0.14x0.086” matches well to the bright 

orange clumps. The full LUMOS field of 2x3 microshutter arrays could be used to tile the full 

scale of the outflow. At right, we use the zoomed in region from at left to show how these 

clumps of interacting gas can be mapped at the level of individual shutters, offering access to the 

appropriate physical scales with unique UV diagnostics and low sky backgrounds.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 5-13

M82. Four separate footprints of the 2 x 3–

array microshutter field of view are overlaid in 

light green on the galaxy and its outflow. The 

false color zoom shows small clumps that 

trace the interaction of the galaxy’s outflow 

with the CGM. Did these clumps cool and 

form where they are? Or is this material 

directly ejected? If so, what does it imply 

about the mass ejection rate, and the mass 

rate of recycling? This is galactic feedback in 

action, but only at UV wavelengths can we 

prove the relevant energy scales to answer 

these questions. Multiply this small region 

many-fold, and it becomes clear that we 

must be able to observe hundreds of such 

places in this complex flow to understand 

its true dynamics. At the distance of M82 

and other nearby galaxies, the 0.14 x 0.07" 

shutters subtend parsec-scale sizes and so 

may be ganged together into larger “virtual 

apertures” to map the faint light emitted by 

gas entering and leaving galaxies, resolving 

these complex flows at sub-kiloparsec scales. 

At z < 2, where the relevant diagnostics are 

still in the UV, each shutter still reaches subkpc scales, as shown by a rescaling of the 

M82 outflow clumps in the right panel of 

Figure 5.6. The ability to resolve gas flows 

at parsec to kiloparsec scales in the key UV 

diagnostics lines is a unique ability of a large 

UV-sensitive space telescope.

Using the same array of shutters binned 

into larger “virtual apertures,” LUVOIR can 

also seek the extremely faint emission from 

the widely distributed diffuse CGM and 

structure within it. Figure 5.7 shows such a 

Program at a Glance – High-Definition View of a Local Galaxy CGM (Figure 5.6) 

Program details: 30 QSOs within 2.2 deg of Cen A (R < 150 kpc at 3.86 Mpc).

Instrument + Configuration: LUMOS FUV and NUV spectroscopy, G120M, G150M, 

G300M. 

Key observation requirements: S/N = 10 at O VI (G120M), 20 at Lyα, S/N = 10 at C 

IV (G150M), S/N = 10 at Mg II (G300M), for a total of approximately 100 total hours of 

integration. 

Synergies with other ground/space: Radio and X-ray telescopes trace the cold and 

hot components of the outflow, while the UV traces warm gas and determines accurate 

kinematics.

Program at a Glance – Emission Line Maps 

of Nearby Galaxy Gas Flows (Figure 5.6) 

Program details: Observe M82 outflows. 10 multi-shutter array tiles, 100 objects / tile, 

10 hours per tile, 100 hours. Exact shutter placement determined from LUMOS UV preimaging. Total ~100 hr.

Instrument(s) + Configuration: LUMOS FUV and NUV spectroscopy, R ~ 30-40,000, 

G120M, G150M, G180M, G300M

Key observation requirements: S/N ~ 5 or higher in key lines, with exposure times 

tuned to achieve this in multi-shutter apertures of varying size. 

Synergies with other ground/space facilities: Ground-based (Ha) or LUMOS UV maps 

can provide additional diagnostics; radio detections might be achieved for dense clumps 

in nearby galaxies. 
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hypothetical map from a new hydrodynamical 

simulation of a Milky Way progenitor galaxy 

at z = 1 (FOGGIE; Corlies et al. 2018). This 

diffuse gas will be challenging to detect even 

for LUVOIR, but appropriate integration times 

could be fitted in as parallels to the weeklong exoplanet visits, for example. In deep 

LUMOS exposures, the structure of the CGM 

can be detected in multiple lines, allowing us 

to count up the heavy element content of this 

gas, to watch the flows as they are ejected 

and recycled, and to witness their fate when 

galaxies quench their star formation, all as a 

function of galaxy type and evolutionary state. 

LUVOIR could map galaxies in fields where 

deep imaging identifies filaments in the largescale structure, and where ground-based 

ELTs have made deep redshift surveys to 

pinpoint the galactic structures and sources 

of metals to be seen in the CGM. Because this 

radiation is far weaker than local foreground 

radiation (SB ~ 100–1000 photons cm-2 s-1 

sr-1) ground-based telescopes seeking it at 

redshifts where it appears in the visible (z > 

2) must perform extremely demanding sky 

foreground subtraction to reveal the faint 

underlying signal. These foregrounds are 

considerably lower from space (by factors 

of 10–100), shortening required exposure 

times by an equivalent factor. Using this 

technique and binning up 0.5–1" regions of 

the microshutter array (a few kpc at z < 2), 

Figure 5.7. A rendering of simulated CGM emission in three key UV lines for a Milky-Way 

progenitor galaxy at z = 1. These lines range from 1000-1200 Å in the rest frame, so they are 

uniquely visible in the UV at z < 2, or half of cosmic time. The 15-m LUVOIR+LUMOS will directly 

detect bright emission (blue/dark green regions) and resolve bright knots in the light green regions. 

Significantly smaller telescope apertures will lack the sensitivity for all but the brightest knots, and 

will not resolve them even if they are detected. Credit: L. Corlies (JHU).
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The LUVOIR Interim Report 5-15

LUMOS users can examine the large scale 

distribution of the filaments and extended 

disks they feed, from the peak of cosmic star 

formation down to the present.

When put together, the three essential 

ingredients of UV coverage, spatial 

resolution, and extreme sensitivity will enable 

mapping of gas flows through the CGM in 

its rich detail. LUVOIR accesses the key 

lines at z < 2, a 10 Gyr era encompassing 

the peak of cosmic star formation, the rise 

of the brightest quasars, the widespread 

quenching of massive galaxies, and the 

emergence of galaxies as we know them 

today. Only an optimized UV mission such 

as LUVOIR has the sensitivity and resolution 

to apply these powerful diagnostics to this 

cosmic era when so much happens. 

5.1.4 Gas flows and quenching

Galaxy quenching is a prime target for 

LUVOIR’s unique power. How galaxies 

quench, and remain so, is a major open 

question. The number density of passive 

galaxies has increased 10-fold over the 10 

billion year interval since z ~ 2 (Brammer et 

al. 2011). Galaxies undergoing quenching are 

the ideal laboratories to study the feedback 

that all galaxies experience: the galactic 

superwinds driven by supernovae and stellar 

radiation, the hot plasma ejected by black 

holes lurking in galactic centers, and the 

violent mergers that transform galaxy shapes 

while triggering the consumption or ejection 

of pre-existing gas. LUVOIR will have the 

collecting area to support deep, wide-field 

UV multi-object spectrograph searches for 

CGM gas at the line emission fluxes that are 

expected, and with the spatial resolution to 

observe the transformation of star forming 

disks to passive spheroids at 50–100 pc 

spatial resolution and closely examine the 

influence of AGN on this process. For galaxies 

identified as quenching, emission maps of 

the surrounding CGM will determine the fate 

of the gas that galaxies must consume or 

eject and powerfully elucidate the physical 

mechanisms that trigger and then maintain 

quenching. Only a diffraction-limited space 

telescope with an aperture of at least 10–12 

meters can achieve such spatial resolution 

in the optical and observe the rest-frame UV 

light necessary to witness the co-evolution 

of stars and gas in galaxies undergoing this 

transition. As most of the development of the 

present-day red sequence occurred since z 

~ 2, and the key diagnostics are rest-frame 

UV lines, this critical problem is a unique and 

compelling driver for LUVOIR’s aperture, UV 

coverage, and capabilities for multi-object 

spectroscopy.

5.1.5 Mapping inflow and outflow 

with down-the-barrel spectroscopy

We can now consider the final steps in our 

view of galactic gas flows with LUVOIR: 

how CGM gas turns into ISM, and how ISM 

gas returns to the CGM. What drives the 

flows that transport mass from the CGM 

into galaxies, and then back? The inflows 

are driven primarily by gravity and cooling 

acting on and within gas that enters the halo 

in filaments, strips off satellites, or becomes 

thermally unstable while orbiting in the halo. 

Gas cooling and gravity are straightforward to 

implement in models and simulations, even if 

the emergent behavior they create is hard to 

simulate. “Feedback”—the general term for 

mass, momentum, and energy from stars 

and AGN that influence the course of galactic 

evolution—is far more complex, because 

the underlying physical mechanisms span a 

huge range of density, temperature, energy, 

and physical scales. Feedback flows such 

as supernovae and AGN winds originate on 

parsec scales, but propagate to hundreds 

of kiloparsecs while interacting energetically, 

hydrodynamically, and radiatively with 
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5-16 The LUVOIR Interim Report

everything they encounter, including current 

inflows and past generations of outflows.

Understanding how feedback operates 

physically to influence galaxies as they grow 

is an active and abiding challenge in the 

astrophysics of galaxies. Theoretical models 

of how flows develop and propagate for 

various sources and physical mechanisms, 

including winds and radiation pressure from 

main-sequence OB stars, the collective 

effects of correlated supernovae, and jets and 

winds from AGN. At their best these models 

make specific predictions for the mass and 

energy transport, velocity and acceleration 

profiles, and time evolution of these flows as a 

function of the source properties (e.g., Murray 

et al. 2011, Thompson et al. 2011). These 

trends are often then implemented as “subgrid” prescriptions in large-scale numerical 

simulations that attempt to recover realistic 

galaxy populations and internal properties. 

These prescriptions are labeled “sub-grid” 

because they occur at sub-kiloparsec scales 

that cosmological simulations still cannot 

resolve.

Observers can test these physical 

models of feedback, but with a major 

limitation. Some bulk properties of galactic 

outflows can be revealed by spectroscopy 

that uses the driving sources themselves—

whether AGN or star-forming regions—as 

background sources, in a so called “downthe-barrel” spectrum. Inflow is detected as 

redshifted absorption and/or emission, while 

outflow is blueshifted. Often the observed 

profiles include absorption and emission 

from separate portions of the gas, which 

must be teased apart to reveal the details 

of the flow. The major limitation of current 

“down-the-barrel” measurements is that 

they typically cover most or all of a galaxy’s 

disk, so the observed profiles average 

over a large number of individual sources, 

erasing the source-by-source variations in 

energy and mass that trace the key physical 

variables. Nevertheless, this technique has 

successfully demonstrated a correlation 

between gas velocity, star formation rates, 

and star formation surface density (Martin 

2005) and star formation surface density 

(Kornei et al. 2012), with galaxies with higher 

star formation rates exhibiting higher outflow 

velocities. These signatures get stronger 

as the sightline approaches the galaxy’s 

semi-minor axis, suggesting that the flows 

are biconical in shape and emerge up out 

of the disk (Bordoloi et al. 2014), as also 

seen in hydrodynamical simulations. Downthe-barrel measurements of small “greenpea galaxies” have examined the escape 

of ionizing radiation from their star-forming 

regions using UV/optical line ratios (Henry et 

al. 2015).

To resolve the physics at scales closer 

to the actual sources, astronomers using 

Hubble’s COS instrument are making 

pioneering measurements of 16 individual 

clusters in the face-on nearby galaxy M83 (D 

= 4.6 Mpc). This program (Program 14681, 

PI Aloisi) is observing 16 UV-bright clusters 

across the face of M83, to map out the 

gas flows emerging from them individually. 

This program requires 40 orbits to execute, 

so doing 10 times as many clusters or a 

few galaxies would be a large or very large 

allocation of Hubble time.

Figure 5.8 shows the potential of the 

LUMOS multi-object mode to transcend 

these limitations and fully resolve these 

outflows. In particular, LUMOS could probe 

the launch-points for winds across the face 

of a spiral galaxy by observing its individual 

OB associations, providing a measure of the 

energy injection needs for launching winds. 

This would resolve the flows at the physical 

scales of individual star-forming regions, 

allowing us to correlate outflow properties 

with their driving sources in detail, rather 
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The LUVOIR Interim Report 5-17

than averaged over the whole disk. Outflows 

can be examined as a function of the starformation region that produced them, as 

a test of specific predictions for how flow 

velocities and mass transfer rates depend on 

time and energy input (Murray et al. 2011, 

Thompson et al. 2005). Not only will LUMOS 

enable the dissection of flows at small scales, 

but the multiplexing of the microshutter 

arrays provides a huge efficiency gain that 

will enable maps of resolved flows for a wide 

range of galaxies in the nearby Universe. It 

is critical to understand the wind-expulsion 

history of galaxies as a function of mass, 

as the winds are likely critical for shaping 

the stellar mass-halo and mass-metallicity 

relationships of galaxies, which depend 

strongly on galaxy mass. This is best 

accomplished by high-S/N, high-resolution 

spectroscopy with broad UV wavelength 

coverage. The wavelength coverage, 

notably to wavelengths as short as λ~1000 

Å, is critical. If we are to understand galaxy 

transformation and the role that winds may 

play in it, the ability to observe these flows at 

the relevant small scales is needed.

Figure 5.8. LUMOS will perform intensive multi-object spectroscopy of star-forming regions, 

ISM gas, and galactic outflows in nearby galaxies. Here we show the footprint of the LUMOS 

multi-object mode overlaid on the nearby galaxy M83 at 4.6 Mpc. At this distance, the LUMOS 

microshutters subtend 2-3 parsecs. At the top, we show three silicon lines that trace multiphase 

gas in outflows, as proxied by Hubble/COS spectra of low-z “green pea” galaxies by Henry et al. 

(2015). LUMOS users will be able to examine a wide range of ionization, metallicity, kinematics, 

and dust diagnostics down to 1-3 parsec scales at the positions of hundreds of individual stellar 

clusters and ISM simultaneously, and for many nearby galaxies in a single program. 
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5-18 The LUVOIR Interim Report

The LUVOIR Deep and Wide Galaxy Fields

Since the original Hubble Deep Field (Williams et al. 1996), large-area surveys at the deepest 

limits have been a mainstay of galaxy evolution studies; the Ultra Deep Field, CANDELS, 

and the Frontier Fields have followed. 

Much of the LUVOIR’s “Signature 

Science” will follow the same model, 

in which multiple scientific objectives 

are enabled by a single set of deep 

exposures over a large area. 

The nominal High Definition Imager 

(HDI) has a field of view of 2 x 3 arcmin, 

or 6 sq. arcmin, sampled by two focal 

plane arrays with Nyquist sampling 

that view the sky at the same time. 

Detailed calculations show that HDI mounted behind LUVOIR’s 15-meter A architecture will 

reach 5σ photometric limits of AB = 33–33.5 for point sources in integrations of 10 hours 

per band. The most basic “LUVOIR Deep Field” is a single field of view deep integration, 

much like Hubble’s Ultra Deep Field (AB ~ 29; Beckwith et al. 2006) or Extreme Deep Field 

(Illingworth et al. 2013), with these 10 bands taking 100 hours of integration to reach the 

tabulated limits. 

A LUVOIR “Wide” Field, inspired by Hubble’s CANDELS program (Grogin et al. 2011), 

exploits LUVOIR’s high mapping speed to cover 720 arcmin2 in 120 tiled fields of view. If 

we limit this program to 1200 hours integration time, or 10 hours per tile, 1 hour per band, 

the limits are AB ~ 31–32 mag, which surpasses Hubble’s deepest limits by > 2 mag and 

matches JWST’s deep limits.

These limits are unique to LUVOIR: not even 30-meter class telescopes on the ground 

will reach such depths, owing to time-variable sky backgrounds. This capability enables 

LUVOIR to detect (1) a single Sun-like stars (AB = 4.72) out to 5.5 Mpc, (2) a main sequence 

O star to 500 Mpc or redshift z ~ 0.1, nearly the entire volume covered by the SDSS 

spectroscopic survey, and (3) a 0.001 L* galaxy at z= 6, deep enough to detect the early 

seeds of galaxies like our own Milky Way. This is a broadly applicable capability that will 

advance the many areas of science we contemplate as “Signature” for LUVOIR.
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The LUVOIR Interim Report 5-19

5.2 Signature science case #2: 

The multiscale assembly of galaxies

The galaxies we see in our neighborhood 

today have spent billions of years growing and 

evolving from their first faint seeds at Cosmic 

Dawn. Over this time, some grow by forming 

stars and merging with their neighbors to 

become big galaxies with prominent bulges 

and massive disks hosting violently active 

black holes. Those that grow too big too 

fast can “quench," ceasing to form stars 

and then evolving passively as their stars 

age. Other galaxies remain small, perhaps 

eventually merging into larger ones. Nature 

makes galaxies of these kinds and every 

type in between, and no theory of origins will 

be complete without a full understanding of 

how this happened. 

Why will galaxy formation remain 

Signature Science in the era of LUVOIR, even 

after JWST and the ELTs? In short, none of 

these highly capable facilities will be able to 

achieve LUVOIR’s unique combination of 

depth, resolution, and mapping efficiency. 

Depth allows us to see the smallest building 

blocks of galaxies. With enough resolution 

we can look inside galaxies at small physical 

scales, breaking down their formation at a 

level of detail that isolates the key physical 

processes. Finally, any observational insights 

must be backed by statistically significant 

samples collected with a high mapping 

efficiency or speed. Optimizing for these 

three figures of merit will enable LUVOIR’s 

users to make revolutionary advances in the 

Signature Science of galaxy formation and 

evolution.

5.2.1 Galaxy assembly at the faint 

frontier

Pushing back the faint frontier has been a 

constant theme of galaxy formation from the 

Program at a Glance – Gas, Radiation, and Dust Feedback in Nearby Galaxies 

(Figure 5.8) 

Program details: 1000 sources / galaxy (100 sources per config, 10 configs / galaxy, 

4 hr / config) on 5 nearby galaxies over a range of mass / metallicity. Total 200 hours of 

integration.

Instrument(s) + Configuration: LUMOS multi-object FUV and NUV spectroscopy, R ~ 

30–40,000, G120M, G150M, G180M, G300M, 1000–4000 Å.

Key observation requirements: S/N > 30 in UV continuum over 1000–4000 Å (FUV 

mag < 19).

Synergies with other ground/space facilities: ALMA and radio telescopes can 

constrain ISM gas, dust, metal content, and cluster age/metallicity.

Figure 5.9. Integration time required to achieve 

S/N = 10 for point-source photometry in three 

broad bands for the 15-m LUVOIR A (solid 

lines) and the 38-m E-ELT (dashed lines). If we 

take the 10-hour exposure as a nominal “large 

program” to which many nights or orbits would 

be devoted, LUVOIR reaches AB = 32.5 mag 

in V and J, 3.5 magnitudes deeper than E-ELT. 

The LUVOIR limits were derived from the HDI 

ETC available at luvoir.stsci.edu/hdi_etc
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5-20 The LUVOIR Interim Report

earliest days, through the Hubble Deep Field, 

and into the present. LUVOIR will expand 

the faint frontier to the smallest relevant 

scales over nearly the whole of cosmic time.

In the modern universe, galaxies occupy 

an enormous range of mass from giant 

ellipticals at M* > 1012 M to ultra faints at 

M* < 104 M. In the prevailing hierarchical 

paradigm, all galaxies of any substantial size 

grow from the steady accumulation of gas 

and by the successive mergers of many 

smaller galactic components in what is 

known as a merger tree. Every giant galaxy 

has many dwarfs in its past; our own Milky 

Way has evidently accumulated many such 

dwarfs over its history. Within the next few 

hundred million years, it will acquire the two 

Magellanic Clouds in a minor merger with 

the even more dramatic major merger with 

Andromeda to follow. Telling the full story of 

galaxy formation starts with being able to 

work backwards through this tree of galactic 

origins to see the roots, and this requires 

pushing back the faint frontier. 

The brightest galaxies at any redshift will 

tend to be the seeds of the brightest galaxies 

at any later time. The galaxies we see at the 

dawn of time with Hubble, and soon with 

JWST, are not the earliest seeds of galaxies 

like our Milky Way. While we can probably 

reach Milky Way-like progenitors as far back 

as z = 6 with JWST (Okrochkov & Tumlinson 

2010), what we see will be the main trunk of 

the tree, not the hundreds of other branches 

that existed at that time. But what is the 

minimum threshold for galaxy formation that 

LUVOIR should try to reach?

The faint frontier expanded in an 

unexpected direction with the discovery 

of ultra-faint dwarf (UFD) galaxies in the 

halo of the Milky Way. These galaxies are 

gravitationally bound but possess only ~105 

M of stars, or even less, with characteristic 

radii of only 100–300 parsecs. They were 

first detected as slight over-densities in the 

all-sky star map produced the Sloan Digital 

Sky Survey (Willman et al. 2005). The UFDs 

are now believed to be the ancient “fossils” 

of tiny galaxies that formed 80–90% of 

their stars before or during the epoch of 

reionization (Ricotti & Gnedin 2005; Brown 

et al. 2012; Weisz et al. 2014), like those first 

galaxies that preceded our own Milky Way. If 

so, then the ultimate faint frontier lies where 

we can detect the UFD scale over the full 

sweep of cosmic time. 

We demonstrate the power LUVOIR will 

bring to such a study by deriving the plausible 

mass limits reached as a function of redshift 

in a single very deep exposure. Figure 5.10

shows the stellar mass range that defines 

the dwarf populations of galaxies, with the 

UDFs at the minimum threshold (as we 

currently know it). The figure also shows 

the limits achievable by telescope of varying 

size, including Hubble, JWST, and the two 

LUVOIR architectures. The High Definition 

Imager will enable LUVOIR users to reach 

the extreme low mass end (M* < 105.5 M) 

of the stellar mass limit at many redshifts in a 

deep survey (AB ~ 33–34) and wider surveys 

of 106–107 M systems in much shorter 

exposures.

Only LUVOIR will be able to detect 

galaxies at the scale of the Ultra-Faints when 

they are still forming stars. This capability 

will reveal the entire grand sweep of galaxy 

formation to the earliest times, permitting 

full reconstruction of mass functions and 

merger trees. LUVOIR will also be able to 

trace the relationships between massive 

galaxies and their faintest satellites, which 

respond to details of the gas and star 

formation physics in complex ways that are 

still poorly understood. LUVOIR can, for 

instance, probe the relationship between L* 

galaxies and their faintest satellites, looking 

for evidence of when and now dwarf galaxies 

can be quenched by their larger, quenched 

neighbors (“conformal quenching”) for 
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The LUVOIR Interim Report 5-21

Figure 5.10. At left,the stellar mass from Boylin-Kolchin & Bullock. Three stellar mass ranges for 

dwarf galaxies are shown. At right, the limits for detection by four observatories in a 500 ksec 

observations, assuming a 200 pc extended source and S/N = 5. LUVOIR’s Architecture A can 

reach the UFD scale out to redshift z ~ 10.

Figure 5.11. Physical size corresponding to the spatial resolution element for Hubble (black, 0.5 

micron), JWST (red, 2 micron), and LUVOIR-A (green 0.5 micron), which has a resolution of 80 

pc or smaller at all redshifts.
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5-22 The LUVOIR Interim Report

indications that galaxies at the UFD scale 

continue to form stars after reionization.

5.2.2 Seeing inside galaxies as they 

form and transform

Galaxy assembly is a messy, complex 

process. Using Hubble, astronomers have 

surveyed large samples of galaxies during 

the rise and fall of cosmic star formation 

(Madau & Dickinson 2014). These studies 

have mapped out the relationship between 

stellar mass and star formation rate (Whitaker 

et al. 2012), and traced the rise of quenched 

galaxies on the red sequence almost back to 

its beginnings (Kriek et al. 2009). Perhaps the 

greatest surprise from this work is how small 

galaxies appear to be at early times. Figure 

5.12 shows that the red sequence is already 

identifiable at z ~ 2–3, but with most of its 

quenched galaxies occupying only a few kpc 

in size. Indeed, star forming galaxies at these 

redshifts average several times larger than 

passive galaxies at the same mass. How do 

these galaxies quench, and do they actually 

get smaller as they do? 

Hubble itself has struggled to address 

what happens inside galaxies at these 

early times because its diffraction limit 

corresponds to physical scales of 300–400 

pc at z = 2–3. Hubble images place only a 

few pixels across these small yet massive 

galaxies at high redshift. This is sufficient 

to marginally resolve galaxy disks, but not 

the smaller-scale processes at work within 

them. It is also sufficient to detect ~1 kpc 

star-forming clumps (Elmegreen et al. 2007, 

2009; Forster Schreiber et al. 2011), but 

large clumps may be blended collections of 

smaller star-forming regions (Tamburello et al. 

2017; Rigby et al. 2017; Bordoloi et al. 2016), 

even if the clumps are like the largest and 

brightest in our immediate neighborhood, 30 

Doradus in the LMC (diameter d ~200 pc) 

and the Carina Nebula (d ~ 140 pc).

We know that galaxies have structure 

on 100 pc scales because of evidence from 

the few cases where natural gravitational 

telescopes are provided by foreground 

galaxy clusters (Figure 5.13). In these 

rare cases, Hubble probes individual starforming regions in ways that illustrate the 

power of LUVOIR. Figure 5.13 shows an 

example, where Hubble reveals two dozen 

star-forming clumps in a lensed galaxy, with 

radii of 30–50 pc. These clumps have the 

sizes and luminosities of the brightest starforming regions in the nearby universe, and 

none would be resolvable to Hubble without 

lensing. The lensing reconstruction of this 

lensed galaxy provides a rare view of what 

distant star-forming galaxies actually look 

Figure 5.12. Galaxy sizes and masses from the Hubble CANDELS survey. Even massive galaxies 

at z > 1 have sizes of only a few kpc. Adapted from van der Wel et al. (2014). 
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The LUVOIR Interim Report 5-23

Figure 5.13. A gravitationally lensed galaxy at z = 2.481, as imaged by Hubble/WFC3 in F105W, 

F606W, and F390W (top panels), as reconstructed in the source plane (middle panel), and 

as it would look to Hubble were it not lensed (right panel). The left panel shows the source 

reconstruction, blurred to the normal spatial resolution of Hubble. The far right panel shows 

the source reconstruction with lensing. The lower panels simulate images from a large space 

telescope, without lensing, using a PSF scaled from an empirical PSF for Hubble WFC3-UVIS 

F390W. Adapted from Johnson et al. (2017b) and Rigby et al. (2017).

like. Convolving this truth image with the 

scaled empirical Hubble PSF shows which 

features LUVOIR can recover, as a function 

of aperture. LUVOIR’s users will be able to 

recover almost all the star-forming clumps 

in the F390W filter. This simulation shows 

that for LUVOIR, any distant galaxy can 

be imaged with the sharpness that Hubble 

can now achieve for the most favorable 

gravitationally lensed systems, which cover 

only a tiny fraction of the sky. LUVOIR would 

therefore be able to survey star formation 

down to 100 pc scales for thousands of 

galaxies, which is terra incognita.

Galaxy death, or quenching and 

compactification: The mystery of how 

galaxies “die”—or cease to form stars at any 

significant level—is one of the most abiding 
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5-24 The LUVOIR Interim Report

in astrophysics. Edwin Hubble’s original 

tuning fork (1926) captured the basic truth 

that some galaxies are disky, blue, and 

star-forming, while others are spherical, 

redder, and quiescent. Numerous complex 

and overlapping mechanisms from stellar 

feedback to AGN and mergers have been 

proposed to turn star-forming galaxies into 

quenched ones, but there is still no definitive 

physical understanding of this basic 

phenomenon. 

The presence of fully quenched galaxies 

at high redshifts z > 2 is surprising enough, 

but even more so are the extremely compact 

sizes of many of these galaxies. Ultracompact galaxies pack a Milky Way’s worth 

of stars, 1011 M or more, into a kiloparsec 

or less. These galaxies are only marginally 

resolved by Hubble, with > 50% of their total 

light falling onto just one or a few pixels. 

Yet these galaxies are possibly the earliest 

progenitors of today’s massive ellipticals, 

and if so they are a key part of the galaxy 

formation puzzle. 

To unravel the origins of these mysterious 

galaxies, we must be able to resolve their 

internal structures, to measure stellar content 

and ages of their stellar populations, to look 

for internal gradients in age, to follow internal 

dynamics, and to trace all these quantities 

over time as this population arises. This is 

Signature Science for LUVOIR owing to its 

exquisite spatial resolution, 100 pc or better 

at all redshifts, which will allow us to map the 

internal structures and permit age dating of 

stellar populations at small scales. LUVOIR’s 

broad UV/optical coverage and collecting 

area are also essential to this problem: restframe UV and optical colors are much more 

sensitive indicators of population age than 

are the optical/IR colors available with JWST. 

LUVOIR will surpass JWST on both counts, 

both in terms of physical resolution and the 

diagnostics that can be measured at that 

resolution. The same is true of ground-based 

telescopes, over small fields, which will 

exceed LUVOIR’s raw spatial resolution with 

extreme NIR extreme AO, but will struggle 

to reach the same depths and cannot apply 

rest frame UV star formation diagnostics.

Figure 5.14. Simulated galaxies showing the rich interior detail that is visible in deep exposures 

with telescopes of varying size. In all cases we assume extremely deep 500 ksec exposures. The 

109 M galaxy is at the top and 106 M galaxy is at the bottom. Credit: G. Snyder (STScI)
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The LUVOIR Interim Report 5-25

Figure 5.15. At z ~ 2, massive galaxies of surprisingly small size are already “red and dead." 

These “ultracompact” red galaxies are likely the progenitors of today’s massive ellipticals. They 

may have formed in a monolithic collapse, by compaction of star-forming progenitors, or gas 

rich major mergers are possibilities. Why did they stop forming stars so early, and why have they 

not resumed in the 10+ billion years since? At z ~ 2, these galaxies are very poorly resolved by 

Hubble, and JWST will do little better. LUVOIR will resolve these galaxies’ internal structures to 

measure age gradients and dynamics that reflect their origins.

5.3 Dissecting galaxies one star at 

a time

Our ability to determine when galaxies 

assemble their stellar populations, and how 

that process depends on environment is a 

fundamental component to any robust theory 

of galaxy formation. By definition, the dwarf 

galaxies we see today are not the same as 

the dwarf galaxies and proto-galaxies that 

were disrupted during assembly. Our only 

insight into those disrupted building blocks 

comes from sifting through the resolved field 

populations of the surviving giant galaxies 

to reconstruct the star-formation history, 

chemical evolution, and kinematics of their 

various structures (Brown et al. 2010). 

Resolved stellar populations are cosmic 

clocks and assay meters that can assess 

the age and metallicity of their galaxies 

using well-defined relationships obeyed by 

stellar luminosity and color. Their most direct 

and accurate age diagnostic comes from 

resolving both the dwarf and giant stars, 

including the main sequence turnoff. As 

demonstrated by the Panchromatic Hubble 

Andromeda Treasury program (PHAT, 

Dalcanton et al. 2012), fitting stellar evolution 

models to CMDs can reveal not only when 

star formation occurred in Andromeda, but 

how episodic it is, and how it varies across 

the disk (e.g., Williams et al. 2014). This work 

highlights both the need for the survey depth 

required to obtain a CMD accurate enough 

to differentiate between models and the 

ability to map across as much of a galaxy as 

possible.
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5-26 The LUVOIR Interim Report

Unfortunately, the main sequence turnoff 

is too faint to detect with any existing 

telescope for galaxies beyond the Local 

Group, and beyond this the increasingly 

severe effects of crowding become an 

even more fundamental limit. Both effects 

greatly hinder our ability to infer much about 

the details of galactic assembly because 

the galaxies in the Local Group are not 

representative of the galaxy population at 

large. LUVOIR will transform our ability to 

determine stellar histories by leveraging both 

light gathering power and spatial resolution, 

extending our reach beyond the Local Group, 

and thus significantly increasing the diversity 

of galaxies we can study. 

We consider two types of surveys by way 

of example. First, we consider the distances 

that can be reached for characterization of 

diffuse (i.e., non-crowded) stellar populations 

in nearby galaxies. Figure 5.16 shows a 

cartoon of the local Universe with galaxies 

in their actual positions and marked by 

type. In this view, 24 Mpc across, a 15-m 

LUVOIR can reach the main-sequence 

turnoff at SNR = 5 in 100 hours (V and I). 

For diffuse populations, this capability can 

be used to probe faint dwarfs or age-date 

the outer regions of massive galaxies. At 

closer distances, these observations can be 

collected over time to build up the timeline of 

proper motion and work out galaxy motions 

in 3D (see Chapter 6).

For a second type of survey, we consider 

regions where crowding comes into play. 

LUVOIR’s spatial resolution will enable 

usable photometry at much higher stellar 

densities than smaller telescopes. The PHAT 

program has empirically determined that 

usably accurate photometry can be obtained 

in UVOIR images up to a surface density of 

~15 stars per square arcsecond with Hubble. 

Scaling this limit up by 1/D2, we find that 

a 15-meter LUVOIR could detect the main 

sequence turnoff out to 5 Mpc, working with 

up to 400 stars per square arcsecond before 

crowding becomes prohibitive (Figure 5.17). 

LUVOIR will work in concert with 

30m-class ground-based telescopes 

expanding our reach to other well-populated 

galaxy groups, with LUVOIR obtaining 

photometry of G dwarfs stars down to V~34 

mag, and the ground obtaining kinematics 

of much brighter giants out to the Coma 

Sculptor Cloud. The dwarf stars in the Coma 

Sculptor Cloud are effectively inaccessible 

to the ground, requiring gigaseconds of 

integration even for an isolated star. 

This capability for studies of resolved and 

semi-resolved stellar populations has a wide 

range of applications from mapping the history 

of star formation in galaxies to assessing the 

impact or reionization at the smallest scales. 

Figure 5.16. Map of local universe (24 Mpc 

across) shown with the distances out to which 

HST (yellow), JWST (orange), and LUVOIR 

(9-m (green), 15-m(purple)), can detect the 

main sequence turnoff from a CMD in V and 

I passbands at SNR=5 in 100 hours. Giant 

spirals, like M31, are indicated by the blue 

galaxy symbols, giant ellipticals as orange 

blobs, and dwarf galaxies as small green dots. 

Derived using the HDI exposure time calculator 

at luvoir.stsci.edu/hdi_etc.
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The LUVOIR Interim Report 5-27

Figure 5.17. Simulated CMDs for a single-age solar-metallicity stellar population. The simulation 

uses a 10 Gyr-old isochrone from FSPS (Conroy et al.), placed at 5 Mpc and “observed” in 50 

hours each in the g and r bands. To compute the crowding limit, we assume AB = 23 per square 

arcsecond. The Hubble / WFIRST analogue (2.4 meters) cannot overcome the crowding and poor 

photometry. The 9-m LUVOIR can resolve the giant branch before crowding becomes too severe 

at the sub giant branch. The 15-m LUVOIR can reach to the top of the main sequence to apply 

population age and metallicity diagnostics. These figures were made with the LUVOIR CMD 

simulation tool at luvoir.stsci.edu/cmd.

The LUVOIR HDI ETC and the CMD tool, 

linked here, provide prospective users with 

the opportunity to derive the requirements 

for their own applications.

References

Berg, T., et al. (2018). ApJ, in press

Bertone, S., Aguirre, A., & Schaye, J. 

(2013). MNRAS, 430, 3292 

Bordoloi, R., et al. (2014). ApJ, 796, 136 

Bordoloi, R., et al. (2016). 458, 1891

Borthakur, S., et al. (2016). ApJ, 833, 259 

Brammer, G. B., et al. (2011). ApJ, 739, 24

Brown, T. M., et al. (2012). ApJ, 753, 21

Burchett, J., et al. (2016). ApJ, 832, 124

Cantalupo, S., et al. (2014). Nature, 506, 63 

Corlies, L., & Schiminovich, D. (2016). ApJ, 

827, 148 

Dalcanton, J. J., et al. (2012). ApJS, 200, 

18 

Elmegreen, B. G., et al. (2008). ApJ, 688, 

67
LUVOIR The Large UV Optical Infrared Surveyor

5-28 The LUVOIR Interim Report

Elmegreen, D. M., et al. (2007). ApJ, 658, 

763 

Förster Schreiber, N. M., et al. (2011). ApJ, 

739, 45

Fukugita, M., Hogan, C. J., & Peebles, P. J. 

E. (1998). 50, 518 

Henry, A., et al. (2015). ApJ, 809, 19 

Johnson, S. D., et al. (2015). MNRAS, 449, 

3263 

Kornei, K. A., et al. (2012). 758, 135

Kriek, M., et al. (2009). ApJ, 750, L71

Lehner, N., et al. (2013). ApJ, 770, 138 

Madau, P. & Dickinson, M., (2014). ARA&A, 

52, 415

Martin, C., (2005). ApJ, 621, 227

Martin, D. C., et al. (2016). ApJ, 824, L5 

Matsubayashi, K., et al. (2012). ApJ, 761, 

55

McGaugh, S. S., (2005). ApJ, 632, 859

Murray, N., Menard, B., & Thompson, T. A., 

(2011). ApJ, 735, 66

Okrochkov, M., & Tumlinson, J. (2010). ApJ, 

716, L41

Oppenheimer, B. D., et al. (2016). MNRAS, 

460, 2157

Peeples, M. S., et al. (2014). ApJ, 786, 54

Ricotti, M., & Gnedin, N. Y., (2005). ApJ, 

629, 259 

Rigby, J. R., (2017). ApJ, 843, 79 

Table 5.1. Summary science traceability matrix for Chapter 5

Scientific Measurement Requirements Instrument Requirements

Objectives Measurement Observations Instrument Property Value

Measure the 

baryons over 

cosmic time 

from 10–107 K

Column density, 

velocity width, 

and redshift for 

multiple ions

S/N >=20 

spectra of 100 

QSOs from 

1000 to 4000Å

LUMOS R~30,000 

Gratings

G120M, G150M 

G180M, 

G300M

Provide a 

high definition 

exploration of 

the CGM of 

local galaxies

Column density, 

velocity width, 

and redshift for 

multiple ions

S/N >=10 

spectra of 30 

QSOs within 

R=150 kpc of 

the galaxy 1000 

to 3000Å

LUMOS R~30,000 

Gratings, MOS

G120M, G150M 

G180M, 

G300M

Trace the CGM 

in emission

Emission line 

maps of key 

ions (e.g., HI, 

CIII, OVI)

S/N >5 

emission 

spectra in 

multi-shutter 

apertures 1000 

to 3000Å

LUMOS R~30,000 

Gratings, MOS

G120M, G150M 

G180M, 

G300M

Characterize 

nearby galaxy 

inflows and 

outflows in 

detail

Spatially map 

inflowing/

outflowing gas 

and dust in 

absorption

S/N >=30 

spectra of 1000 

sources/galaxy 

for 5 nearby 

galaxies from 

1000 to 4000Å

LUMOS R~30,000 

Gratings, MOS

G120M, G150M 

G180M, 

G300M

Explore star 

formation 

histories across 

the Hubble 

sequence

ColorMagnitude 

diagrams for 

stars in galaxies 

out tens of Mpc 

across the full 

range of galaxy 

type

S/N > 5 

imaging of 

resolved stellar 

populations

HDI Multi-band UVIS 

imaging

UBVRI filters
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 5-29

Somerville, R. S., & Davé, R. (2015). 

ARA&A, 53, 51 

Tamburello, V., et al. (2017). MNRAS, 468, 

4792

Thompson, T. A., et al. (2005). ApJ, 630, 

167

Tripp, T. M. (2013). arXiv:1303.0043

Tumlinson, J., et al. (2013). ApJ, 777, 59

Tumlinson, J., Peeples, M. S., & Werk, J. K., 

(2017). ARA&A, 55, 389 

Weisz, D. R., et al. (2014). ApJ, 789, 147 

Werk, J. K., et al. (2014). ApJ, 792, 8 

Willman, B., et al. (2005). ApH, 626, L85

Williams, B., et al. (2014). ApJS, 215, 9 

Wotta, C. B., et al. (2016). ApJ, 831, 95
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-1

Chapter 6
LUVOIR The Large UV Optical Infrared Surveyor

6-2 The LUVOIR Interim Report

6 What are the building blocks of cosmic structure?

Our understanding of how the universe 

works is sophisticated but incomplete. On 

cosmological scales, the total mass-energy 

density is almost precisely equal to the critical 

value. Most of the mass-energy density is in 

the dark energy component and one quarter 

of the total is a cold dark matter (CDM) 

component. There is a complex interplay 

between the cosmic expansion history, dark 

matter, radiation, and baryons, which yield 

many of the observable elements in the 

universe. Yet we are still working towards 

ascertaining the true nature of dark energy 

and dark matter. Furthermore, our models 

for galaxy formation and evolution from the 

birth of the first stars through the era of 

reionization, and on to the present, remain 

rudimentary. 

Using ground- and space-based 

observatories through the early 2030s, 

we will make significant inroads in our 

understanding of the cosmic “dark sector” 

and the properties of galaxy building blocks 

in the early universe. This drives many of 

the science objectives of JWST, Euclid, 

LSST, WFIRST, and the planned 20-m to 

40-m ground-based telescopes. Yet these 

telescopes will not reach beyond a specific 

frontier that we already recognize as critical for 

a comprehensive understanding of structure 

formation in the universe. This frontier exists 

at the scales of the lowest mass galaxies 

(with total masses ≤ 106 M that we know 

are there, from the first sparks of galaxy 

formation at z>10 to the least luminous dwarf 

galaxies in the present day. These galaxies 

map to physical scales as small as 10 kpc 

and with stellar concentrations as small as 

100 parsecs. At these size and mass scales, 

we enter regimes where competing scenarios 

for the evolution of the dark matter density 

field, and its associated baryonic structures, 

make distinguishable predictions that can be 

robustly tested with observations that reach 

fainter than AB = 33 mag. 

To explore this ultra-faint regime, we 

require LUVOIR, which (for the 15-m 

architecture) will be capable of resolving 

< 100 parsec scales at all redshifts while 

reaching a 5-s point source limiting AB 

magnitude of 33 (0.23 nJy) in just 10 hours 

and ~35 mag (0.04 nJy) in ~10 days. Three 

novel observational regimes that require 

LUVOIR are described in this chapter:

1) The dark matter power spectrum 

on scales below 100 kpc: Between the 

universe’s horizon scale and galactic scales, 

the structure we measure is consistent with 

dark matter being entirely non-relativistic 

and non-interacting. This does not uniquely 

identify a specific fundamental particle 

for dark matter, however. Indeed, several 

candidate particle classes are allowed by 

existing observations. This has motivated 

work on dark matter particle candidates 

that, on astronomical scales, have ensemble 

behaviors consistent with being “warm” 

(mildly relativistic) and/or “self-interacting.” 

The microphysics of the interactive or 

radiative properties of dark matter influences 

the nature of structure on various scales 

and epochs in time. Specifically, the imprint 

of dark matter physics manifests at scales 

below 100 kpc (corresponding to the scale 

of self-gravitating halos of a few million solar 

masses) in the statistics and shapes of these 

structures as functions of size and mass over 

cosmic time.

2) The limits of galaxy assembly in the 

pico-Jansky regime: Associating galaxies 

with the underlying dark matter structure 

requires modeling the interplay of cooling 

mechanisms, the epoch of reionization, and 

feedback effects from star formation and 

AGN. The most massive dark matter halos 

have long baryonic cooling times, with stars 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-3

subsequently forming late. Very low mass 

halos are not able to retain a steady pool of 

star-forming baryons due to ionizing radiation 

that is keeping the universe transparent to 

this light. The source of the ionizing photons 

likely comes from a great abundance of starforming, low mass galaxies. The bulk of the 

physical processes that dominate the early 

formation and evolution of cosmic structure 

all take place on scales below 10 kpc. This 

corresponds to minuscule volumes within 

massive galaxies, or in dwarf galaxies, 

The State of the Field in the 2030s

By the year 2035, HST, Euclid, WFIRST, and JWST will have completed their missions 

and LSST will have achieved its 10-year survey depth. At least two 20–40 meter class 

ground-based optical-NIR observatories will be operational. SKA should be online and 

creating deep maps of the neutral hydrogen distribution out to high redshifts. LUVOIR 

will be following on the success of these facilities by bringing unparalleled sensitivity and 

angular resolution in the ultraviolet and optical regime.

James Webb Space Telescope (JWST): Deep NIRCam surveys with JWST will exist 

down to AB = 31.5 mag (for 0.1” diameter source with 500 ksec exposure time). This 

will allow JWST users to discover many new ultra-faint sources but will still not be deep 

enough for many of the luminosity regimes we need to reach to fully map the assembly of 

structure from small-scale components at 6 < z < 10.

LSST, Euclid, and WFIRST: These sky survey telescopes will detect many new faint 

dwarf galaxies in the local universe but will not have the resolution to characterize their 

morphological structure in detail nor measure their proper motions. However, these targets 

can be studied with LUVOIR.

Extremely Large Telescopes on the Ground: These facilities will have similar angular 

resolution as LUVOIR in the NIR but will have difficulty detecting sources fainter than AB 

~ 31 mag due to the brightness of the night sky. The spectroscopic capabilities of such 

20–40 meter telescopes will be a superb complement to the imaging power of LUVOIR.

Square Kilometer Array: The SKA will perform wide-area deep imaging of the neutral 

hydrogen gas at cosmological distances (z < 10) and will provide the knowledge of where 

to target deep imaging with LUVOIR to study the role of reionization suppression of star 

formation.

LUVOIR: In the context of studying the building blocks of galaxies, LUVOIR will reach 

stellar mass limits typical of ultra-faint dwarf galaxies at all redshifts. LUVOIR observers 

will detect sources with fluxes fainter than AB = 33 mag, where reionization suppression of 

star formation is predicted to be important. LUVOIR will also extend dwarf galaxy detection 

out to 25 Mpc in the local universe, and allow observers to detect directly the sources of 

ionizing radiation in the cosmos and characterize the energy distribution of that ionizing 

radiation. Its high precision astrometric stability and calibration, will allow the measurement 

of transverse velocities of stars within Local Group galaxies to directly measure mass 

density profiles and to map the gravitational fields around nearby galaxies by measuring 

satellite orbits within their central halos. This information will provide additional constraints 

on the kinematics and the nature of dark matter.
LUVOIR The Large UV Optical Infrared Surveyor

6-4 The LUVOIR Interim Report

the smallest galaxies that are able to form 

and retain stars. Observing these scales 

across a full range of galaxy properties from 

the current epoch to the end of the era of 

reionization will require sensitivities down to a 

few tens of picoJanskys (AB ≈ 35 mag). Such 

observations will be transformational as they 

will reveal star and galaxy populations not yet 

seen with current or planned telescopes. 

3) The history of ionizing light from 

observations shortwards of rest-frame 

900 Å: To complete our understanding 

of the emergence of structure, we must 

characterize the influence that ionizing 

radiation has on early galaxies. Directly 

observing ionizing photons at the epoch of 

reionization is effectively impossible due to 

the opacity of the intergalactic medium to 

the ionizing photons of that epoch. LUVOIR 

will leverage its FUV and UV sensitivity and 

resolution to directly detect weak Lyman 

continuum radiation escaping from z<1 

galaxies in a spatially resolved manner for 

multiple objects per pointing, revealing the 

environmental factors that favor the escape 

of radiation, providing crucial data on how 

light escaped galaxies during the epoch of 

reionization at z ≥ 7.

With LUVOIR, the community acquires a 

unique and unmatched combination of sensitivity and volume to observe, across most 

of cosmic time and in many different intergalactic environments, what the universe 

actually does on spatial scales where the 

confluence of the matter power spectrum, 

dark matter physics, and baryonic processes all interplay. LUVOIR will accomplish this 

by extending our census of dwarf galaxies 

to much fainter limits in the current epoch 

and extend that census, at brighter limits, 

across the last 13 billion years. This will allow 

LUVOIR users to establish the direct connection between the current-epoch matter power spectrum with its early progenitors. With 

such information, we can directly distinguish 

between competing dark matter models that 

show significant differentiation on scales below 100 kpc. LUVOIR investigators will also 

directly observe the impact of reionization on 

early star formation by measuring the galaxy 

luminosity function well below limits achievable with current or planned telescopes. 

Finally, LUVOIR will enable a direct characterization of that ionizing radiation by acquiring 

the spectra of UV light escaping from galaxies at wavelengths below rest-frame 900 Å, 

something only a large-aperture UV-sensitive 

telescope can accomplish. 

The Signature Science Cases discussed 

in this chapter represent some of the most 

compelling types of observing programs 

on the building blocks of structure that 

scientists might do with LUVOIR at the limits 

of its performance. As compelling as they 

are, they should not be taken as a complete 

specification LUVOIR’s future potential in 

these areas. We have developed concrete 

examples to ensure that the nominal design 

can do this compelling science. We fully 

expect that the creativity of the community, 

empowered by the revolutionary capabilities 

of the observatory, will ask questions, 

acquire data, and solve problems beyond 

those discussed here—including those that 

we cannot envision today.

6.1 Signature Science Case #1: 

Connecting the smallest scales 

across cosmic time

Dwarf galaxies are the smallest luminous, 

dark-matter dominated objects known. The 

least luminous dwarf galaxies contain just 

dozens of stars, although their host dark 

matter halos have total masses around 

107 M. A small number of these ultra-faint 

dwarf galaxies have been detected in orbit 

around our Milky Way Galaxy to date. The 

SDSS Segue-1 system, at a distance of 23 

kpc, has an absolute magnitude of MV = -1.5 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-5

Figure 6.1. The matter power spectrum as a function of spatial scale, as depicted by the “transfer 

function” for different dark matter particle scenarios at the current epoch (z=0). In each panel, 

smaller mass and physical scales are to the right. The standard cold dark matter (CDM) model 

prediction is shown for comparison. Left panel: Decaying dark matter model. Middle panel: A 

warm dark matter (WDM) model. Right panel: A self-interacting dark matter model (e.g., Buckley 

& Peter 2018). Credit: R. Massey 

mag (Simon et al. 2011). The Virgo-I dwarf 

galaxy has MV  =  -0.8 mag and is 87 kpc 

away (Homma et al. 2016). Even at this 

distance, we are only reaching one third 

of the distance to our Galaxy’s virial radius 

(Walsh et al. 2009), and are far from having 

a complete survey of the accessible area 

above and below the Galactic plane. 

There is evidence that the ensemble 

of dwarf galaxies is primarily responsible 

for the reionization of the universe and for 

maintaining its transparency to ultraviolet 

radiation. The space density and internal 

structure of these galaxies are closely 

connected to the fundamental properties 

of the dark matter particle, the level and 

duration of reionization, and the granular 

limits of the galaxy formation process. To 

date, limits on the thermal signature of dark 

matter have been placed by studies of the 

intergalactic medium (IGM) through Lymanalpha forest statistics (e.g., Viel et al. 2013), 

strong gravitational lensing (e.g., Li et al. 

2016), and dwarf-galaxy-scale statistics 

(e.g., Kim et al. 2017). One of the most 

constraining measurements, by Jethwa et 

al. (2018), places a 95% confidence limit on 

dark matter particles having a mass greater 

than 2.9 keV. Dark matter particles may be 

significantly colder (more massive) yet be 

interacting (either self-interacting or with a 

dark matter-baryon interaction mechanism), 

or have more nuanced properties that fall 

outside of the currently popular prescriptions 

for dark matter candidates (e.g., Buckley 

& Peter 2018). Each type of dark matter 

property manifests itself on astronomical 

scales through a gravitational signature, 

in the statistical description of dark matter 

structure. Our measure of this signature is 

through the matter power spectrum today, 

as well as at earlier cosmic times.

Figure 6.1 illustrates the matter power 

spectrum behavior for several dark matter 

cases compared to CDM. At wave numbers 

above k ≈ 10h/Mpc, corresponding to scales 

well below that of the Milky Way Galaxy, the 

distinctions between these power spectra 

are significant. The k ≈ 10 h/Mpc scale 

corresponds to total mass scales in the 109 to 

1010 M range. These correspond to classical 

dwarf galaxies, which have stellar masses of 

less than 105 to 106 M and are fainter than MV

= -10 mag. As we approach smaller scales, 
LUVOIR The Large UV Optical Infrared Surveyor

6-6 The LUVOIR Interim Report

near k ≈ 100 h/Mpc, we reach the regime of 

107 M (total mass) halos. This approaches 

the lower mass limit where gravitationally 

bound halos can host star formation, before 

the necessary radiation cooling for forming 

stars becomes too inefficient. Halos of total 

mass of 107 M have stellar masses as low 

as just tens of solar masses. In the current 

epoch, such systems have visual absolute 

magnitudes between -2 and 0. This defines 

the capability level LUVOIR targets.

By the early 2020s, the Dark Energy 

Survey and LSST will have completed the 

census of dwarf galaxies in the vicinity of 

the Milky Way Galaxy. In the spirit that each 

massive galaxy’s satellite galaxies provide 

just one “draw” of the underlying physics that 

gives rise to this population, we must expand 

on that sample by surveying the satellite 

dwarf population associated with massive 

galaxies similar to our own Milky Way. By 

the late 2020s, JWST and ELTs will have the 

sensitivity to extend the dwarf satellite galaxy 

search out to several Mpc. Dwarf galaxies 

are identified through detection of their 

constituent stars, as long as the membership 

of those stars can be confidently determined, 

through colors or spectroscopy over an 

extended field of view. LUVOIR will extend 

this sensitivity, using horizontal branch stars, 

to far beyond the Virgo Cluster’s 16.5 Mpc 

distance. This has multiple advantages. First, 

a sample of massive and Milky Way-analog 

galaxies provides multiple “draws” similar 

to our well-studied Galaxy. Second, this 

much larger volume gives us access to an 

enormous dynamic range of environments, 

so that dwarf galaxies can be studied in both 

volatile and quiescent states. 

LUVOIR’s sensitivity, angular resolution, 

and Nyquist sampling will deliver 

transformative measurements on the smallest 

galactic scales. We highlight the potential 

breakthroughs in this field by presenting 

two dramatically different (but connected) 

periods in the history of the universe: the 

extended local volume in the current epoch 

(out to a luminosity distance of ~30 Mpc, or 

z  =  0.008), and the early universe, 13 Gyr 

in the past (at z~7), about half-way through 

the reionization transition era from a visually 

opaque to a transparent universe.

6.1.1 Dwarf galaxies in the local 

universe: testing dark matter models

Dwarf galaxies are predicted to exist around 

all massive galaxies and in all environments. 

Galaxies out to 40 Mpc from us, with masses 

comparable to the Milky Way, are being 

systematically surveyed for dwarf galaxies as 

faint as MV = -12.3 mag (comparable to the 

Leo-I Galactic dwarf; e.g., Geha et al 2017). 

These surveys are identifying dwarf galaxies 

that are current-epoch analogs to the early 

galaxies responsible for re-ionizing the 

universe (e.g., Boylan-Kolchin et al 2015). 

The potential connections are complex, 

however. Many of the faintest local dwarf 

galaxies detected are chemically enriched 

relative to their early-universe counterparts. 

The stars in these low-mass neighbors of our 

Galaxy are created, in part, through inefficient 

molecular cooling while, at the same time, 

experience additional quenching effects from 

an intense ionizing radiation field. Exploring 

the connections between local dwarf 

galaxies and corresponding properties in the 

early universe suggests that even JWST will 

not be able to detect the full complement 

of dwarf galaxies at early times by several 

magnitudes of sensitivity (see Figure 6.6

and Figure 6.8).

To make a major breakthrough in understanding the connection between dwarf 

galaxies and the mass-scales of their dark 

matter halos will require the detection and 

characterization of significant samples of 

faint and ultra-faint dwarf galaxies in a range 

of environments over a wide span of cosmic 

time. Then, with subhalo-abundance match-
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-7

Figure 6.2. The matter power spectrum for samples of four and sixteen Milky Way analog 

systems. The error bars shown are magnified by a factor of 7 to make them visible in this figure. 

The abscissa positions of the points have been shifted slightly for clarity. For illustration, the 

underlying power spectra for a 4.5 keV warm dark matter model and a self-interacting dark matter 

model (Vogelsberger et al. 2016) are shown to illustrate the scales where these different scenarios 

exhibit detectable effects.

ing techniques, the luminous components 

can be connected with their dark matter halo 

mass properties and, in turn, connected to 

the underlying matter power spectrum that 

they are ultimately drawn from. LUVOIR will 

allow us to do this both locally and at high 

redshift.

We can calculate the fidelity of the power 

spectrum measurements that will be possible from a completeness-corrected sample 

of dwarf galaxies around several Milky Way 

Galaxy-scale systems. The results are shown 

in Figure 6.2 for sample sizes of 4 and 16 

Milky Way-like satellite systems. The error 

bars shown in Figure 6.2 have been magnified by a factor of 7 to make them visible in 

this plot. These calculations begin by adopting an empirical relation between dark matter 

halo masses and their stellar mass (Bullock 

& Boylan-Kolchin 2017) (see Figure 6.3), although how reliable this is in the regime of 

the true ultra-faint galaxies is the focus of 

ongoing work (e.g., Kim et al. 2017). At the 

low-mass end of the galaxy distribution, the 

effects of star formation suppression due 

to an intergalactic ionizing background flux 

can be significant. Barber et al (2014) develop a semi-analytic description of this effect 
LUVOIR The Large UV Optical Infrared Surveyor

6-8 The LUVOIR Interim Report

(see also Dooley et al. 2017). In a rigorous 

treatment of the physics of galaxy formation, 

many other evolutionary effects can affect the 

survival and apparent properties of individual dwarf galaxies. However, as we are considering the possibility of acquiring broadly 

complete samples of dwarf galaxies around 

massive systems, including those that extend towards the parent halos’ virial radii, we 

begin by including only this dominant suppression signature of reionization. Finally, a 

relation between the adjusted differential 

mass function and the corresponding power spectrum within specified scale-ranges 

is needed. While aspects of this connection 

have long been explored (e.g., Eisenstein & 

Hu 1999), the method derived by Schneider 

(2015) is particularly practical and that is 

what we have used for this calculation.

A LUVOIR program to obtain the needed 

constraints on the small-scale matter power 

spectrum would begin with two contiguous 

chevrons of HDI observations for each 

target. Each strip extends from the central 

galaxy out to about half the virial radius (~100 

kpc); one strip is along the analog’s major 

axis, and the other is along its minor axis. 

In addition, we would observe a number of 

random positions within the central 100 kpc 

to ensure we achieve a 50% filling fraction 

of the survey region (see Figure 6.4 for a 

survey layout centered on NGC 3810). This 

survey approach samples one quarter of the 

total virial-radius volume.

The dwarf galaxies discovered from 

these observations are then used to estimate 

the census of the compete population 

within the entire virial volume of each 

central galaxy targeted in the survey. The 

advantage of LUVOIR for this study is that 

the combination of its sensitivity and survey 

efficiency is unachievable by any current 

or planned platform, on the ground or in 

Figure 6.4. The LUVOIR image tiling strategy 

(orange rectangles) to sample the dwarf galaxy 

population around the Milky Way analog NGC 

3810 at a distance of ~14 Mpc. The 156 HDI 

pointings cover 50% of a region that extends 

out to half the virial radius (~100 kpc). Full 

sampling is performed along the major and 

minor axes of NGC 3810.

Figure 6.3. The correspondence between 

stellar mass and halo (virial) mass can be 

made through abundance matching in N-body 

simulations. The stellar mass ranges for typical 

dwarf galaxy populations are shown in this 

reproduction from Bullock & Boylan-Kolchin 

(2017). 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-9

space, by orders of magnitude. With these 

representative surveys of luminous dwarfs in 

each system, we then use the abundancematching framework to transform this to 

a mass function. The mass function is 

then compared with that predicted for the 

CDM-derived matter power spectrum (e.g., 

Schneider 2015). By targeting multiple 

systems that are as similar to the Milky Way 

as possible, and assuming that each one 

is an effective “draw” on the properties of 

the underlying physics that results in the 

populations we observe, we can estimate 

both the power spectrum’s amplitude and its 

measurement uncertainty (see Figure 6.2) 

on several spatial scales corresponding to 

those in the derived mass functions.

A LUVOIR survey to characterize the 

dwarf galaxy mass function around Milky Way 

analogs will provide the degree of accuracy 

we need to distinguish between different 

dark matter scenarios at >4s significance 

as Figure 6.2 demonstrates. The targeted 

galaxies will be in hand as several Milky-Way 

scale galaxies have already been identified 

out to 40 Mpc (e.g., Danieli et al. 2017; Geha 

et al 2017). Indeed, dwarfs within the nearer 

systems have been identified to MV=-9 mag, 

and to MV=-12 mag in the more distant 

systems. A LUVOIR deep imaging survey 

around Milky Way analog systems within 15 

Mpc will reach down to be to the horizontal 

branch (MV=0 mag) in dwarf galaxies detected 

in these systems, reaching the limits of dwarf 

galaxy formation. The LUMOS instrument 

could be operated in parallel during this 

type of survey to simultaneously map the 

absorption features from the circumgalactic 

medium of the central galaxy.

Figure 6.5 demonstrates the potential 

of a Nyquist-sampled imager to detect and 

characterize different stellar populations on 

space telescopes of various aperture sizes. 

The individual horizontal branch or red giant 

branch stars (depending on distance) will 

be detectable within each candidate dwarf 

galaxy. Many of the dwarf galaxy systems 

identified with LUVOIR will be superb targets 

for ELTs, given the precise positions and 

properties to aim for. The least luminous 

candidates in many target Milky Way analogs 

will be fainter than AB = 31 mag and would 

likely require additional LUVOIR observations.

The internal structural profiles of dwarf 

galaxies in the local universe will also be 

feasible as these systems will be extremely 

well resolved with LUVOIR. For example, the 

FWHM of the PSF of the 15-m LUVOIR HDI 

instrument in the r-band (650 nm) is 9 milliarcseconds. An individual dwarf galaxy 

analogous to the Virgo-I dwarf with a halflight radius size of ~20 parsecs would, at 

the Virgo Cluster’s distance, subtend ~0.50 

Figure 6.5. The distance to which three 

representative stellar types can be detected, 

versus aperture size of LUVOIR+HDI-like 

platforms. The distances shown here are for 

a S/N = 5 optical detection with a fiducial 

10-hour integration in the V-band. In such 

an observation, observers using the 15.1m 

LUVOIR could detect RGB stars out to 16 Mpc 

and horizontal branch stars out to 40 Mpc. With 

a 100-hour integration, the 15.1-m LUVOIR will 

have the ability to detect individual RGB stars 

out to 29 Mpc.
LUVOIR The Large UV Optical Infrared Surveyor

6-10 The LUVOIR Interim Report

arcsecond and would therefore be sampled 

with ~3000 distinct resolution elements. This 

means we will be able to fully characterize the 

dwarf galaxy’s stellar light profile, its stellar 

populations, and its internal structure—feats 

that would not be possible with any other 

current or upcoming telescopes.

6.1.2 Dwarfs in the distant universe: 

probing the impact of reionization

The reionization era is the point in cosmic 

history where the universe transitions 

from being opaque to UV radiation to its 

present largely transparent state. The faintend slope of the galaxy luminosity function 

becomes quite steep by z~7 (e.g., Bouwens 

et al. 2015; Finkelstein et al. 2015), which 

implies that if galaxies exist at luminosities 

below the current detection limits, then 

these ultra-faint dwarf galaxies should be 

the dominate sources for reionizing the 

universe. Gravitationally magnified Hubble 

Frontier Field data do show high-redshift 

galaxies, which are at least 100 times fainter 

in luminosity than previously observed 

(Livermore, Finkelstein and Lotz 2017). 

However, the steep faint-end slope cannot 

continue to indefinitely faint galaxies—there 

must be a turnover or cutoff. As such, the 

behavior of the luminosity function at the 

faint end should reveal the degree to which 

faint galaxies powered cosmic reionization. 

As in the local universe, the dark matter 

power spectrum defines the scaffolding 

upon which high-redshift dwarf galaxies are 

built. Star formation within dwarf galaxies 

then plays multiple roles, not the least of 

which is as a source of ionizing photons. 

While there are not yet enough identified z 

≥ 7 sources of ionizing photons to account 

for the universe’s far UV transparency at z < 

6 (e.g., Schenker 2015), the balance would 

tip in favor of high-z galaxies if there are large 

numbers of them that are fainter than what 

we presently can detect.

After reionization, the UV background 

should suppress star-formation and gas 

accretion onto halos at log (M/M) < 9 (see 

e.g., Iliev et al. 2007; Mesinger & Dijkstra 

2008; Alvarez et al. 2012, Jaacks et al. 2013). 

Additionally, halos with virial temperatures 

less than 104 K (log [M/M] < 8; Okamoto 

et al. 2008; Finlator et al. 2012) will not be 

able to cool gas via atomic line emission, 

and so are not expected to host efficient star 

formation. These processes should result in 

a turnover in the number density of very faint 

Program at a Glance - Dwarf Galaxies and the Matter Power Spectrum

Goal: Measure matter power spectrum on small scales (< 100 kpc) to discriminate between 

standard cold dark matter, interacting dark matter, and several warm dark matter models 

at a statistical significance of 4-s or better (at k = 40 h/Mpc). 

Program details: Observations of 4 Milky-Way analogs in the 5–15 Mpc distance range to 

gain a representative sample of the entire luminosity range of dwarf galaxies by detecting 

their constituent horizontal branch stars at high significance. Total time required is ~390 

hours. 

Instrument(s) + Configuration: HDI instrument on LUVOIR-A (15-m) observatory. 

Optional: LUMOS MOS operated in parallel to study CGM.

Key observation requirements: Imaging in V, R passbands in multiple pointings to cover 

50% of the central 100 kpc region. Depth required corresponds to reaching S/N=5 per 

horizontal branch star with MV = 0.0. Survey fill factor is set to 50% to ensure reliable 

estimates of completeness. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-11

galaxies at z~7 (e.g., Jaacks et al. 2013). 

Existing Hubble observations of the Frontier 

Fields clusters have found no evidence for 

such a turnover down to MUV ≈ -14 mag, 

which corresponds to log (M/M) ≈ 9.5. A 

direct test requires pushing fainter, down to at 

least M = -13 mag. The apparent luminosity 

of such faint sources at z ~ 7 corresponds to 

tens of picoJanskys. LUVOIR will be capable 

of detecting extended sources at such low 

flux levels with a survey of several hundred 

hours per pointing, which would directly 

test the hypothesis that the UV background 

suppresses star formation and produces a 

significant turnover in the low-mass galaxy 

luminosity function.

Finkelstein (priv. comm.) and Jaacks et 

al. (2013) have placed important constraints 

on the shape of the faint-end of the z=7 

luminosity function. Using the modified form 

of the Schechter luminosity function with 

a turnover as described by Jaacks et al. 

(2013), the rest-frame UV luminosity function 

may exhibit a turnover that is detectable at, 

and fainter than, an absolute UV magnitude 

around M = -13.5 mag. Abundancematching with current best estimates of 

the UV luminosity function implies that this 

corresponds to halo masses log (Mh/M) 

≈ 9 at z = 7, consistent with theoretical 

expectations for the suppression mass. 

Observationally, this deviation would start to 

be seen in NIR passbands at AB ≈ 33 mag. 

LUVOIR-enabled deep imaging surveys 

that reach the turnover in galaxy LF that is 

predicted to occur due to UV background 

feedback suppression, will directly test 

this hypothesis. However, there are two 

additional observational constraints that 

must be accommodated. The first is that 

while the luminosity function must turn over, 

it cannot go to zero, at least not until well 

below the regime of dwarf galaxy progenitors. 

The second is the emissivity of ionizing 

photons in the intergalactic medium (IGM), 

which has been estimated using absorption 

features in z > 6 quasar spectra (e.g., Bolton 

et al. 2007; Becker & Bolton 2013). These 

two constraints set limits on the number of 

ionizing photons emanating from galaxies at 

a particular epoch.

Figure 6.6 shows the predicted 

number of galaxies per square arcminute 

per magnitude detected in scenarios with 

and without a turnover in the differential 

UV luminosity function. Such a survey 

would deliver 40% more galaxies (down to 

-13.5 mag) if no turnover is present. Users 

of the 15-meter LUVOIR could detect this 

difference in the number of observed galaxies 

in a single HDI pointing at a 3-s confidence 

level in about 340 hours of total exposure 

time, which includes enough time to observe 

each field in 3 passbands and is within the 

time allocation range of a large or multi-cycle 

treasury program. We have assumed that 

these ultra-faint z = 7 galaxies are resolved 

(~0.1 arcsec) with radii in the range of 200–

300 parsecs (e.g., see Ono et al. 2013). The 

9-m LUVOIR would require about 950 hours 

per pointing to achieve a similar detection.

Ideally, LUVOIR users would want to 

probe to these depths across both ionized 

and neutral regions (which in the 2030s, we 

should know of from SKA 21cm mapping, 

and WFIRST Lyman-a grism mapping; and/

or potential NASA Probe mission Lyman-α 

intensity mapping). Current theories predict 

that we should see this turnover in ionized 

regions, but not in neutral regions. Models 

of reionization are extremely sensitive to 

this, as the galaxies near this limit dominate 

the ionizing emissivity. They dominate the 

UV luminosity density due to their numbers, 

but they are also more likely to have larger 

ionizing escape fractions than their more 

massive counterparts. A survey consisting of 

such multiple pointings may only be feasible 

with the LUVOIR-A design.
LUVOIR The Large UV Optical Infrared Surveyor

6-12 The LUVOIR Interim Report

To observe directly the build-up of the 

extreme low mass end of the galaxy mass 

function from the epoch of reionization 

onwards LUVOIR users will require a 

telescope that can conduct a census of 

low-luminosity galaxies over a broad range 

of redshifts. Dwarf galaxies are faint even in 

the nearby universe and become much more 

so at higher redshifts. LUVOIR, however, will 

probe remarkably small stellar mass systems 

across a wide redshift range. To demonstrate 

this, we employ the Gonzalez et al. (2012) 

stellar mass–luminosity (M-L) relation for highredshift galaxies and derive the expected 

stellar mass scales that can be probed for 

space telescopes of different apertures. 

Figure 6.7 shows the results for a 500 ksec 

exposure with a Nyquist-sampled imager. 

LUVOIR architectures enable the study of 

the extreme low mass end (M* < 105.5 M) 

of the halo mass function at many redshifts 

in a single deep survey and enables wider 

surveys of 106–107 M systems in much 

shorter exposures.

Studying the faint end of the luminosity 

function at high-redshift also allows users 

of LUVOIR to seek the progenitors of the 

local dwarf galaxy populations, and identify 

their role in the process of reionization, in the 

context of the underlying dark matter power 

spectrum properties that allows them to 

form in the first place. There must be selfFigure 6.6. Predictions for the number of z=7 galaxies per square arcminute per magnitude with 

reionization suppression (red histogram) and without reionization suppression (black histogram). 

The effects of reionization suppression manifest as a turnover in the luminosity function at an 

absolute UV magnitude around M ≈ -13.5 mag. The depths reached with the two LUVOIR 

architectures are shown as a function of survey time. Such a survey could deliver up to 40% 

more galaxies if no turnover is present.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-13

consistency between the galaxy populations 

expected at early times with the total galaxy 

populations that we detect subsequently. 

Part of the consistency check is against the 

dwarf galaxy census LUVOIR will perform in 

the local universe. As noted above, the faint 

end of the UV luminosity function can have 

a steep slope; a low-luminosity cutoff; or a 

combination of these. At z~7, the ultraviolet 

luminosity of local dwarf galaxy progenitors 

can be calculated (Boylan-Kolchin et al. 

2015). This is shown in Figure 6.8. The most 

luminous local dwarf is the Large Magellanic 

Cloud, and its progenitor at z~7 has a restframe absolute ultraviolet magnitude of 

-15.8. A LUVOIR ultra deep field (500-hour 

exposure) will reach AB = 33.5, corresponding 

to MUV = -13.5 mag at z~7 for resolved dwarf 

galaxies enabling not only the detection of 

LMC progenitors but progenitors of the Small 

Magellanic Cloud (SMC) as well.

6.2 Signature Science Case #2: 

Constraining dark matter via high 

precision astrometry

Dwarf spheroidal galaxies (dSph) in the local 

universe are extraordinary sites to explore 

the properties of non-baryonic dark matter. 

First, their mass is dominated by dark 

matter—they have mass-to-light ratios 10 to 

100 times higher than the typical L* galaxy 

like the Milky Way (Martin et al. 2007; Simon 

& Geha 2007; Strigari et al. 2008). Second, 

they are relatively abundant—over 40 dSph 

galaxies have been found in the Local Group 

(Simon et al. 2011; Torrealba et al. 2016) 

and more will be discovered in the LSST era. 

Third, and perhaps most striking, is that all 

Figure 6.7. The sensitivity to detecting galaxies of a particular stellar mass (y-axes) as a function 

of redshift and telescope aperture for a fiducial 500 ksec observation that returns a S/N = 5 

detection of a 200-parsec diameter source. The notional stellar mass ranges for bright, classical, 

and ultra-faint dwarf galaxies are indicated.
LUVOIR The Large UV Optical Infrared Surveyor

6-14 The LUVOIR Interim Report

well-studied dSph galaxies, covering more 

than four orders of magnitude in luminosity, 

inhabit dark matter halos with the same mass 

(~107 M) within their central 300 pc (Strigari 

et al. 2008; see Figure 6.9 here).

The ability of dark matter to cluster in 

phase space is limited by intrinsic properties 

such as mass and kinetic temperature. Cold 

dark matter particles have negligible velocity 

dispersion and very large central phase-space 

Program at a Glance - The High Redshift Luminosity Function

Goal: Detect the turnover in z = 7 galaxy luminosity function predicted by models for the 

impact of reionization on early star formation.

Program details: Observations of 4 fields, 2 in regions with HI emission detected via SKA 

or other deep radio survey. LUVOIR imaging must be deep enough to allow detection of 

turnover at 3-sigma confidence level. Total time required for 4 fields is 1400 hours. 

Instrument(s) + Configuration: HDI instrument on LUVOIR-A (15-m) observatory.

Key observation requirements: Broadband imaging in I, J, H passbands for a total of 

~340 hours per field (S/N = 5 per object at a mag limit of 33.5). The time per field is 70% in 

I-band and 15% each in the J and H-bands. Galaxies at z~7 selected using Lyman break 

method. 

Figure 6.8. The luminosities of Local Group dwarf galaxies, extrapolated to z~7, based on Figure 

4 of Boylon-Kolchin et al. (2015). The observed z~7 ultraviolet luminosity function and the best 

fit to the data (solid line) and an extrapolation of that fit (dashed line) are also shown. The arrows 

show limits that can be reached with unlensed deep HST imaging (HUDF), anticipated JWST 

deep fields with a limiting magnitude of AB = 31.5 mag, and with LUVOIR-A 15-m which, in 100 

hours, reaches a 5-s limiting magnitude of AB = 33.5 mag (corresponding to MUV(z~7) = -13.45 

mag) for extended 0.10” sources and AB = 34.4 mag for point sources.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-15

density, resulting in steep central density 

profiles. In contrast, dark matter halos with 

highly relativistic particles (i.e., warm dark 

matter) have smaller central phase-space 

densities, so that density profiles saturate 

to form systems with flat central density 

profiles (constant central cores). Owing to 

their small masses, dSphs have the highest 

average phase space densities of any galaxy 

type, which implies that for a given dark 

matter model, phase-space limited cores 

will occupy a larger fraction of the virial radii. 

Hence, the mean density profile of dSph 

galaxies is a fundamental constraint on the 

nature of dark matter.

Current observations are unable to 

measure the density profile slopes within dSph 

galaxies because of a strong degeneracy 

between the inner slope of the dark matter 

density profile and the velocity anisotropy 

of the stellar orbits. Radial velocities alone 

cannot break this degeneracy even if the 

present samples of radial velocities are 

increased to several thousand stars (Strigari 

et al. 2007). The only robust way to break 

the anisotropy—inner slope degeneracy is 

to combine proper motions with the radial 

velocities. The required measurements 

include proper motions for ~100 stars per 

galaxy with accuracies better than ~10 km/

sec (i.e., < 40 mas/yr at 60 kpc) and ~1000 

line-of-sight velocities. 

Furthermore, the orbital motions of dwarf 

satellite galaxies within the halo of their more 

massive central galaxy can reveal important 

properties of the dark matter distribution of 

galaxies like our own Milky Way and M31 (e.g., 

see Massari et al. 2018). While such studies 

have been done with existing telescopes 

for our Galaxy and M31, LUVOIR users will 

be able to extend such investigations out 

to several Mpc and thus expand the range 

of galaxies whose gravitational potential 

is mapped in this way. Measuring orbital 

parameters for dwarf satellite galaxies 

requires proper motion measurements with 

Figure 6.9. The integrated mass within the inner 300 pc of local dwarf spheroidal galaxies as a 

function of their luminosity. The central density of these galaxies remains relatively constant over 

many orders of magnitude in luminosity. Figure credit: Strigari et al. (2008).
LUVOIR The Large UV Optical Infrared Surveyor

6-16 The LUVOIR Interim Report

accuracy similar to that noted above in order 

to see typical transverse orbital velocities of 

about 10 km/sec.

In the case of the brightest of these 

dSph galaxies such as Fornax and Sculptor, 

sufficient velocities and proper motions can 

be obtained using stellar giants. Groundbased large-aperture (~10–m) telescopes 

could measure the spectra, and Gaia or 

JWST can measure the proper motions. 

For the less massive dwarfs, where the 

dark matter dominance is the greatest, 

main sequence stars will have to be used to 

obtain the numbers of velocity and proper 

motion measurements needed. This will 

require larger (30-m class) ground-based 

telescopes for the velocities. The 30-m 

class telescopes may also be able to obtain 

the necessary proper motions but it will be 

extremely challenging: it will require precisely 

stitching many fields together, most of which 

are unlikely to contain enough background 

quasars of sufficient brightness to be useful 

as astrometric references.

However, the necessary astrometric 

precision can be readily achieved by LUVOIR 

users, given its multi-arcminute field of 

view, high stability, and the pixel geometry 

calibration system in the HDI instrument. 

Figure 6.10 shows the predicted transverse 

velocity error as a function of distance for 

LUVOIR-A. With a baseline of 5 years and 

100 stars per galaxy, a transverse velocity 

error of 10 km/s can be achieved out to 

4  Mpc (corresponding to a proper motion 

error of 0.5 mas/yr) using RGB stars as 

tracers and out to 5.5 Mpc (~0.4 mas/yr) 

using horizontal branch stars as tracers. 

Thus, LUVOIR and ELTs working together will 

provide some of the best constraints on the 

density profiles of satellite dwarf galaxies and 

the mass distribution around central galaxies 

Figure 6.10. The transverse velocity error from proper motion measurements achievable with the 

15-m LUVOIR as a function of galaxy distance. Green, red, and blue lines show the accuracies 

expected for solar-type main sequence stars, RGB stars, and HB stars, respectively, assuming a 

5-year baseline and 100 stars per epoch. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-17

in the nearby universe, revealing important 

knowledge about the kinematic nature of 

dark matter.

6.3 Signature Science Case #3: 

Tracing ionizing light over cosmic 

time

During the epoch of reionization, somewhere 

between a redshift of z=12 and z=6, 

corresponding to 360 Myr to 920 Myr after 

the Big Bang, the universe underwent a 

phase change from being radiation-bounded 

to density-bounded. The cause of this phase 

change was ionizing radiation1 emitted by 

the first collapsed objects. In the densitybounded phase the neutral material became 

enveloped in an expanding meta-galactic 

ionizing background (MIB). The emergence 

and sustenance of the MIB along with the 

accompanying evolution of large-scale 

structure are dependent on a key parameter, 

for which we have very little theoretical 

or observational guidance, namely the 

fraction of ionizing radiation, fe

LyC, escaping 

from these first and subsequent collapsed 

1 By ionizing radiation, we mean photons in the 

Lyman continuum (LyC) with wavelengths below the 

hydrogen ionization edge at 912 Å

objects. Uncertainty in the evolution with 

redshift of fe

LyC is a major systemic unknown 

(Finkelstein et al. 2015; Ellis 2014; Haardt & 

Madau 2012), impeding our understanding 

of the strength of ionizing radiation feedback 

on the formation of structure on hierarchical 

and secular timescales from reionization 

through to the present day.

At low redshifts, the uniformity of the 

MIB appears to be tied to the low end of the 

neutral hydrogen column density distribution 

of Lya forest absorbers, (12.5 < log[NH I (cm2

)] < 14.5), effecting the “ionizing photon 

underproduction crisis,” where a paucity in 

the number of low column absorbers found 

by Danforth et al. (2016) requires a MIB ≈ 2 

to 5 times higher than theoretical estimates 

(cf. Haardt and Madau 2012; Kollmeier et al. 

2014; Shull et al. 2015; Puchwein et al. 2015; 

Khaire et al. 2015; Gaikwad et al. 2017; 

Tonnesen et al. 2017). At higher redshifts, 

it is an open question whether the primary 

source of the MIB was the first black holes 

or the first stars (Madau & Haardt 2015). 

However, it is estimated that small galaxies 

could have dominated the ionizing radiation 

budget provided: the faint end slope of the 

galaxy luminosity function was steep, that it 

Program at a Glance - Connecting Proper Motions to Dark Matter

Goal: Measure proper motions of stars in 20 Local Group Dwarf galaxies.

Program details: Imaging of 20 dwarf galaxies done in conjunction with ELT groundbased spectroscopy. Total baseline will be 5 years with 3 epochs of observations for each 

target.

Instrument(s) + Configuration: HDI instrument on LUVOIR, with pixel geometry 

calibration using the onboard solid-state laser interferometer.

Key observation requirements: Broadband imaging in V and I passbands. The total 

exposure time per dwarf galaxy will depend on its distance but expected to be about 

20 hours per epoch. With up to 20 such galaxies and a total of 3 epochs per galaxy, we 

estimate this program would require about 1,200 hours of LUVOIR time spread over 5 

years (i.e., ~400 hours per epoch). A companion spectroscopic survey using a 20 to 40-m 

ground-based telescope will be needed to acquire ~1000 line of sight stellar velocities 

within each dwarf galaxy
LUVOIR The Large UV Optical Infrared Surveyor

6-18 The LUVOIR Interim Report

extended to absolute UV magnitudes M1500

< -13 mag, and that the escape fraction 

was 5% < fe

LyC < 40% (Bouwens et al. 2015; 

Finkelstein et al. 2015; Khaire et al. 2016, and 

references therein). As shown above, LUVOIR 

provides the ability to determine the faint-end 

shape of the luminosity function at z~7. Here, 

we discuss a different, complementary role 

for LUVOIR: characterization of the ionizing 

radiation itself.

Although JWST is designed in part to 

discover the source(s) behind the reionization 

epoch, it cannot directly observe the escape 

of ionizing radiation because of attenuation 

along the line-of-sight by overlapping HI 

clouds in the IGM with large neutral hydrogen 

column densities (log[NH I (cm2)] > 17.2, 

a.k.a. Lyman limit systems). The cosmic 

evolution of the IGM opacity is stark: the 

mean transmission as a function of redshift 

at z = [0.5, 1, 2, 3, 4, 5, 6] is TIGM = [0.97, 

0.96, 0.8, 0.5, 0.3, 0.08, 0.01] (Inoue et al. 

2014; McCandliss & O’Meara 2017), leaving 

less than 1% transmission by the IGM at 

the epoch of reionization. Therefore, the 

low-z universe, accessed through the UV, 

has a clear advantage for direct detection 

of escaping LyC, requiring only minuscule 

corrections for IGM attenuation. The ultimate 

goal is a statistically significant determination 

of LyC luminosity function evolution across 

cosmic time as envisioned by Deharveng et 

al. (1997) and Shull et al. (2015), providing 

a full accounting of the LyC escape budget 

from star-forming galaxies, AGN, and 

quasars of all types. These observations will 

allow determination of the sources and sinks 

of the MIB.

LUMOS will provide LUVOIR users with 

the ability to perform spatially resolved 

sampling of a large number of sources in a 

single observation over an extended field, 

allowing an in-depth characterization of the 

environmental factors that favor LyC escape. 

Spatial resolution is an important diagnostic, 

as models indicate that LyC escape exhibits 

gross variations, depending on the lineof-sight to an emitting region with respect 

to intervening neutral and ionized material 

in surrounding disks, superbubbles and 

circumgalactic streams (Dove & Shull 1994, 

Bland-Hawthorn & Maloney 1999, Dove et 

al. 2000, Shull et al. 2015).

McCandliss & O’Meara (2017) have 

calculated the intensities of LyC leakage 

expected from star-forming galaxies with 

characteristic UV L*1500(1+z), as provided by 

Arnouts et al. (2005) (z < 3), attenuated by 

various H I column densities. Examples are 

shown in Figure 6.11 for three galaxies 

spanning the range of redshifts accessible 

by the low resolution grating in LUMOS. 

The spectra show the characteristic sharp 

drop in flux at the Lyman edge followed by a 

recovery in transmission that is proportional 

to (l/911.8)3, which we refer to as a Lyman 

“drop-in.” McCandliss & O’Meara (2017) 

find that the absolute escape fraction, 

integrated over the entire EUV bandpass, 

is significantly greater than the relative 

escape fraction, fe

900, measured in a narrow 

window close to the Lyman edge, for log[NH I

(cm2)] > 17.25. An effort to spectroscopically 

resolve the Lyman drop-in signature with 

deep observations in the redshifted EUV will 

provide enhanced fidelity to determinations of 

f

e

LyC. Complementary high spectral resolution 

observations at longer wavelengths will 

be especially important for objects at 

progressively higher redshift to account for 

IGM attenuation due to Lya forest absorbers 

along the line-of-sight.

GALEX has shown that UV emitting 

sources are intrinsically faint and are widely 

separated in angle. LUMOS, with its 5 

arcmin2 (Wmsa = 3.0 x 1.6 arcmin2) FoV, high 

effective area and low background equivalent 

flux is the ideal instrument for carrying out 

a survey of LyC emitting objects. France 

et al. (2017) have outlined the expected 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-19

science return from a one hundred-hour pilot 

observing program devoted to probing LyC 

leakage in star-forming galaxies at redshifts 

z ≤ 1 in the far-UV. These expectations are 

informed by the estimated effective area for 

the G145LL grating of LUMOS (R ≈ 500) in 

the 15-m architecture (shown in Section 

9.3.2), where a peak Aeff(1150Å) ≈ 105 cm2 

and low dispersion produce a background 

limit of AB ≈ 34 mag.

The top panel of Figure 6.12 shows the 

cumulative number of star-forming galaxies 

observed to the indicated escape fraction, 

f

e

900, at the Lyman edge (5s detection limit) 

as a function of the apparent UV magnitude, 

m(1+z)900Å. The result is also summarized in 

Table 6.1. The bottom panel shows the 

cumulative number of galaxies sampled 

for LyC leak as a function of the rest frame 

absolute UV magnitude, M1500Å.

Figure 6.11. Attenuated SEDs at redshifts z = [0.2,0.5,0.8] for star-forming galaxies with 

characteristic apparent UV magnitudes of M1500 x (1+z) AB = [23, 20,18] mag, attenuated by 

various HI column densities. The green overlays on the curve indicate regions where a 10 hour 

LUMOS integration returns a S/N of 5 or greater.

Table 6.1. Number of observable galaxies in FOV 3.0 x 1.6 arcmin2 as a function of redshift interval 

Dz to the indicated, fe

900 limit at 5s, Dt = 10 hours, Dl = 30 Å. A one hundred-hour program will 

observe 10 such fields.

N(Dz) Totals

f

e

900 Dz=(0–0.2) Dz=(0.2–0.4) Dz=(0.4–0.6) Dz=(0.6–0.8) Dz=(0.8–1.2)

1.00 9 72 292 358 295 1028

0.64 8 64 225 270 217 786

0.32 6 54 152 175 135 525

0.16 5 46 102 113 82 350

0.08 4 38 68 72 48 232

0.04 3 31 45 45 27 153

0.02 3 25 29 27 14 100

0.01 2 20 18 15 6 64
LUVOIR The Large UV Optical Infrared Surveyor

6-20 The LUVOIR Interim Report

The construction of high-fidelity luminosity 

functions places a requirement on sample 

size, where 25 objects per luminosity bin 

per redshift interval will yield an approximate 

rms deviation of ~20% for each point. For 

the z < 1 redshift range proposed here, the 

luminosity range should be 2 to 3 orders of 

magnitude with ~20 to 40 bins covering 5 

redshift intervals. Redshifts are required for 

each object. These considerations suggest 

Figure 6.12. Top: The cumulative number of galaxies detected in a single LUMOS field of view 

as a function of redshift (colored lines) and escape fraction (different size circles) as a function 

of the 5-s limiting flux in a 10-hour observation over a 30 Å interval shortward of (1+z) * 911.8 

Å. Bottom: The cumulative number of galaxies detected in a single LUMOS field of view as a 

function of redshift (colored lines) and the absolute rest-frame 1500 Å magnitude.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-21

an observing program with 500 to 1,000 

objects per redshift interval, yielding 5,000 

to 10,000 objects total. Table 6.1 and 

Figure 6.13 demonstrate that LUVOIR 

users can meet this goal with 10 LUMOS 

fields observed for 10 hours each. Such a 

program would be a definitive exploration of 

LyC leakage at the faint end of the luminosity 

function for the 0 < z < 1 regime.

Quantification of how fe

900 changes with 

M1500Å is of the highest priority, especially 

for those objects with intrinsically low fe

900. 

In Figure 6.13, we show an estimate of 

how the number of detectable objects with 

f

e

900 limits would be affected by a change in 

aperture, under the assumption of a 10-hour 

observation in a Wmsa = 5 arcmin2 FOV. In the 

top panel, we plot the number of objects 

sampled to fe

900 < 0.1 as a function of redshift 

for the indicated telescope diameters. In 

the bottom panel, we display the total time 

required to populate the redshift bins of 

Figure 6.13. The upper panel shows how the number of detectable objects with low (< 10%) 

escape fraction rises with increasing telescope aperture. The lower panel shows the amount of 

time required to obtain at least 500 galaxies per redshift bin over the range 0.1 < z < 1.1. A 100-

hour LUMOS program on LUVOIR meets this threshold for nearly all redshifts.

Program at a Glance - Precision Measurements of Lyman Continuum Radiation

Goal: Detect and quantify the evolution of ionizing radiation of galaxies at 0 < z < 1

Program details: Low resolution spectroscopy of galaxies below the Lyman break over 

extended fields with LUMOS. 

Instrument(s) + Configuration: LUMOS G145LL 

Observations requirements: 500-1000 galaxies per z=0.2 bin detected at S/N = 5 or 

greater over a 30 Å window below the Lyman break. These requirements translate to 

acquiring 10 separate LUMOS fields, each with an exposure time of 10 hours.
LUVOIR The Large UV Optical Infrared Surveyor

6-22 The LUVOIR Interim Report

Table 6.1 with at least 500 galaxies, also 

as a function of telescope aperture. Easily 

seen is the precipitous drop in the number 

of objects sampled for the smaller telescope 

diameters at the highest redshifts and at the 

lowest levels of fe

900. These figures emphasize 

that a thorough quantification of the LyC 

escape phenomenon will require the largest 

telescope possible.

References

Arnouts, S., Schiminovich, D., Ilbert, O., et 

al. (2005). ApJ, 619, L43 

Alvarez, M. A., Finlator, K., & Trenti, M. 

(2012). ApJ, 759, L38

Barber, C., Starkenburg, E., Navarro, J. F., 

et al. (2014). MNRAS, 437, 959 

Baur, J., Palanque-Delabrouille, N., Yéche, 

C., et al. (2017). JCAP, 12, 013

Bland-Hawthorn, J., & Maloney, P. R. 

(1999). ApJL, 510, L33

Bouwens, R. J., Illingworth, G. D., Oesch, P. 

A., et al. (2015). ApJ, 811, 140

Boylin-Kolchin, M., Weisz, D. R., Johnson, 

B.D., et al. (2015). MNRAS, 453, 1503 

Buckley, M. R. & Peter, A. H. G. (2017). 

arXiv:1712.06615

Bullock, J. S. & Boylan-Kolchin, M. (2017). 

ARA&A, 55, 343

Danieli, S., van Dokkum, P., Merritt, A., et al. 

(2017). ApJ, 837, 136 

Danforth, C. W., Keeney, B. A., Tilton, E. M., 

et al. (2016). ApJ, 817, 111

Dooley, G. A., Peter, A. H. G., Yang, T., et al. 

(2017). MNRAS, 471, 4894 

Dove, J. B., & Shull, J. M. (1994). ApJ, 430, 

222

Dove, J. B., Shull, J. M., & Ferrara, A. 

(2000). ApJ, 531, 846

Table 6.2. Summary science traceability matrix for Chapter 6

Scientific Measurement Requirements Instrument Requirements

Objectives Measurement Observations Instrument Property Value

Determine the 

turnover in the 

high redsfhift 

galaxy luminosity 

function

Incidence 

frequency of z~7 

galaxies at faint 

magnitudes

3-band imaging at 

5σ to J > 33.5 for 

at least 4 fields

HDI UVIS & NIR 

filters

I (UVIS) J, H 

(NIR)

Constrain the 

nature of dark 

matter via dwarf 

galaxies

Matter power 

spectrum on 

small scales

Multi-band imaging 

down to MV = 0.0 

at 5σ to cover 

50% of central 100 

kpc region around 

N ≥ 4 Milky Way 

analogues

HDI UVIS filters V, R

Constrain the 

nature of dark 

matter via 

astrometric 

methods

Proper motion 

determination 

for stars in Local 

Group galaxies

Visible band 

imaging of 20 Local 

Group galaxies at 

high astrometric 

precision over 5 

years i+ ground for 

line of sight velocity 

spectra.

HDI UVIS filters, 

ultra-precise 

astrometry

V, I band. 

HDI precision 

astrometric 

mode.

Determine the 

escape fraction of 

ionizing radiation 

out to z~1

Amount of 

flux below the 

Lyman break for 

multiple galaxies

FUV spectroscopy 

for N>5000 0.2 < z 

< 1.2 galaxies from 

1000 to 2000 Å

LUMOS R~500 

Gratings

G145LL
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 6-23

Eisenstein, D. J., & Hu, W. (1999). ApJ, 511, 

5

Ellis, R. S. (2014). arXiv:1411.3330

Fan, X., Carilli, C. L., & Keating, B. (2006). 

ARA&A, 44, 415

Finkelstein, S. L., Ryan, R. E., Papovich, C., 

et al. (2015). ApJ, 810, 71

France, K., Fleming, B., West, G., et al. 

(2017). SPIE, 10397, 1039713

Gaikwad, P., Khaire, V., Choudhury, T. R., et 

al. (2017). MNRAS, 466, 838 

Geha, M., Wechsler, R. H., Mao, Y. Y., et al. 

(2017). ApJ, 847, 4 

Haardt, F. & P. Madau (2012). ApJ, 746, 125

Homma, D., Chiba, M., Okamoto, S., et al. 

(2016). ApJ, 832, 21

Iliev, I. T., Mellema, G., Shapiro, P. R., & 

Pen, U. L. (2007). MNRAS, 376, 534 

Inoue, A. K., Shimizu, I., Iwata, I., et al. 

(2014). MNRAS, 442, 1805 

Inoue, K. T., Takahashi, R., Takahashi, T., et 

al. (2015). MNRAS, 448, 2704 

Irsic, V., Viel, M., Haehnelt, M.G., et al. 

(2017). PhysRevD, 96, 023522

Jaacks, J., Thompson, R., & Nagamine, K. 

(2013), ApJ, 766, 94

Jethwa, P., Erkal, D., & Belokurov, V. (2018). 

MNRAS, 473, 2060 

Khaire, V. & Srianand, R. (2015). MNRAS, 

451, L30 

Khaire, V., Srianand, R., Choudhury, T. R., et 

al. (2016). MNRAS, 457, 4051 

Kim, S. Y., Peter, A. H. G., & Hargis, J. R. 

(2017). arxiv:1711.06267

Kollmeier, J. A., Weinberg, D. H., 

Oppenheimer, B. D., et al. (2014). ApJ, 

789, L32 

Li, R., Frenk, C. S., Cole, S., et al. (2016). 

MNRAS, 460, 363 

Livermore, R. C., Finkelstein, S. L., & Lotz, 

J. M. (2017). ApJ, 835, 113

Madau, P., & Haardt, F. (2015). ApJ, 813, L8 

Martin, N. F., Ibata, R. A., Chapman, S. C., 

et al. (2007). MNRAS, 380, 281 

Massari, D., Breddels, M. A., Helmi, A., et 

al. (2018). Nature Astronomy, 2, 156 

McCandliss, S. R. & J. M. O'Meara (2017). 

ApJ, 845, 111 

Menci, N., Grazian, A., Castellano, M., et al. 

(2016). ApJ, 825, L1 

Mesinger, A., & Dijkstra, M. (2008). MNRAS, 

390, 1071 

Ono, Y., Ouchi, M., Curtis-Lake, E., et al. 

(2013). ApJ, 777, 155

Puchwein, E., Bolton, J. S., Haehnelt, M. 

G., et al. (2015). MNRAS, 450, 4081 

Schenker, M. (2015). PhD Thesis, California 

Institute of Technology, Publication 

Number: AAT 3636854, ISBN: 

9781321187793, https://thesis.

library.caltech.edu/8626/

Schneider, A. (2015). MNRAS, 451, 3117 

Simon, J. D., & Geha, M. 2007, ApJ, 670, 

313

Simon, J. D., Geha, M., Minor, Q. E., et al. 

(2011). ApJ, 733, 46

Shull, J. M., Moloney, J., Danforth, C. W., et 

al. (2015). ApJ, 811, 3

Strigari, L. E., Bullock, J. S., & Kaplinghat, 

M. (2007). ApJ, 657, L1

Strigari, L. E., Bullock, J. S., Kaplinghat, M., 

et al. (2008). Nature, 454, 1096 

Tonnesen, S., Smith, B. D., Kollmeier, J., et 

al. (2017). ApJ, 845, 47 

Torrealba, G., Koposov S. E., Belokurov V., 

et al. (2016). MNRAS, 459, 2370 

Walsh, S. M., Willman, B., & Jerjen, H. 

(2009). AJ, 137, 450

Viel, M., Becker, G. D., Bolton, J. R., et al. 

(2013). PhysRevD, 88, 043502

Vogelsberger, M., Zavala, J., Cyr-Racine, F. 

Y., et al. (2016). MNRAS, 460, 1399 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-1

Chapter 7
LUVOIR The Large UV Optical Infrared Surveyor

7-2 The LUVOIR Interim Report

7 How do stars and planets form?

The Signature Science Cases discussed 

in this chapter represent some of the most 

compelling types of observing programs on 

stars, stellar evolution, and the local universe 

that scientists might do with LUVOIR at the 

limits of its performance. As compelling 

as they are, they should not be taken as 

a complete specification LUVOIR’s future 

potential in these areas. We have developed 

concrete examples to ensure that the nominal 

design can do this compelling science. 

We fully expect that the creativity of the 

community, empowered by the revolutionary 

capabilities of the observatory, will ask 

questions, acquire data, and solve problems 

beyond those discussed here—including 

those that we cannot envision today.

7.1 Signature science case #1: 

How do stars form?

The shape, nature, origin, and variability 

of the stellar initial mass function

Stars form from the fragmentation of the 

dense parts of molecular clouds, in regions 

referred to as “cores.” For all the simplicity of this description, significant challenges 

remain for a quantitative formulation of star 

formation. The relation between cores and 

stars is still matter of debate, as is the origin 

of the distribution of stellar birth masses (the 

stellar initial mass function, or IMF), the influence of the surrounding environment, and 

the effects of early dynamical evolution. The 

reasons for these debates are the significant 

observational challenges faced by measurements of the relevant quantities (cores and 

stars at different stages of formation and 

early evolution), and the fact that we do not 

have yet a predictive theory covering all 

stages of star formation. Extant models are 

able to describe individual observational results, but not link them. For instance, massive stars are notoriously difficult to form in 

simulations, and observations have not been 

able to determine how (fragmentation, coalescence) and where they form. Mass segregation has not been tested as a function 

of environment, nor is known whether it is a 

dynamical or primordial effect (Kryukova et 

al. 2012; Kirk et al. 2012; Pang et al. 2013; 

Elmegreen et al. 2014). Star clusters eject 

stars via N-body relaxation; the frequency of 

these runaways, which is around 4% in the 

Milky Way, has neither been established in 

other galaxies, nor studied as a function of a 

galaxy’s properties and environment. We are 

also still struggling to establish whether age 

spreads exist in young clusters, although 

some evidence for this exists for the R136 

cluster in the Large Magellanic Cloud (LMC), 

with a spread of a few Myr.

The two problems feed into each other: 

without a theory, we cannot unequivocally 

interpret the observations of resolved or 

unresolved stars; and without unambiguous 

measurements, we cannot formulate 

a theory. In order to break this circular 

ambiguity, we need to perform unambiguous 

measurements of the early stages of star 

formation and the stellar IMF in a vast range of 

environments and parent galaxy properties, 

to guide models and finally achieve one of 

the key goals of modern astrophysics: a 

theory of “how stars form.”

Accomplishing the characterization and 

quantification of star formation and the stellar IMF will require the synergistic combination of observations from a broad wavelength 

range, UV to millimeter. This combination is 

partially offered by current (Hubble, ALMA, 

VLT, etc.), upcoming (JWST), and planned 

(30m-class ground-based telescopes) facilities. Among the capabilities that are neither 

available nor planned, and which will be re-
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-3

quired to accomplish the overarching goal of 

formulating a theory of star formation, are:

1. Angular resolution for imaging: 0.007" 

at 500 nm (Figure 7.1 and Figure 7.2), 

with stable point spread function (PSF) 

and stable astrometry across the entire 

field of view (FoV).

2. UV and optical imaging spectroscopy 

over 1'–3' FoV, with 0.01"–0.02" angular 

resolution.

3. High-sensitivity UV spectroscopy with 

resolving power R > 3000, to resolve 

atomic and molecular lines from winds 

and photospheres in stellar spectra.

Determining the nature, average properties and relevant physical parameters, and 

potential environmental dependencies of star 

formation (and its corollary: the stellar IMF) 

across the full stellar mass range, from the 

high to low mass ends, will require synergy 

across multiple wavelength regions, from the 

UV to the mm. Important characteristics are 

poorly constrained, including merger rates, 

binary fractions below O stars, multiplicity, 

fraction of runaway stars, and whether very 

massive stars (VMSs, i.e., stars with masses 

> 150 M) exist and are common. In addition 

to its primary and unique role in achieving a 

full understanding of star formation, LUVOIR 

will play a complementary role in characterizing the shocks, jets, and outflows from the 

individual protostars and protoclusters, and 

their natal cores, identified by ALMA.

Figure 7.1. The resolvable separation, in AU, 

between two stars, as a function of telescope 

diameter, for binaries in the Westerlund 2 starforming region (at a distance of 4 kpc) in the 

Milky Way and in the Large Magellanic Cloud 

(50 kpc). A 15-meter telescope fully separates 

binary systems down to ~30 AU in the starforming region and down to ~350 AU in the 

Large Magellanic Cloud. Massive binary stars 

at different stages of evolution are separated 

by distances between a few tens and a few 

thousand AU (Portegies Zwart et al. 2010), 

and a 15-m telescope will resolve binaries at 

critical stages of evolution. With this telescope, 

spectral signatures of very massive stars within 

dusty star clusters will be detectable in ~200 

luminous infrared galaxies and ~20 ultraluminous infrared galaxies out to about 150 

Mpc.

Figure 7.2. An inner 8.4" by 8.4" region of 

the galaxy IC 4247 from Calzetti et al. (2015), 

as viewed by a 15-m LUVOIR and a 4-m 

telescope if IC 4247 was a distance of 30.8 

Mpc. At this distance, each box is 1.3" on a 

side, corresponding to 31 pc. The 4-m image 

would need ~16 times the exposure time as the 

LUVOIR image.
LUVOIR The Large UV Optical Infrared Surveyor

7-4 The LUVOIR Interim Report

7.1.1 Very massive stars

VMSs are stars that exceed the standard 

limit of 150 M, and one needs to observe 

very young (<~1.5 Myr), massive star clusters 

(>105 M) in order to detect their presence, 

due to small number statistics at the high end 

of the IMF, and rapid evolutionary timescales. 

Yet these stars can heavily influence their 

surrounding environment; for instance, they 

can provide between 25% and 50% of the 

ionizing photon flux from the host cluster.

A solid case for the presence of VMSs 

in R136, the central cluster in 30 Dor in the 

Large Magellanic Cloud, has been recently 

made by Crowther et al. (2010, 2016). UV 

spectroscopy has provided evidence for the 

potential presence of VMSs in an additional 

two unresolved star clusters in NGC 

3125 (Wofford et al. 2014) and NGC 5253 

State of the Science in 2030

By 2030, JWST and WFIRST will have observed the stellar populations down to 0.3 solar 

masses for all galaxies within the local 0.5 Mpc, with lower limits for nearer galaxies. This 

will test theories of low-mass star formation, and constrain variations of the low-end of the 

stellar initial mass function in low-density environments. ALMA will have characterized the 

populations of gas cores in several nearby galaxies, and will have shed light on the relation 

between the core mass function and the stellar initial mass function around the low mass 

turnover. Improved models will help interpret the complex chemistry of the pre-stellar 

cores. In parallel, adaptive optics-assisted 30-m class telescopes will have assembled 

large samples of binary stars at a range of masses out to the Magellanic Clouds, using 

both radial velocity (RV) and proper motion techniques. Combinations of these facilities will 

have increased the number of candidate galaxy hosts of very massive stars, i.e., stars with 

masses > 150 M, by at least tenfold, up from the current census of a handful.

By 2030 NASA’s TESS mission will have identified a thousand planets smaller than 

Neptune with orbital periods less than 40 days around nearby stars (Ricker et al. 2014); 

we can expect that many will have been further characterized using ground-based (e.g., 

ELTs) and space-based facilities (e.g., JWST). NASA’s WFIRST and ESA’s PLATO are 

expected to launch around 2024 and will extend exoplanet detections out to and beyond 

the habitable zone, with the latter mission focusing on bright solar-type stars. Groundbased RV monitoring will have extended the statistics of planets larger than Super-Earths 

out to a few AU (e.g., Pepe et al. 2010). Thus, by 2030 the exoplanet population of nearby 

stars within a few AU will be fairly well characterized. In relation to planet-forming disks, 

ALMA will have delivered hundreds of continuum disk images at a resolution of ~5–10 AU 

in nearby star-forming regions, thus fully characterizing the mid-plane dust emission in the 

giant planet-forming region. Many of these disks will also have AO-assisted ground-based 

and/or WFIRST scattered light imagery at similar spatial resolution, probing the population 

of small grains and the overall disk structure. JWST will have surveyed hundreds of these 

young stars at a resolution of > 20 AU, and probed material ejected in jets/outflows as 

well as warm (~500 K) disk gas inside the snowline via medium-resolution (Dv~100 km/s) 

spectroscopy.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-5

(Figure 7.3; Smith et al. 2016). Detection 

of supernovae that have characteristics 

typical of pair-instability SNe provide indirect 

evidence for the presence of VMSs. Whether 

the VMSs are the result of birth conditions 

or of mergers is still matter of debate, but 

models can successfully reproduce many 

observational properties using a rotating 

single VMS (Yusof et al. 2013, Kohler et 

al. 2015). Data on the VMS numbers and 

Figure 7.3. Identifying very massive stars (VMSs) in star clusters. NGC 5253 (left panel) is an 

amorphous dwarf located at 3.15 Mpc distance, and hosting a starburst within its central ~250 

pc region (orange rectangle in the left panel, and central-top panel, which shows a three-color 

combination from HST imaging). Most of the dust and molecular gas in this galaxy are concentrated 

in a ~20 pc region located at the north end of the starburst (yellow rectangle in the center panel, 

and right-top panel), where also the two main radio peaks are located (yellow circles in the topright panel; Turner et al. 2000). The two radio peaks coincide with two clusters identified in the 

HST images (magenta circles in the top-right panel; called 5 and 11 by Calzetti et al. 2015). UVto-K HST photometry shows that the two clusters are ~1 Myr old, with masses 1–3 x 105

 M, 

and with AV ~2–50 mag of dust. UV spectroscopy of cluster 5 further reveals presence of VMS 

signatures: P Cygni NV (1240 Å) and CIV (1550 Å) profiles, broad He II (1640 Å) emission, blueshifted OV (1371 Å) wind absorption, and absence of SiIV (1400 Å) P Cygni emission/absorption 

(bottom panel; Smith et al. 2016), as confirmed from the comparison with the UV spectrum of 

the LMC cluster R136a, also containing VMSs (Crowther et al. 2016). The HST UV spectrum of 

cluster 5 required 3 hours of exposure. The same amount of time with LUMOS on a 15-m will 

detect with S/N~10 a 106 M cluster with similar dust properties as cluster 5 out to a distance 

of 80 Mpc (25 times further away than NGC 5253). LUVOIR will increase by at least tenfold the 

parameter space in distance and galactic properties (morphology, dust content, star formation 

rate, etc.) of the systems whose VMS content can be identified and characterized.
LUVOIR The Large UV Optical Infrared Surveyor

7-6 The LUVOIR Interim Report

frequency in young star-forming regions are 

lacking, due to observational limitations: they 

can only be recognized in the UV, and the 

cluster needs to be resolved into individual 

stars in order to obtain a census of VMSs. 

Individual cluster stars can only be resolved 

to the distance of the LMC with Hubble 

(Crowther et al. 2016). Reaching beyond the 

Magellanic Clouds, to resolve into individual 

stars the young populations in the starforming galaxies of the Local Group out to ~1 

Mpc (e.g., NGC 6822, IC 1613, NGC 3109, 

Gr 8, the unusual nearby bright starburst 

IC 10) will offer a significant increase in the 

potential number of individually detected 

VMSs.

In addition to a census of VMSs within 

individual clusters, a census of the frequency 

of VMSs in different environments is required 

to gauge their overall impact on the evolution 

of galaxies, and to constrain models for 

their formation and evolution. Ideally, one 

would sample a wide range of very young 

star clusters in as wide a range of galaxies 

as possible, including luminous infrared 

galaxies (LIRGs) and ultra-luminous infrared 

galaxies (ULIRGs). Presence of VMSs can be 

inferred using unique UV spectral signatures: 

P Cygni NV (1240 Å) and CIV (1550 Å) 

profiles, broad He II (1640 Å) emission, blueshifted OV (1371 Å) wind absorption, and an 

absence of SiIV (1400 Å) P Cygni profiles. In 

the optical, a VMS would be easily confused 

with WR emission from a lower mass O-star. 

Thus, UV spectroscopy is a key requirement 

for obtaining a census of VMSs and other 

massive stars in the nearby Universe, 

together with quantitative measurements of 

their properties.

UV multiplexing (multi-object spectrograph or integral field spectrograph) to obtain 

spectra of resolved massive stars in clusters 

out to 0.7–1 Mpc, and spectra of unresolved 

young, massive star clusters out to 150 Mpc 

(Arp 220, the prototype ULIRG, is at 77 Mpc 

distance) is a minimum requirement, and will 

secure about 200 LIRGs and ~20 ULIRGs. 

Large detector formats increase efficiency by 

covering entire star clusters/star-forming regions and/or entire star cluster populations in 

a single pointing. A large telescope aperture, 

~15-m, will ensure both enough sensitivity 

(~2 hours exposure to reach S/N = 10 at 160 

nm on a compact cluster with mAB(FUV) = 23 

mag; this is a 1 Myr old, 106 M cluster at 

a distance of 80 Mpc and embedded in 20 

mags of extinction, as expected in Arp 220) 

and angular resolution for resolving individual 

stars within clusters out to 0.7–1 Mpc (0.01" 

resolution in the UV corresponds to a separation of 0.03 pc at 0.7 Mpc distance). As 

such, LUVOIR+LUMOS represents an ideal 

combination for VMS studies.

7.1.2 Stellar multiplicity

Stellar multiplicity and binary frequency 

constrain models of star formation and the 

IMF (Offner et al. 2014). Hydro-dynamical 

simulations show that massive stars require 

dense and massive accretion disks to 

form, as these are needed to overcome 

the radiation pressure barrier. The disks 

tend to be unstable and break into complex 

systems. The resulting properties of the 

multiple systems (number of companions, 

distribution of separations [i.e., short vs. 

long orbital periods], distributions of mass 

ratios, and their dependence on the stellar 

mass) are model-dependent. Furthermore, 

the dynamics of binaries drive the evolution 

of star clusters while, simultaneously, the 

combination of cluster dynamics and internal 

stellar processes determine the internal 

evolution of each binary (Portegies Zwart 

et al. 2010), but, again, there is enormous 

dependency on uncertain parameters. 

Short-period (spectroscopic) binaries 

will remain the domain of ground-based 

telescopes, especially the upcoming 

integral field spectrographs on adaptive 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-7

optics-assisted (AO-assisted) 30-m+ class 

telescopes. Long-period binaries require 

high angular resolution, a very stable PSF, 

and high precision photometry across 

the entire FoV, which needs to be of a few 

arcminutes, in order to increase efficiency by 

targeting each cluster with as few pointings 

as possible. Upcoming AO-assisted red/

near-IR cameras (e.g., ELT/MICADO) may 

achieve the required level of stability over 

large FoVs, and deliver proper motions as 

accurate as 10 mas/yr over 4 years. However, 

the physical properties of the stars need to be 

characterized in order to constrain models. 

This demands measuring the resolved

massive stars’ winds and photospheric 

parameters (including bolometric luminosities 

and masses), which can be accomplished 

only with spectroscopy of individual stars in 

the 100–200 nm range (Wofford et al. 2012). 

Binaries at different stages of evolution 

within a star cluster have mean separations 

that change between a few tens and a few 

thousand AUs (Portegies Zwart et al. 2010). 

Resolving binaries with UV spectroscopy 

at all stages of evolution requires resolving 

mean separations across a wide range from 

tens to hundreds of AU; probing a range of 

environments requires reaching at least the 

distance of the Magellanic Clouds. LUMOS 

on LUVOIR-A will resolve binary stars with 

separations down to ~40 AU in the iconic 

high-mass star cluster Westerlund 2 (~4 kpc 

from the Sun) and ~500 AU in the LMC.

The diffraction-limited angular resolution 

HDI on LUVOIR-A (0.007") will resolve and 

image all binary stars down to ~30 AU 

separation in Westerlund 2 (Figure 7.1), and 

down to smaller separations in the many 

dozens of intervening star-forming regions 

spanning a wide range of densities and other 

properties. A 30 AU separation is found for a 

system of two equal-mass late-type B stars 

(~3 M each) with a 25-year orbital period. 

The tens-AU regime is interesting for the 

investigation of the progenitors of peculiar 

objects and GRBs, as models suggest 

massive stars may be interacting in the postMS phase. This will also address numerous 

additional related questions, including the 

IMF of binary stars, and the role of binary 

stars in determining the upper end of the IMF 

(Figure 7.4).

7.1.3 The low-end of the IMF in 

unresolved populations

Recent observational results suggest 

variations at the low mass end of the 

stellar IMF (< 1 M), although small number 

statistics and degeneracies plague all current 

studies. Analysis of spectroscopic features 

of the integrated light from elliptical galaxies 

indicates that the slope of the low-end of 

the IMF becomes steeper, i.e., more bottom 

heavy, for increasing velocity dispersion (e.g., 

van Dokkum & Conroy 2010; Conroy & van 

Dokkum 2012; Spiniello et al. 2014, 2015; 

van Dokkum et al. 2017), in agreement with 

results from analyses of the mass-to-light 

ratio in the same systems (Cappellari et al. 

2012). However, measurements of lensed 

early type galaxies and of IR spectral features 

have challenged the robustness of IMFsensitive spectral features and of mass-tolight ratio determinations (Smith et al. 2014, 

2015).

Direct IMF determinations, i.e., counting 

stars in resolved stellar populations, yield 

a cleaner result, although current facilities 

cannot reach far enough to probe a rich 

range of galaxy properties. Geha et al. (2013) 

have investigated the stellar content of two 

ultra-faint dwarf galaxies (UDGs) using HST 

multi-color imaging, concluding that their 

IMFs are significantly flatter than either 

those of Kroupa or Salpeter in the 0.5–0.75 

M range. When evaluated together with 

direct low-mass IMF determinations in the 

Magellanic Clouds and the Milky Way, Geha 

et al. conclude that there is a systematic 
LUVOIR The Large UV Optical Infrared Surveyor

7-8 The LUVOIR Interim Report

trend for lower velocity dispersion and lower 

metallicity systems to have flatter IMFs, in 

agreement with the trends observed for the 

early type galaxies. Conversely, Wyse et al. 

(2002) determine that the low-mass IMF 

down to 0.3 M in Ursa Major, another of 

the Milky Way satellites, is similar to the IMF 

of dense Milky Way globular star clusters. 

These contradictory results may stem from 

the fact that the mass range directly probed 

in the Milky Way satellites is too close to the 

turnover of a Kroupa/Chabrier IMF for an 

unambiguous determination of the faint-end 

slope, and the measurements need to be 

pushed below 0.2–0.3 M to yield reliable 

results (Offner et al. 2014).

Near-future facilities (JWST and WFIRST) 

will make in-roads for direct star counting in 

nearby galaxies, mostly in the satellites of 

the Milky Way. JWST will resolve individual 

stars down to 0.3 M for all Milky Way 

dwarf satellite galaxies out to ~0.5 Mpc, 

and down to the hydrogen burning limit for 

the nearest dwarfs, thus enabling testing 

of models of low mass star formation and 

probing variations of the low-end of the IMF. 

The IR capabilities on JWST will provide a 

factor of ~2 improvement in efficiency over 

HST for detection of low-mass stars (Geha 

2014), and repeat observations to separate 

populations (foreground from external ones) 

via proper motions will further increase the 

efficacy of the approach.

Conversely, the central regions in elliptical 

galaxies will remain unresolved even with the 

most powerful telescope we can conceive; so 

Figure 7.4. Simulations of binary systems under different assumptions of binary fractions and 

mass ratios. In all cases the goal is to quantify the input IMF of the resolved stars, in order to 

reproduce, for the unresolved binary systems, the observed Salpeter stellar IMF (slope = -2.35) in 

the mass range 0.5–150 M. The left panel shows the case of equal mass binaries, while the right 

panel shows the case in which the binaries have randomly selected mass ratios (down to stellar 

masses of 0.01 M). In both panels, the black line shows the case of 0% binary fraction, i.e., 

the output (`observed’ for unresolved binaries) IMF, expressed as number of stars per mass bin. 

The purple and blue lines show the cases of fixed binary fractions, 20% and 70%, respectively. 

The orange line assumes mass-dependent binary fractions: 70% for M > 10 M, 20% for 2 M

< M < 10 M and 10% for M < 2 M. Equal mass binaries (left panel) are drawn from an intrinsic 

IMF that is not dissimilar from the output one, with the exception of a mass break at a few M. 

Conversely, binaries with random mass ratios (right panel) need to be drawn from intrinsic IMFs 

that are potentially very different from the observed one: the cases with fixed binary ratios display 

a dramatic flattening of the intrinsic slope for masses below 10 M, while the case with variable 

binary fractions could be drawn from an intrinsic IMF with a steeper high-end slope than the 

observed one.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-9

indirect (integrated population) methods will 

remain the only option. The central regions 

of ellipticals appear to be special places in 

many respects: they host supermassive 

BHs; show high velocity dispersion/deep 

potential wells and high [α/Fe] abundance 

patterns; and are the centers of the first 

collapsing structures, with very high inferred 

star formation rates when the bulk of the stars 

formed, etc. Several physical models for the 

IMF seem to suggest that these would be 

prime sites for IMF variation. Among these 

galaxies are archetypes like M87, M60, and 

M49 (all Virgo ellipticals), but also the nearby 

field ellipticals Centaurus A and NGC 3377.

One of the most promising avenues 

for constraining the low-end IMF is the 

exploitation of the surface brightness 

fluctuation technique at the spectroscopic 

level, called “fluctuation spectroscopy” (e.g., 

van Dokkum & Conroy 2014). The basic idea 

is the following: for nearby elliptical galaxies 

the number of giants per pixel is small and 

so their numbers will fluctuate (via Poisson 

statistics) from pixel to pixel. One can therefore 

identify pixels containing relatively few giants. 

By focusing on “low fluctuation” pixels one 

can obtain a relatively unobstructed view 

of main sequence stars, which are easier 

to model in integrated light than the entire 

population. The breakpoint is when the mean 

number of stars per resolution element is < 

105, which is where the pixels are dominated 

by MS stars, and there are very few, if any at 

all, giants. These resolution elements need to 

be observed with multi-object spectroscopy 

between 400 nm and 1000 nm (an extension 

to the LUMOS bandpass currently under 

consideration), in order to separate M dwarfs 

from F and G dwarfs, and infer variations at 

the low-mass end of the IMF.

Currently, HST delivers a mean number 

of stars per resolution element (~0.2") of 

~107 in the central regions of Virgo ellipticals, 

implying that an improvement of about a 

factor 100 in diffraction-limited region area 

is required. This demands ~0.01" angular 

resolution spectroscopy over arcminutesized fields. Lack of blue coverage (JWST) 

or large FoV (e.g., HARMONI on ELT) makes 

these future facilities inadequate for achieving 

this goal. For future AO-assisted integral field 

spectrographs, the main challenge will be to 

control the stability of the PSF and plate scale 

over a large FoV at blue wavelengths. To 

accomplish a major breakthrough, 400–1000 

nm multi-object spectroscopy over several 

arcminutes FoV, with a 10x improvement 

Program at a Glance - The Nature of the IMF

Goal: Constrain massive star formation and the stellar initial mass function. 

Program details: UV and optical spectral signatures of individual stars and star clusters / 

groups in nearby galaxies (d < ~150 Mpc). Imaging of stellar systems to identify multiplicity, 

and follow-up multi-object spectroscopy in the 100–200 nm range to characterize stars 

in multiple systems. NUV-optical multi-object spectroscopy of the centers of ellipticals to 

constrain variations at the low-end of the IMF. 

Instrument(s) + Configuration(s): HDI NUV and optical imaging; LUMOS multi-object 

UV spectroscopy (with extension into optical)

Key observation requirements: About 250 targets; Imaging resolution of 0.007"; 

Imaging SNR = 10–300; Multi-object spectroscopy angular resolution of 0.01"–0.02"; 

Spectroscopy SNR ≥ 10 over 100–1000 nm; Spectroscopy resolution R > 3000 to resolve 

stellar wind and photospheric lines.
LUVOIR The Large UV Optical Infrared Surveyor

7-10 The LUVOIR Interim Report

in spatial resolution over HST at 500 nm is 

required. Independent resolution elements 

are 2 PSF widths in size, translating to a 

resolution requirement of ~0.01" at 500 nm.

7.2 Signature science case #2: 

How and in what environments do 

planets assemble?

Detecting protoplanets and tracing the 

composition and evolution of planetforming material

One of the main science objectives of LUVOIR 

will be to detect and characterize Earthsize planets in the habitable zone of nearby 

stars (Chapter 3). Understanding how 

and in what environment planets assemble 

contributes to this science objective in two 

critical ways: i) by determining which nearby 

planetary systems are most likely to host lifebearing planets and ii) by constraining planet 

properties that are not directly observable 

(e.g., bulk composition), which are important 

in interpreting the spectra of potential Earthlike planets.

Recent high-resolution images of 

circumstellar disks around young (1–10 Myr) 

stars revealed complex structures (e.g., 

ALMA Partnership 2015; Wagner et al. 2015; 

Andrews et al. 2016; Perez et al. 2016; Pohl et 

al. 2017; Hendler et al. 2018), some of which 

point to advanced planet formation. A few 

candidate giant planets have been recently 

detected around Myr-old stars (e.g., Sallum 

et al. 2015). Short timescales to assemble 

asteroid-size objects and giant planets are 

well in line with the evolution and dispersal 

of gas and dust in the solar nebula (Pascucci 

& Tachibana 2010). Thus, 1–10 Myr-old 

circumstellar disks provide an opportunity to 

study planet formation in action.

Constraints on planet formation also come 

from the properties of Gyr-old exoplanets 

and planetary systems (Chapter 4). Indeed, 

scaling relations in disks and exoplanets with 

the mass of the central star strongly suggest 

that sharp edges and gaps in young disks 

remain imprinted in the exoplanet population 

(e.g., Alexander & Pascucci 2012; Mulders 

et al. 2015). In addition, the composition of 

early planetary atmospheres should inform 

the formation history of planetary systems 

(e.g., Cridland et al. 2017).

At the distances of typical star-forming 

regions (e.g., Taurus-Auriga or Chamaeleon 

I), 1 AU corresponds to an angular scale 

of ~10 mas and gas probing ~1 AU radii 

will have FWHMs of ~30 km/s. Therefore, 

the high angular and spectral resolution 

of LUVOIR is a unique tool for mapping 

the inner disks where terrestrial planet 

formation occurs (Figure 7.5). Such highresolution observations will directly link birth 

environments to the final architecture of the 

exoplanetary systems. LUVOIR observations 

of young stars will detect and characterize 

protoplanets (Section 7.2.1), trace the 

composition, evolution, and dispersal of 

planet-forming material (Section 7.2.2), and 

map protoplanetary material dispersed via 

disk winds (Section 7.2.3).

Figure 7.5. Terrestrial planet-forming radii 

resolved at 500 nm as a function of telescope 

aperture for nearby star-forming regions with a 

range of massive star content.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-11

7.2.1 Detect and characterize 

protoplanets

As protoplanets are likely accreting gas 

through a circumplanetary disk, multiwavelength observations are necessary to 

separate disk emission from that produced 

in an accretion shock and thereby constrain 

the properties of forming planets. Accreting 

giant planets will glow in U- and B-band 

as well as UV and Balmer emission lines 

(e.g., Ha at 656.3 nm; Figure 7.6), while 

the disk will start to dominate at ~1000 nm 

(e.g., Zhu 2015). 30m-class ground-based 

telescopes with extreme AO systems will 

be diffraction limited at near-infrared and 

visible wavelengths over small field of view 

(~few square arcseconds) and will mostly 

probe re-processed emission from one 

circumplanetary disk at a time.

Jovian mass planets at 5–50 AU 

separations will carve deep gaps in their natal 

disks that can be readily resolved in LUVOIR 

coronagraphic scattered light images in 

the visible. UV/visible imaging in HDI filters 

sensitive to these accretion diagnostics will 

enable the direct detection of protoplanets 

within these gaps. Figure 7.6 shows an 

example of a simulated face-on disk + 

actively accreting protoplanet, at the distance 

of the Taurus star-forming region, in units of 

the signal-to-noise per resolution element of 

HDI’s narrow-band Ha filter. Conservative 

estimates of the accretion luminosity indicate 

that such intermediate distance Jovian/

Saturnian mass planets should be detectable 

in all nearby star-forming regions. Since these 

accreting planets can be detected without 

coronagraphy in narrow-band Ha imagery, 

nearby star-forming regions can be surveyed 

with HDI’s 3'  ×  2' field-of view, enabling a 

statistical study of protoplanets in both 

mass and semi-major axis. Furthermore, the 

accreting protoplanet survey will identify the 

brightest targets for follow-up spectroscopy 

in the FUV and NUV with LUMOS—enabling 

us to characterize the circumplanetary 

accretion environment in detail, a capability 

unique to LUVOIR.

Neptune-mass planets may be too faint 

to be directly detected but can dynamically 

carve detectable narrow (~1–10 AU) gaps 

in circumstellar disks. High-resolution 

scattered light images probe the population 

of small (sub-micron) grains that are coupled 

to the gas and, as such, can trace dynamical 

perturbations induced by planets. The 

narrow gap at ~80 AU in the nearby disk 

of TW Hya imaged with HST is a candidate 

for a sub-Jovian carved gap (Debes et al. 

2013). JWST will detect similar gaps at large 

radial distances (> 30 AU), but LUVOIR’s 

observational capability is unique for the 

Figure 7.6. Simulated protoplanetary disk (a

= 10-3; h/r = 0.04) scattered light image and 

accreting protoplanet (located in the dust gap at 

the upper left) at the distance of the Taurus starforming region. The 1 Jupiter mass protoplanet 

is located 30 AU from the host proto-K star, 

and is accreting with 1/100th the Hα luminosity 

of more massive, large-separation protoplanets 

(Zhou et al. 2014). In a 9000 s exposure with the 

HDI narrow-band Hα filter, the protoplanet is 

detected at S/N~20, the outer disk is detected 

at S/N between 1–2 per resolution element 

(easily binned to increase contrast), and the 

inner disk is detected at S/N between 2–12. 

Disk simulation courtesy of R. Dong (Steward 

Observatory/University of Arizona), based on 

work published in Dong & Fung (2017).
LUVOIR The Large UV Optical Infrared Surveyor

7-12 The LUVOIR Interim Report

study of planet forming regions inside ~10 

AU while still capturing structures at large 

distances thanks to its field of view.

While star-light suppression techniques 

(e.g., coronagraph, angular differential 

imaging) will be necessary for all instruments 

to detect exoplanets at ~1 AU, only LUVOIR 

can extend these studies into the UV, fully 

probe the accretion component, and help 

determine the properties of forming planets 

at Solar System scales. By comparing where 

exoplanets form in disks and their properties 

at birth with the location and properties of 

exoplanets around Gyr-old stars, we will 

be able to assess which processes shape 

planetary architectures and constrain the 

luminosity evolution of giant planets. In 

addition, we can test if the pile-up of single 

RV giants at ~1 AU results from initial 

conditions in the disk (e.g., the location of 

the snowline; Ida & Lin 2008) or rather disk 

dispersal processes (e.g., migration at the 

gap created by photoevaporation; Alexander 

& Pascucci 2012). These studies will clarify 

where and how planets form, constrain the 

mechanisms for gap formation and disk 

dispersal, and provide the initial conditions 

for the direct exoplanet characterization 

performed by LUVOIR.

7.2.2 Trace the composition, 

evolution, and dispersal of planetforming material

UV spectroscopy is a unique tool for 

observing the molecular gas in the inner 

disk; the strongest electronic band systems 

of H2 and CO reside in the 100–170 nm 

wavelength range (e.g., Herczeg et al. 2002; 

France et al. 2011). UV fluorescent H2 spectra 

are sensitive to gas surface densities lower 

than 10-6 g cm-2, making them an extremely 

useful probe of remnant gas at r < 10 AU. In 

cases where mid-IR CO spectra or traditional 

accretion diagnostics (e.g., Hα equivalent 

widths) suggest that the inner gas disk has 

dissipated, far-UV H2 observations can offer 

unambiguous evidence for the presence 

of a remnant molecular disk (Ingleby et al. 

2011; France et al. 2012; Arulanantham et 

al. 2018). 

LUVOIR’s multi-object, high-resolution 

capability (R > 30,000, 4.8 square arcminutes 

per LUMOS field) enables emission-line 

surveys and absorption-line studies of 

high-inclination (i > 60°) disks. Absorption 

line spectroscopy through high-inclination 

disks, currently limited to a small number 

of bright stars (e.g., Roberge et al. 2000, 

2001; France et al. 2014), are especially 

important because the large wavelength 

coverage of LUVOIR provides access to 

important molecular species in the UV and 

NIR, such as H2, CO, OH, H2O, CO2, and 

CH4. UV absorption line spectroscopy is 

the only direct observational technique to 

characterize co-spatial populations of these 

molecules with H2, offering unique access 

to absolute abundance and temperature 

measurements without having to rely on 

molecular conversion factors or geometrydependent model results as with emissionline spectroscopy.

Uniform spectral surveys of local starforming regions are required for a systematic 

determination of disk abundances (a direct 

measurement of the initial conditions for 

planet-formation) and gas disk lifetimes 

(the timescales for gas envelope accretion 

and migration of planetary cores through 

their natal disks). For star-forming regions 

beyond Taurus-Auriga, the efficiency of 

these surveys goes up dramatically with the 

introduction of multi-object spectroscopy, 

greatly reducing the total observing time 

and increasing the survey efficiency in a 

census of star-forming regions at distances 

less than 1 kpc. Combining these surveys 

with the detailed characterization of debris 

disks discussed in Chapter 4 enables us to 

trace the physical and chemical evolution of 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-13

the bulk of the disk mass, constraining the 

composition of the gas that can be accreted 

onto the core of giant planets as well as that 

of the planetesimals that could constitute the 

bulk of the icy planets.

7.2.3 Map protoplanetary material 

dispersed via disk winds

Recent theoretical work suggests that 

disk evolution and dispersal are driven by 

a combination of thermal and MHD disk 

winds (e.g., Alexander et al. 2014; Gorti 

et al. 2016; Ercolano & Pascucci 2017). 

Disk winds can affect all stages of planet 

formation: from planetesimal formation, by 

reducing the disk gas-to-dust mass ratio 

(e.g., Carrera et al. 2017); to the mass of 

giant planets, by starving gas accretion 

onto late-forming planet cores (e.g., Shu et 

al. 1993); to planetary orbits, by dispersing 

gas from preferential radial distances and 

halting planet migration (e.g., Alexander & 

Pascucci 2012; Ercolano & Rosotti 2015). 

However, computational challenges and 

poorly constrained input parameters make it 

difficult to predict how basic disk properties 

evolve and to ascertain the role of different 

winds in dispersing protoplanetary material.

On the observational side, high-resolution (R > 30,000) optical and infrared spectroscopy yields growing evidence of slow 

(<40 km/s) disk winds around Myr-old stars 

(e.g., Pascucci & Sterzik 2009; Sacco et al. 

2012; Natta et al. 2014; Simon et al. 2016). 

Modeling of the line profiles and line flux ratios suggest that emission arises at disk radii 

between 0.1 and 10 AU. On similar spatial 

scales, jets (ejected material moving at ~100 

km/s) are also expected to be accelerated 

and collimated (e.g., Ray et al. 2007). Many 

of the brightest jet diagnostics are accessible to high-resolution imaging spectroscopy 

at UV and optical wavelengths (see Frank et 

al. 2014 for a recent review).

The high spatial resolution and 

wavelength coverage of LUVOIR are 

therefore needed to understand the origin 

of disk winds, the relative roles and interplay 

of MHD and photoevaporative winds, as 

well as to clarify the physical mechanism 

by which jets are launched and collimated. 

In addition to high-resolution spectroscopy, 

LUVOIR slitless spectroscopy (i.e., open 

microshutter array) and narrow-band 

images in UV and optical forbidden lines will 

map for the first time the launching region 

of jets and disk winds, and reveal their 

interaction. POLLUX spectropolarimetry will 

complement the science case discussed 

above by uniquely constraining the strength 

and topology of stellar magnetic fields during 

pre-main sequence evolution, with important 

implications for stellar and disk formation 

and evolution (e.g., Gómez de Castro et al. 

2016).

7.2.4 Star and planet formation in 

Orion

The Orion complex, at a distance of ~400 

pc (e.g., Kounkel et al. 2017), includes rich 

star-forming clusters that are more typical 

birth environments for stars than nearby 

low-density star-forming regions such as 

Taurus. Most stars in the Galaxy, including 

our own, formed in rich clusters (e.g., Lada 

& Lada 2003; Adams 2010) that contain 

massive stars whose high-energy UV and 

X-ray photons contribute to disperse planetforming material around young stars (e.g., 

Johnstone et al. 1998; Clarke & Owen 2015). 

The Orion complex covers different external 

UV fields and the critical 1–10 Myr age range 

over which planet-forming disks disperse. 

It represents one of the best sites to study 

planet formation and several of its regions 

have already a rather complete stellar and 

disk census (e.g., Fang et al., in prep.).
LUVOIR The Large UV Optical Infrared Surveyor

7-14 The LUVOIR Interim Report

LUMOS is ideal for ultraviolet spectral 

surveys of the Orion complex (Figure 7.7). 

First, LUMOS has 40 times higher sensitivity 

and 2–3 times higher spectral resolution than 

HST/COS. Second, it carries a multi-object 

imaging spectroscopy mode that enables 

the simultaneous observation of ~tens-tohundreds of targets (depending on source 

field density). Each LUMOS FUV pointing in 

dense stellar environments like Orion would 

collect a dataset comparable to the mediumresolution UV spectroscopic archive of 

protoplanetary disks observations from 

the entire 28 years of HST observations. In 

about an hour, LUMOS will reach S/N = 10 

per resolution element (R = 40,000) over a 

3' x 1.6' field to Fλ = F(1100 Å) = 2 x 10-16

erg/cm2/s/Å, typical of the continuum flux of 

young stars in Taurus scaled to the distance 

of the Orion complex. This means that in 

about 2–3 hours per field LUMOS will cover 

the entire FUV and NUV spectral range of 

Orion stars enabling the detection of the 

most abundant molecular species in disks, 

Figure 7.7. Multi-object FUV spectroscopy of protoplanetary disks in the Orion Nebula. Upper 

left: HST-ACS image of the Orion Nebula (color credit, Robert Gendler), showing the full FoV 

of the LUMOS spectrograph (France et al. 2017). Lower panels zoom in on a ~0.8 x 0.8 arcmin 

region showing ~30 protostellar/protoplanetary systems (Bally et al. 1998) and the apertures of 

the LUMOS microshutter array overplotted (slits are oversized for display). The two-dimensional 

spectra of the protoplanetary disk and accreting protostar are shown at right. In the upper right, 

we show a zoom in on the 1111–1132 Å spectral region containing strong lines of H2 and H2O. 

The combination of spectral coverage, large collecting area, and multiplexing capability make 

LUVOIR ideal for surveying the composition of the planet-forming environments around young 

stars.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-15

e.g., H2, CO, H2O, and OH. With pre-defined 

maps of targets, LUMOS can cover most 

stars in the Orion Nebula Cluster (~1 Myr, 

~24' size), 1980 (~1–2 Myr, ~16' size), σ Ori 

(~3–5 Myr, ~33' size), λ Ori (4–8 Myr, 49' 

size), 25 Ori (~7–10 Myr, 33' size) in < 150 

hours at FUV and NUV wavelengths. This 

program would allow us to trace the evolution 

and dispersal of the main molecular carriers 

of C, H, and O during planet assembly 

in the terrestrial and giant-planet forming 

regions, trace molecular and low-ionization 

metals from disk winds, and determine the 

absolute abundance patterns in the disk as 

a function of age. This survey will reveal how 

the changing disk environment impacts the 

size, location, and composition of planets 

that form around other stars.

7.3 Signature science case #3: 

The rise of the periodic table

7.3.1 The first stars and the first 

metals

The nucleosynthetic signatures of the first 

stars and supernovae are imprinted in the 

compositions of the most metal-poor stars 

found today. No first-generation (Pop III) 

stars are known at present, but dozens of 

candidate second-generation stars are 

known. These stars have iron abundances 

less than 10–4.5 times the solar abundance 

(i.e., [Fe/H] < -4.5), or are iron-poor but 

highly enhanced in carbon. Hundreds more 

are expected to be found among ongoing 

and future surveys (e.g., LAMOST, LSST). 

When compared with predicted model 

yields, these abundance patterns reveal the 

nature of the elusive Pop III stars, providing 

the only direct tests of the evolution and end 

states of individual Pop III stars (e.g., Frebel & 

Norris 2015). Their locations and kinematics 

reveal the nature of the environments and 

the epochs when they formed and released 

the first metals into the Universe.

Only a few tens of absorption lines are 

commonly found in the optical spectra of 

these second-generation stars, so only ~5–

10 elements are regularly detected. Many 

others (Be, B, Si, P, S, Sc, V, Cr, Mn, Co, Ni, 

and Zn) are expected to be present but are 

rarely detected, and the upper limits derived 

from their optical non-detections are often 

uninformative. The UV part of the spectrum 

is an unexplored window that would allow all 

of these elements to be detected if present 

in the most metal-poor stars known (Figure 

7.8). Key lines include those of boron (B; 

atomic number Z=5; BI lines at 2088, 2089 

Å), phosphorus (P; Z=15; PI lines at 2135, 

2136 Å), sulphur (S; Z=16, SI lines at 1807, 

1820, 1826 Å), chromium (Cr; Z=24; CrII lines 

at 2055, 2061, 2065 Å), and zinc (Zn; Z=30; 

ZnII lines at 2025, 2062 Å) (e.g., Roederer et 

al. 2016).

A large-aperture telescope with a 

high-resolution UV spectrometer enables 

observations of these elements in all known 

Program at a Glance - The Environments of Planet Assembly

Science goal: Trace the main molecular carriers of C, H, and O during planet assembly

Program details: Multiple pointings per star-forming region to study different ages and 

UV environments in the Orion complex 

Instrument(s) + configuration(s): LUMOS high-resolution (R=40,000) multi-object 

spectroscopy

Key observation requirements: S/N of 10 per spectral resolution element. FUV imaging 

parallels offer complementary information.
LUVOIR The Large UV Optical Infrared Surveyor

7-16 The LUVOIR Interim Report

second-generation stars in the halo field, and 

many more found in nearby dwarf galaxies. 

Presently, with COS on HST, only one or 

two of the brightest candidate secondgeneration stars can be observed, but even 

in these cases the data quality is insufficient 

to detect all elements that could, in principle, 

be detected. With LUVOIR, however, highquality UV spectra (R~30,000; S/N~80; 

1800 < λ < 3100 Å) could be obtained for 

F or G-type dwarfs or subgiant stars at 

V~15.5 mag (9-m) or V~16.6 mag (15-m) in 

~1 hour each. This advance would enable us 

to increase the stellar sample sizes by about 

two orders of magnitude, revolutionizing 

our understanding of the first stars, the 

first supernovae, and the first metals in the 

Universe. In other words, LUVOIR+LUMOS 

would enable the acquisition of high-quality 

UV spectra of the vast majority of stars whose 

optical spectra can be observed today from 

the ground.

7.3.2 The origin of elements heavier 

than iron

The elements heavier than iron, which 

have been detected in the ancient stars of 

the Galactic halo, in the ISM, dust grains, 

meteorites, and on Earth, are formed by 

neutron-capture reactions (e.g., Sneden, 

Cowan, & Gallino 2008). Relatively high 

neutron densities (~1022–1028 cm-3) lead 

to heavy-element nucleosynthesis via the 

rapid neutron-capture process (r-process) in 

Figure 7.8. Comparison of the optical and UV spectral domains of a normal G dwarf star at solar 

metallicity (top row), a typical metal poor G subgiant (middle row), and one of the candidate 

second generation stars (bottom row). The red panel is from Aoki et al. (2006). LUVOIR could 

observe the UV spectrum of every candidate second-generation star in the field, and many more 

in nearby dwarf galaxies, obtaining data similar to the lower right panel across a much wider 

spectral range.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-17

supernovae or neutron star mergers. Lower 

neutron densities (~107–1010 cm-3) lead 

to nucleosynthesis via the slow neutroncapture process (s-process) in AGB stars 

or the late evolutionary stages of massive 

stars. Neutron densities intermediate 

between these two extremes (~1015 cm-3) 

lead to nucleosynthesis via the intermediate 

neutron-capture process (i-process), which 

may occur in a variety of sites including 

super-AGB stars, post-AGB stars, He-core 

and He-shell flashes in low- metallicity lowmass stars, and massive stars.

Optical spectra obtained from the ground 

can reveal the nature of the enrichment (e.g., 

the r-process), but key elements useful 

to discriminate between models have no 

absorption lines in the optical domain. Highresolution UV spectroscopy enables a 40% 

improvement, compared to optical/near-IR 

spectra, in the number of elements (~15 to 20) 

that can be detected in the atmospheres of 

late-type (FGK) stars that retain the chemical 

signatures of nucleosynthesis in earlier 

generations of stars. These include elements 

like germanium (Ge; atomic number Z=32; 

Figure 7.9. Comparison of distances at which LUVOIR can obtain high-precision chemical 

abundances in metal-poor giants to examine the origins of the first heavy elements. These 

elements are best studied in the UV; many of these stars are so metal poor that the key lines in 

the optical are intrinsically too weak, but stronger UV lines of Fe, Mg, etc. can still be detected. 

These are challenging, expensive observations for Hubble since the stars are ancient and not 

bright in the UV. LUMOS will push these limits to ~20 kpc, a significant fraction of the total mass 

of the galactic halo.
LUVOIR The Large UV Optical Infrared Surveyor

7-18 The LUVOIR Interim Report

GeI line at 3039 Å), selenium (Se; Z =34; SeI 

line at 2074 Å), cadmium (Cd; Z=48; CdI line 

at 2288 Å), tellurium (Te; Z=52; TeI line at 

2385 Å), platinum (Pt; Z=78; PtI line at 2659 

Å), and mercury (Hg; Z=80; HgII line at 1942 

Å), among others (e.g., Roederer & Lawler 

2012). These abundances provide new, 

critical constraints on stellar nucleosynthesis 

mechanisms like the r-, s-, and i-processes.

A large-aperture telescope with a highresolution UV spectrometer would enable 

observations of these elements in samples 

of stars best suited to revealing the complex 

physics that drives the nucleosynthesis 

of the heaviest elements. Presently, with 

STIS or COS on Hubble, only the handful 

of brightest stars in the solar neighborhood 

can be observed, and these are sub-optimal 

representatives of stars reflecting dominant 

contributions from the r-, s-, or i-processes. 

With LUVOIR, high-quality UV spectra 

(R~30,000; S/N~80; 1800 < λ < 3100 Å) 

could be obtained for metal-poor red giants, 

which typically present the richest line pattern 

for analysis, at V~15.0 (9-m) or V~16.1 (15-

m) in 10 hours, mimicking an observing mode 

Program at a glance - New Insights on Nucleosynthesis

Science goal: Determine elemental abundances in very metal poor stars to constrain 

Pop III nucleosynthesis. Determine elemental abundances in late type stars to constrain 

R-, S, and I-process nucleosynthesis.

Program details: High-resolution FUV and NUV spectroscopy of stars in the Milky Way, 

predominantly in the outer Milky Way halo. 

Instrument(s) + configuration(s): LUMOS high-resolution (R=40,000) multi-object 

spectroscopy. G180M and G300M gratings.

Key observation requirements: S/N of >80 per spectral resolution element. 

Table 7.1. Summary science traceability matrix for Chapter 7

Scientific Measurement Requirements Instrument Requirements

Objectives Measurement Observations Instrument Property Value

Constrain 

massive star 

formation and 

determine the 

stellar IMF

Imaging and 

spectra of 

individual 

stars and star 

clusters

Imaging S/

N=10-300; 

Spectroscopy 

S/N > 10, 

R>3000 

HDI LUMOS UVIS imaging, 

0.007" 

resolution FUV, 

NUV gratings, 

MOS

UVBRI filters 

G120M, 

G150M, 

G180M, 

G300M

Detect and 

characterize the 

composition of 

planet-forming 

material

Spectroscopy 

of molecules in 

protoplanetary 

disks

Spectroscopy 

S/N > 10, 

R=40,000 

LUMOS FUV, NUV 

gratings, MOS

G120M, 

G150M, 

G300M

Follow the first 

metals created 

by stars

Spectroscopy 

of extremely 

metal poor stars 

in the Milky Way 

halo

Spectroscopy 

S/N~80, 

R=30,000 

LUMOS FUV, NUV 

gratings, MOS

G180M, 

G300M

Trace the 

history of R-,S-, 

I- process 

material

Spectroscopy 

of late-type 

stars in the 

Milky Way halo

Spectroscopy 

S/N ~80, 

R=30,000 

LUMOS FUV, NUV 

gratings, MOS

G180M, 

G300M
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 7-19

and strategy routinely used with HST/STIS 

(e.g., Roederer et al. 2012). Examples of all 

kinds of enrichment patterns are found within 

this magnitude limit, enabling for the first 

time observations of the most scientifically 

advantageous stars for the task. In other 

words, LUVOIR+LUMOS would enable the 

acquisition of high-quality UV spectra of the 

vast majority of stars whose optical spectra 

can be observed today from the ground.

References

Adams, F. C. (2010). ARA&A, 48, 47

Alexander, R. D., & Pascucci, I. (2012). 

MNRAS, 422, L82

Alexander, R., Pascucci, I., Andrews, 

S., Armitage, P., & Cieza, L. (2014). 

in Protostars and Planets VI, ed. 

H. Beuther et al. (Tucson, AZ: Univ. 

Arizona Press), 475

ALMA Partnership, Brogan, C. L., Pérez, L. 

M., et al. (2015). ApJ, 808, L3

Andrews, S. M., Wilner, D. J., Zhu, Z., et al. 

(2016). ApJ, 820, L40

Arulanantham, N., France, K., Hoadley, K. 

(2018). AAS, Meeting #231, #147.11

Bally, J., Yu, K. C., Rayner, J., & Zinnecker, 

H. (1998). AJ, 116, 1868

Brown, T.M., Postman, M., & Calzetti, D. 

(2010). arXiv:0902.2973

Calzetti, D., Johnson, K. E., Adamo, A., et 

al. (2015). ApJ, 811, 75

Cappellari, M., McDermid, R. M., Alatalo, 

K., et al. (2012). Nature, 484, 485

Carrera, D., Gorti, U., Johansen, A., & 

Davies, M. B. (2017). ApJ, 839, 16

Clarke, C. J., & Owen, J. E. (2015). 

MNRAS, 446, 2944

Conroy, C., & van Dokkum, P. G. (2012). 

ApJ, 760, 71

Cridland, A. J., Pudritz, R. E., Birnstiel, T., 

Cleeves, L. I., & Bergin, E. A. (2017). 

MNRAS, 469, 3910

Crowther, P. A., Schnurr, O., Hirschi, R., et 

al. (2010). MNRAS, 408, 731

Crowther, P. A., Caballero-Nieves, S. M., 

Bostroem, K. A., et al. (2016). MNRAS, 

458, 624

Debes, J. H., Jang-Condell, H., Weinberger, 

A. J., et al. (2013). ApJ, 771, 45

Dong, R., & Fung, J. (2017). ApJ, 835, 146

Elmegreen, B. G., Hurst, R., & Koenig, X. 

(2014). ApJL, 782, L1

Ercolano B., Rosotti G., (2015). MNRAS, 

450, 3008

Ercolano, B., & Pascucci, I. (2017). RSOS, 

4, 170114

France, K., Schindhelm, E., Burgh, E. B., et 

al. (2011). ApJ, 734, 31

France, K., Schindhelm, E., Herczeg, G. J., 

et al. (2012). ApJ, 756, 171

France, K., Herczeg, G. J., McJunkin, M., & 

Penton, S. V. (2014). ApJ, 794, 160

France, K., Fleming, B., Hoadley, K. (2016) 

SPIE, 9905, 990506

Frank, A., Ray, T. P., Cabrit, S., et al. (2014). 

in Protostars and Planets VI, ed. H. 

Beuther (Tucson, AZ: Univ. Arizona 

Press), 451

Geha, M., Brown, T. M., Tumlinson, J., et al. 

(2013) ApJ, 771, 29

Geha, M.C. (2014). AAS, Meeting #223, 

#314.03

Gómez de Castro, A. I., Gaensicke, B., 

Neiner, C., et al. (2016). JATIS, 2, 

041215 

Gorti, U., Liseau, R., Sándor, Z., & Clarke, 

C. (2016). SSRv, 205, 125

Hendler, N. P., Pinilla, P., Pascucci, I. et al. 

(2018). MNRAS, in press, https://arxiv.

org/abs/1711.09933

Herczeg, G. J., Linsky, J. L., Valenti, J. 

A., Johns-Krull, C. M., & Wood, B. E. 

(2002). ApJ, 572, 310

Ida, S., & Lin, D. N. C. (2008). ApJ, 685, 

584

Ingleby, L., Calvet, N., Bergin, E., et al. 

(2011). ApJ, 743, 105

Johnstone, D., Hollenbach, D., & Bally, J. 

(1998). ApJ, 499, 758
LUVOIR The Large UV Optical Infrared Surveyor

7-20 The LUVOIR Interim Report

Kirk, H., & Myers, P. C. (2012). ApJ, 745, 

131

Köhler, K., Langer, N., de Koter, A., et al. 

(2015). A&A, 573, A71

Kounkel, M., Hartmann, L., Mateo, M., et al. 

(2017). ApJ, 844, 138

Kryukova, E., Megeath, S. T., Gutermuth, R. 

A., et al. (2012). AJ, 144, 31

Lada, C. J., & Lada, E. A. (2003). ARA&A, 

41, 57

Mulders, G. D., Pascucci, I., & Apai, D. 

(2015). ApJ, 798, 112

Natta, A., Testi, L., Alcalá, J. M., et al. 

(2014). A&A, 569, A5

Offner, S. S. R., Clark, P. C., Hennebelle, P., 

et al. (2014). in Protostars and Planets 

VI, ed. H. Beuther et al. (Tucson, AZ: 

Univ. Arizona Press), 53

Pang, X., Grebel, E. K., Allison, R. J., et al. 

(2013). ApJ, 764, 73

Pascucci, I., & Sterzik, M. (2009). ApJ, 702, 

724

Pascucci, I., & Tachibana, S. (2010). in 

Protoplanetary Dust: Astrophysical 

and Cosmochemical Perspectives, ed. 

D. Apai & D. S. Lauretta (Cambridge: 

Cambridge Univ. Press), 263

Pérez, L. M., Carpenter, J. M., Andrews, S. 

M., et al. (2016). Science, 353, 1519

Pohl A., Sissa, E., Langlois, M. et al., 

(2017). A&A, 605, A34

Portegies, S. F., McMillan, S. L. W., & 

Gieles, M. (2010). ARA&A, 48, 431

Ray, T. P. (2007). in IAU Symp. 243, StarDisk Interaction in Young Stars, ed. J. 

Bouvier & I. Appenzeller (Cambridge: 

Cambridge Univ. Press), 183

Sacco, G. G., Flaccomio, E., Pascucci, I., et 

al. (2012). ApJ, 747, 142

Simon, M. N., Pascucci, I., Edwards, S., et 

al. (2016). ApJ, 831, 169

Shu, F. H., Johnstone, D., & Hollenbach, D. 

(1993). Icarus, 106, 92

Roberge, A., Feldman, P. D., Lagrange, A. 

M., et al. (2000). ApJ, 538, 904

Roberge, A., Lecavelier des Etangs, A., 

Grady, C. A., et al. (2001). ApJ, 551, 

L97

Sallum, S., Follette, K. B., Eisner, J. A. et al. 

(2015). Nature, 527, 342

Smith, R. J. (2014). MNRAS, 443, L69

Smith, R. J., Lucey, J. R., & Conroy, C. 

(2015). MNRAS, 449, 3441

Smith, L.J., Crowther, P.A., Calzetti, D., & 

Sidoli, F. (2016). ApJ, 823, 38

Spiniello, C., Trager, S., Koopmans, L. V. E., 

et al. (2014). MNRAS, 438, 1483

Spiniello, C., Trager, S. C., & Koopmans, L. 

V. E. (2015). ApJ, 803, 87

Turner, J. K., Beck, S. C., & Ho, P. T. P. 

(2000). ApJ, 532, 109

van Dokkum, P. G., & Conroy, C. (2010). 

Nature, 468, 940

van Dokkum, P., Conroy, C., Villaume, A., et 

al. (2017). ApJ, 841, 68

Yusof, N., Hirschi, R., Meynet, G., et al. 

(2013). MNRAS, 433, 1114

Wagner, K., Apai, D., Kasper, M., et al. 

(2015). ApJ, 813, L2

Wofford, A., Leitherer, C., Walborn, N.R., et 

al. (2012). arXiv:1209.3199

Wofford, A., Leitherer, C., Chandar, R., & 

Bouret, J.-C. (2014). ApJ, 781, 122

Wyse, R. F. G., Gilmore, G., Houdashelt, M. 

L., et al. (2002). NewA, 7, 395

Zhu, Z. (2015). ApJ, 799, 16

Zhou, Y., Herczeg, G. J., Kraus, A. L., et al. 

(2014). ApJ, 783, L17
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-1

Chapter 8
LUVOIR The Large UV Optical Infrared Surveyor

8-2 The LUVOIR Interim Report

8 The LUVOIR architectures

LUVOIR’s compelling science objectives 

define a set of high-level mission capabilities: 

sensitivity, resolution, flexibility, high-contrast 

imaging, and mission duration. The LUVOIR 

Study team has used these capabilities to 

define a set of fundamental performance 

goals that include: 

• a large (8–15 meter) aperture; 

• broad wavelength sensitivity from the 

far-ultraviolet to the near-infrared; 

• a suite of instruments with imaging, 

spectroscopic, and high-contrast 

capabilities necessary to achieve the 

science observations discussed in this 

report; and 

• a potentially multi-decade mission 

lifetime enabled through on-orbit 

serviceability and upgrading of 

instruments.

The LUVOIR Study Team is working 

to develop two classes of feasible and 

executable mission architectures that meet 

these goals at either end of the aperture 

range: LUVOIR-A is a 15-meter segmentedaperture telescope, while LUVOIR-B is an 

8-meter segmented-aperture telescope. 

Both architectures will have a suite of modular 

instruments and a total facility wavelength 

range from 100 nm to 2.5 mm. Four 

instrument concepts are being developed as 

part of these architecture studies: 

1. The Extreme Coronagraph for Living 

Planetary Systems (ECLIPS), a near-UV / 

optical / near-IR coronagraph capable of 

10-10 contrast at inner working angles as 

small as 2 l/D.

2. The LUVOIR UV Multi-Object 

Spectrograph (LUMOS), a low- and medium-resolution multi-object imaging spectrograph spanning the far-UV (~100–200 

nm) to the near-UV (~200–400 nm), with 

a separate far-UV imaging channel.

3. The High Definition Imager (HDI), a nearUV / optical / near-IR imager with a wide 

field-of-view and high spatial resolution.

4. POLLUX, a high-resolution UV spectropolarimeter, contributed by a European 

consortium under the leadership of 

the Centre National d’Etudes Spatiales 

(CNES).

In this chapter we will describe the overall 

design philosophy, as well as mission-level 

aspects of the architectures. The following 

chapter goes into greater detail on the 

science payload, including the optical 

telescope element and science instruments. 

The POLLUX instrument, and its science 

case, are described in Chapter 10.

8.1 Study philosophy

The goal of the LUVOIR design study is to 

develop a mission concept that achieves 

the science objectives described in this 

report, subject to traditional engineering and 

programmatic constraints of mass, volume, 

cost, schedule, and technical risk. Ultimately, 

the 2020 Decadal Survey Committee will 

evaluate LUVOIR and the other mission 

concepts on the following merits: 

• Is the science compelling? 

• Is the mission executable and feasible? 

• Does the science yield justify the cost 

and risk?

The study team faces several challenges 

when trying to plan and scope a mission 

that may not launch for two decades. At the 

time the Decadal Committee performs their 

evaluation, many of the constraints listed 

above will be unknown. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-3

For example, mass and volume are 

limited by the capability of the launch vehicle. 

However, the fleet of available launch vehicles 

is currently in flux. Existing heavy-lift vehicles 

like the Delta IV Heavy are being phased out 

in favor of new vehicles like Space X’s Falcon 

Heavy, Blue Origin’s New Glenn, and NASA’s 

own Space Launch System. While it is certain 

that these vehicles will be in operation within 

the next decade or so, what is less certain 

are the limits of their capability, the cadence 

of their launch schedule, and the cost of their 

service.1

Another unknown constraint is the 

mission cost. An independent cost estimate 

of all large mission concepts will be 

completed as part of the Decadal review 

process; each is likely to be a multi-billiondollar large-scale mission, similar in scope 

to HST, the James Webb Space Telescope 

(JWST), and the Wide-Field Infrared Survey 

Telescope (WFIRST). What will not be well 

understood is how those costs will fit into 

the future budgetary environment at NASA. 

While the science cases may fully justify the 

cost, the available budget may require long 

and protracted development schedules, 

potentially reducing the value and increasing 

the risk to the missions.

Finally, the Decadal Committee will be 

performing their evaluation just as JWST is 

nearing launch, WFIRST is being developed, 

and a new generation of ground-based 

observatories preparing to come online. It 

is impossible to predict what discoveries 

these facilities will make that may change the 

scientific landscape in the next two decades.

1 See Appendix D for more details on current and 

anticipated launch vehicles.

In response to these uncertainties, the 

LUVOIR STDT has decided to put forth for 

evaluation two mission architectures that 

bracket a range of capability, cost, and risk. 

LUVOIR-A is the larger of the two concepts 

with a 15-m primary mirror diameter that 

maximizes science yield while accepting 

moderate technical and programmatic risk. 

LUVOIR-A is the largest, most capable 

observatory that can be deployed from an 

anticipated 8.4-m diameter fairing, launched 

with NASA’s Space Launch System Block 

2 heavy lift vehicle. Conversely, LUVOIR-B 

represents a more conservative approach 

where science yield is balanced with technical 

and programmatic risks. LUVOIR-B will be 

designed to fit within an industry standard 

5-m-class fairing.

At the time of writing this report, the 

study team has completed an initial design 

iteration for the complete LUVOIR-A concept 

and has begun exploring a number of design 

alternatives in order to maximize system 

performance. The team has also begun to 

evaluate several designs for LUVOIR-B. To be 

clear, both mission concepts will continue to 

evolve as each architecture gains definition. 

Over the course of the next year, detailed 

designs for both concepts will be finalized for 

evaluation by the Decadal Committee. Both 

of these final point designs will be presented 

in the Final Report; for this Interim Report we 

focus primarily on LUVOIR-A and its design 

evolution.

LUVOIR’s two design concepts demonstrate a scalable mission architecture 

with a range of science yield, cost, and technical risks that can be adapted 

to an uncertain future.
LUVOIR The Large UV Optical Infrared Surveyor

8-4 The LUVOIR Interim Report

8.2 Architecture A

8.2.1 Design overview

An initial concept for LUVOIR-A, shown 

in Figure 8.1, builds upon the legacies of 

HST, JWST, and WFIRST. Similar to HST, 

LUVOIR covers a bandpass spanning the 

UV to the near-IR, has a suite of imagers 

and spectrographs, and is designed to 

take advantage of potential future servicing 

capabilities that are currently being 

developed by NASA and other agencies. 

Like JWST, LUVOIR has a segmented 

primary mirror that deploys from a stowed 

configuration, along with the secondary 

mirror support structure, the sunshield, 

and a number of other subsystems. Finally, 

like WFIRST, LUVOIR has a high-contrast 

coronagraph instrument, capable of directly 

imaging and spectroscopically characterizing 

exoplanets. In this sense, LUVOIR builds 

upon the technical legacy of the great space 

observatories that will have preceded it.

LUVOIR-A is divided into three primary 

segments: The Flight Segment consisting of 

the observatory itself, the Ground Segment 

consisting of ground stations and mission 

and science operations centers, and the 

Launch Vehicle.

8.2.1.1 Flight Segment

The Flight Segment, or observatory, is 

shown in Figure 8.2 and is divided into 

the payload and the spacecraft. The 

payload consists of the optical telescope 

element, the backplane support frame, the 

Vibration Isolation and Precision Pointing 

System (VIPPS), the gimbal system, and 

four serviceable instrument modules. The 

spacecraft consists of the payload interface 

tower, the sunshield, and the spacecraft bus. 

The payload is articulated with respect to the 

Figure 8.1. The LUVOIR Observatory. The front view shows the primary mirror, secondary mirror 

support structure, and the deployable aft-optics support structure. The sunshield is rendered 

transparent here in order to show the spacecraft below. The inset shows a rear view, highlighting 

the backplane support frame holding the four instruments, depicted here as simple colored 

volumes: LUMOS (green, lower left), HDI (red, lower right), ECLIPS (blue, upper left), and POLLUX 

(yellow, upper right). The two-axis gimbal is also visible near the middle of the backplane support 

frame.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-5

spacecraft using a two-axis gimbal system. 

Repointing the observatory involves rolling 

the entire observatory about the sun-earth 

axis through 360°, while simultaneously 

pitching the payload with the gimbal from 

0° to 90° (0° corresponds to the telescope 

boresight normal to the sunshield, while 

90° corresponds to the telescope boresight 

parallel to the sunshield). With these two 

degrees of freedom, the payload can point 

anywhere in the anti-sun hemisphere. An 

additional second axis on the gimbal allows 

just the payload to roll about the telescope 

boresight from 0° to 90°. The boresight roll 

allows targets to be aligned to instrumentspecific apertures, and also enables pointspread function roll subtraction. Details on 

the specific elements of the payload are 

provided in Chapter 9.

The payload and spacecraft are 

physically separated from each other via the 

Vibration Isolation and Precision Pointing 

System (VIPPS), a non-contact interface 

that is controlled in six degrees-of-freedom 

by voicecoil actuators. The VIPPS is located 

between the gimbal and the backplane 

support frame and is discussed in more 

detail in Section 8.2.2.1.

The spacecraft, shown in Figure 8.3, 

must provide basic services to the payload: 

power, communications, attitude control, 

command and data handling, propulsion, 

and structural support during launch. The 

spacecraft also thermally isolates the payload 

from solar heat loads via the sunshield. Each 

of these subsystems are designed to not only 

meet the minimum 5-year mission needs, 

but also anticipate future evolution of the 

LUVOIR observatory through serviceability.

8.2.1.1.1 Spacecraft structure

The spacecraft bus is octagonal in shape. A 

central bay holds the propellant tanks, which 

are refuelable. The central bay is surrounded 

by the primary spacecraft structure that 

supports the mass of the payload and 

transfers loads to the launch vehicle during 

launch and ascent. Additional spacecraft 

subsystems are supported by the secondary 

spacecraft structure around the perimeter of 

the primary structure and are accessible for 

serviceability.

8.2.1.1.2 Sunshield

A central component of the LUVOIR thermal 

management architecture is the deployable 

sunshield. The sunshield isolates the payload 

Figure 8.2. The LUVOIR Flight Segment, with identified subsystems. Everything above the 

sunshield (with the exception of the payload interface tower) is considered part of the payload. 

The payload interface tower, the sunshield, and everything below the sunshield are collectively 

considered the spacecraft. The observatory is shown in the Gimbal-90° orientation. 
LUVOIR The Large UV Optical Infrared Surveyor

8-6 The LUVOIR Interim Report

from solar thermal loads and provides a cold 

environment in which the payload can be 

thermally stabilized with active heaters, and 

in which system radiators have sufficient 

cold-sink temperatures.

Nominally, the sunshield remains in a 

fixed attitude normal to the Sun-observatory 

axis, maintaining a constant thermal load 

on the sunward side of the sunshield as the 

payload slews to different targets on the 

leeward side of the sunshield. The payload 

always sees a constant thermal environment, 

regardless of its pointing attitude. Two 

science observations necessitate exceptions 

to this nominal attitude and require the 

entire observatory to pitch toward the sun 

(while still keeping the payload in shadow). 

Exoplanet revisit observations must be 

scheduled at critical time intervals to 

maximize observational completeness and 

may require access to sunward portions of 

the sky. Also, observations of specific solar 

system bodies, such as Venus and comets, 

require pointing the observatory up to 45° 

from the Sun-Earth axis. Figure 8.4 shows 

several different pointing scenarios for the 

observatory.

Although the sunshield appears similar in 

concept and design to that of JWST, it differs 

in two important aspects. First, it is larger. 

While JWST’s sunshield is tennis-court 

sized, the LUVOIR-A sunshield is closer in 

size to a football field, spanning nearly 80-m 

tip-to-tip. And whereas JWST’s sunshield 

is an extremely complex system of 5 thin 

layers that need to be precision deployed to 

tight tolerances on the angle and separation 

between the layers, LUVOIR’s sunshield 

is simpler. A minimum of two sheets of 

single-layer insulation are needed to meet 

the thermal performance requirements, 

although three layers will likely be needed for 

redundancy against micrometeroid strikes. 

The spacing and angle between these layers 

is not critical, so long as they do not touch.

The manner in which the sunshield is 

deployed also differs between LUVOIR and 

JWST. JWST uses two rigid pallets to hold the 

sunshield system. These pallets are stowed 

up around the telescope and instruments, 

cocooning the payload during launch and 

ascent. After launch, the pallets fold down, 

and port and starboard deployable booms 

pull the sunshield layers out from the pallet, 

before spreaders and tensioners fineposition the five sunshield layers. In contrast, 

LUVOIR will package the entire sunshield at 

the base of the spacecraft. Four deployable 

booms will pull the folded sunshield out in 

each of four directions: fore, aft, port, and 

starboard. Details of the deployment are still 

being worked by the Study Team, but we 

Figure 8.3. The LUVOIR-A spacecraft in its 

stowed configuration. To deploy, the two 

solar arrays would first fold down to a position 

perpendicular to their current orientation, and 

the individual panels would further unfold. 

The high gain antenna assembly would also 

fold down to a nominal position below the 

spacecraft. Four deployable booms would 

then pull the sunshield out from its folded 

configuration in the four cardinal directions. 

The propellant tanks are refuelable from below 

the spacecraft. The four control moment 

gyroscopes are mounted to the exterior of the 

spacecraft for accessibility during potential 

servicing.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-7

anticipate several advantages over JWST, as 

well as several new challenges. Advantages 

include a smaller relative mass and volume 

for the stowed sunshield system, the use of 

high TRL deployment mechanisms, such as 

AstroMasts®, and overall lower risk to the 

deployment due to fewer mechanisms. New 

challenges include venting of the stowed 

sunshield during ascent, and the fact that 

the payload will be exposed once the fairing 

is jettisoned. We expect to have more 

detail regarding the sunshield deployment 

advantages and risks available in the final 

study report.

8.2.1.1.3 Attitude control system 

(ACS)

The spacecraft is three-axis stabilized and inertially fixed, with the spacecraft axial vector 

parallel to the sun-spacecraft axis. The ACS 

is responsible for counteracting external disturbances (primarily solar pressure torques), 

as well as changing the yaw axis of the observatory and reacting against gimbal pitch 

changes during payload retargeting maneuvers. The ACS is also responsible for pitching 

the entire observatory sunward for specific 

observations (see Section 8.2.1.1.2).

The ACS sensor suite serves two functions: star trackers, gyroscopes, accelerometers, and coarse sun sensors determine the 

attitude and inertial position of the spacecraft 

relative to the sun, while proximity sensors on 

the VIPPS determine the relative attitude between the spacecraft and the payload. ACS 

actuators, consisting of four control moment 

gyroscopes (CMGs) and sixteen 5-lb. thrusters respond to the sensor suite to maintain 

the absolute attitude and inertial position of 

the spacecraft. The four CMGs are arranged 

to provide full three-axis stabilization with 

redundancy. The thrusters are arranged in 

two sets: the first is used during momentum 

dump maneuvers, the second is used for 

stationkeeping and orbit maintenance.

Figure 8.4. Example payload pointing attitudes. Nominal science observations maintain the 

sunshield normal to the Sun-observatory axis, as shown in the left two figures. The thermal load 

on the sunward side of the sunshield remains constant generating a thermally-stable environment 

on the leeward side, regardless of payload pointing. Exoplanet revisit observations and solar 

system targets of opportunity require the entire observatory to pitch towards the sun to a 45° 

angle with the Sun-observatory axis.
LUVOIR The Large UV Optical Infrared Surveyor

8-8 The LUVOIR Interim Report

8.2.1.1.4 Command & data handling 

(C&DH)

The C&DH system consists of a solidstate recorder, a board-level computer 

for command and control, and dedicated 

controller boards for spacecraft deployment 

and gimbal mechanisms. To ensure the 

payload never sees direct sunlight, a separate 

ACS safe-hold processor takes over in the 

event of a C&DH system failure to maintain 

a safe attitude. Each instrument contains 

enough internal storage for two day’s-worth 

of data. Therefore, the solid-state recorder 

on the spacecraft only needs to act as a 

buffer for each downlink and is sized to hold 

only a single downlink’s worth of data (~10 

Tb, including margin). Figure 8.5 shows a 

block diagram of the spacecraft avionics and 

their interface to spacecraft subsystems and 

payload.

8.2.1.1.5 Communications

The communications system uses Ka-band 

for data downlink and S-band for telemetry 

and housekeeping. A 1.8-m high gain 

antenna is used to connect with ground 

stations. Commands and telemetry during 

launch and ascent will use S-band omni 

antennas. A link budget is provided in Table 

8.1.

8.2.1.1.6 Electrical power systems

Approximately 105 m2 of solar panels provide 

more than 31 kW of power needed by the 

LUVOIR observatory, allocated by system in 

Figure 8.5. Block diagram of the spacecraft (S/C) avionics systems and interface to the payload. 

Solid black lines represent a MIL-STD-1553 bus, solid red lines represent individual SpaceWire 

links, dashed purple lines represent timing (1 pps) links, and solid green lines represent power 

cables. ACS, propulsion, communication, deployment mechanisms, and gimbal control 

electronics are connected to their respective subsystems via appropriate links (RS-422, 28-v, 

1553, etc.). The solid-state recorder provides a buffer for a single downlink’s-worth of data and 

is connected to the payload via redundant and cross-strapped SpaceWire. The solar array and 

electrical power subsystem supplies 120-v power directly to the payload, and 28-v power (via a 

down converter) to the S/C. A 28-v battery provides power during launch and ascent.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-9

Table 8.2. The solar array is divided into two 

deployable wings with three panels each, 

and is located at the base of the spacecraft, 

below the sunshield. A 24 amp-hour battery 

is included for providing power during launch 

and ascent. The power system provides 

28-v to spacecraft subsystems, and 120-

v directly to the payload. A 120-v payload 

bus was chosen primarily to minimize the 

power harness across the VIPPS interface. 

The higher voltage allows for fewer, thinner 

conductors, and therefore more flexible 

cables, to bridge the non-contact interface.

LUVOIR adopts a Class-A mission risk 

posture throughout the observatory (both 

the spacecraft and the payload). All electrical 

system boards are side-A / side-B or boxlevel redundant, all mechanisms have dualTable 8.1. Link budget for LUVOIR science data downlink.

LUVOIR Science Data Downlink @ 26,500MHz to White Sands, New Mexico, 20° elevation

Link Type: Space-Ground 1/2 Angle Half-Power Beam Width (°): 0.22

Science Type: Space Science Polarization: Circular

Modulation Scheme: Quadrature PhaseShifting Key (QPSK)Ground Station Latitude: 32.54

Coding: Low-Density Parity 

Check Rate 1/2

Ground Station Longitude: -106.61

Bit Error Rate: 10-6 Ground Station Altitude: 1.45

Transmit Frequency 26,500 MHz

Transmit Power 20 dBW

Antenna Diameter 1.8 m

Antenna Efficiency 55 %

Antenna Gain 51.38 dBi

Spacecraft Passive Loss 3 dB

Spacecraft Pointing Loss 0.5 dB

Transmitter Equivalent Isotropically Radiated 

Power (EIRP)

67.88 dBWi

Slant Range 1,800,000 km

Min. Elevation Angle 20 deg

Total Propagation Loss Effects 3.025 dB

Power Received -181.256 dB

Ground Station G/T 46.9 dB/K (18.3m antenna)

Received Carrier to Noise Density (C/No) 94.24 dB

Modulation Loss 0 dB

Information (Data) Rate 430 Mbps

Total Information (Data) Rate 86.33 dB

Differential Encoding/Decoding Loss 0 dB

User Constraint Loss 0 dB 

Received Eb/No 7.91 dB

Implementation Loss 3 dB

Required Eb/No At Decoder 1.89 dB (LDPC Rate 1/2 @ 10^-6)

Margin 3.02 dB
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8-10 The LUVOIR Interim Report

windings, and all electrical power and 

data interfaces are either cross-strapped, 

redundant, or both.

8.2.1.1.7 Propulsion

A single 100-lb. thruster is used for the first 

mid-course correction maneuver, which 

occurs ~12 hours after launch and corrects 

the trajectory for variability in the launch 

vehicle performance. A set of four 5-lb. longduration thrusters are used for subsequent 

mid-course correction maneuvers, as well as 

for the orbit insertion burn to enter the SEL2 

halo orbit.

8.2.1.1.8 Flight dynamics

LUVOIR will orbit the second Sun-Earth 

Lagrange (SEL2) point in a quasi-halo orbit 

that does not exceed 25° from the Sun-Earth 

axis at its maximum, or 5° from the SunEarth axis at its minimum. A notional delta-v 

budget is provided in Table 8.3. StationTable 8.2. Power Budget for the LUVOIR-A Observatory. Current Best Estimate (CBE) values are 

grass-roots estimates based on instrument designs. Maximum Expected Values (MEV) include 

a 40% growth allowance that accounts for the early-stage nature of the designs. Maximum 

Permissible Values (MPV) include an additional 25% margin as required by NASA Goddard 

Space Flight Center GOLD Rules. We note that the ECLIPS instrument includes the Control 

System Processor, which is responsible for performing all closed-loop sensing and control for the 

payload, and therefore requires more power than the other instruments.

System / Subsystem CBE Power 

(W)

Growth 

Allowance 

MEV Power 

(W)

GOLD Rules 

Margin

MPV Power 

(W)

Optical Telescope Element 

(OTE)

3,000 40% 5,000 25% 6,667

Backplane Support Frame 

(BSF)

2,650 40% 4,417 25% 5,889

ECLIPS 1,640 40% 2,733 25% 3,644

HDI 480 40% 800 25% 1,067

LUMOS 650 40% 1,083 25% 1,444

POLLUX 550 40% 917 25% 1,222

Spacecraft 5,260 40% 8,767 25% 11,689

Total: 14,230 - 23,717 - 31,622

Table 8.3. Delta-v budget for the LUVOIR-A mission concept. MCC: mid-course correction; LOI: 

L2 orbit insertion.

Maneuver ΔV (m/s) Mission Time Burn Time (min) Notes

MCC-1 45 L +12h 48 Dependent on launch vehicle 

performance reliability

MCC-1a 10 MCC-1 + 5h 11 Clean up for MCC-1 efficiency

MCC-2 20 L +22h 106 (1.6 hr) Clean up for MCC1 and solar radiation 

pressure effects after Sunshield 

deployment Using 4 5lb thrusters 

(Sunshield is deployed)

MCC-3 5 L +132h 27 Using 4 5lb thrusters

LOI 30 L + 98day 159 (2.65 hr) Using 4 5lb thrusters

Orbit 

Maintenance

60 ~every 3 weeks 

after LOI

>1 ΔV for 10 years ~ 4 m/s for 

stationkeeping and 2m/s for 

Momentum Unloading per year
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The LUVOIR Interim Report 8-11

Figure 8.6. A notional LUVOIR ground system. Two ground stations in White Sands, NM and 

South Africa allow for twice-daily downlink passes. The Mission Operations Center (MOC), 

backup MOC, and the Science Operations Center would be located at suitable institutions with 

the experience and infrastructure necessary to support a large-scale mission with a broad user 

base.

keeping and momentum dumps will occur 

nominally once every three weeks and will be 

scheduled to coincide to minimize science 

observation down-time.

8.2.1.2 Ground System

The LUVOIR Ground Segment will be based 

on the models of other large observatories 

such as HST and JWST. Primary and backup 

mission operations centers will be responsible 

for overall control of the observatory, orbit 

determination, telemetry, and mission 

planning. Similarly, a science operations 

center will be responsible for planning the 

observation schedule, supporting a guestobserver program, processing science data, 

and distributing and archiving the data for the 

user base. Two Ka/S-band ground stations 

are also baselined to provide two downlink 

passes per day; a third station would provide 

additional margin against missed passes 

due to weather or other conditions but is not 

fundamentally necessary to meet mission 

requirements. The ground stations consist 

of 18-m dishes in White Sands, NM, and in 

South Africa. Figure 8.6 shows a schematic 

of a notional ground system.

8.2.1.3 Launch Vehicle

As discussed in Section 8.1, LUVOIR-A 

has been designed to take full advantage of 

NASA’s Space Launch System (SLS) Block 

2 vehicle with an 8.4-m × 27.4-m “long” 

fairing. No other anticipated launch vehicle 

is capable of accommodating both the 

mass and volume of the LUVOIR-A design. 

However, the scalability of the architecture, 

as will be demonstrated by LUVOIR-B, 

allows for the mission design and science 

objectives to be tailored to whichever heavylift launch vehicles may be available or 

preferred in the 2030s. Additional details on 

current and anticipated launch vehicles are 

available in Appendix D.
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8.2.1.4 Concept of operations

LUVOIR’s concept of operations will be 

similar to other large observatories of its 

class. There are three general observation 

scenarios, related to the three key elements 

of LUVOIR’s science campaign: general 

astrophysics observations, local solar 

system observations, and high-contrast 

exoplanet observations. Each scenario may 

impose different operational constraints on 

the observatory. Regardless of the scenario, 

each observation will begin with a slew to the 

new target field-of-view.

8.2.1.4.1 Slew and target 

acquisition

Following a science observation, wavefront 

maintenance routine, or stationkeeping/

momentum dump maneuver, LUVOIR 

will be ready to begin a new science 

observation. The observatory will slew to a 

new target based on previously uploaded 

coordinates. Slews will incorporate pitchangle changes using the gimbal system, 

and yaw adjustments using the spacecraft 

ACS. For special sunward observations, the 

spacecraft ACS may also need to pitch the 

entire observatory. During a slew, the VIPPS 

will be actively isolating the payload from the 

spacecraft, reducing the dynamic inputs to 

the payload and thus reducing the amount 

of settling time that will be required once the 

final attitude is achieved.

Star trackers will be used to coarsely 

align the observatory to the target field, after 

which fine guiding will be done by the HDI 

instrument. Acquisition of suitable guide 

stars known to be in the field-of-view will be 

performed autonomously using the control 

system processor. Once fine guidance 

has begun, the observation can proceed, 

depending on the type of observation.

8.2.1.4.2 General astrophysics 

observations

General astrophysics observations using HDI, 

LUMOS, or POLLUX may begin shortly after 

a slew is complete, once system dynamics 

and thermal drift have settled to a level 

commensurate with their required wavefront 

stability (on the order of nanometers). While 

waiting for this settling to occur, internal 

instrument calibrations may be performed, 

if necessary, as well as instrument “set-up”: 

selecting appropriate filters and channels, 

or in the case of LUMOS, aligning the 

microshutter array to the target field.

During general astrophysics observations, parallel observation with other instruments is supported. It is possible to have 

HDI, LUMOS, and POLLUX collecting data 

simultaneously under the condition that secondary instrument operations do not impact 

those of the primary instrument. For example, if a specific LUMOS observation is being 

executed, HDI may collect science data on 

whatever field it happens to be pointed at, 

but HDI would not be able to request pointing dithers to fill in focal plane gaps during 

this time. Similarly, if HDI were operating as 

prime, LUMOS would not be able to request 

roll maneuvers to align targets to the microshutter array.

8.2.1.4.3 Solar system observations

Most solar system observations will likely 

require engaging LUVOIR’s target tracking 

capability (up to 60 mas/s). Following a slew 

to target, the target would first be acquired 

by HDI using in-field guide stars. A trajectory 

would be computed, and tracking would 

begin. For cases in which it is desired to 

observe the object with LUMOS, ECLIPS, 

or POLLUX, an additional offset would 

be computed to place the object in their 

respective fields-of-view. Depending on 
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The LUVOIR Interim Report 8-13

the speed and duration of the observation, 

tracking may be accomplished by the fast 

steering mirror, VIPPS, gimbal system, 

spacecraft ACS, or any combination thereof.

8.2.1.4.4 Exoplanet observations

High-contrast exoplanet observations 

will be the most demanding of LUVOIR’s 

capabilities. Following a slew, it is likely that a 

longer settling period will be required for the 

active control systems to help stabilize the 

wavefront error before acquisition of highcontrast images can begin. Once engaged, 

the “dark-hole” acquisition algorithm will use 

images from the coronagraph focal planes to 

generate updates to the deformable mirrors, 

gradually increasing the contrast in the focal 

plane to the desired level (~10-10 for most 

observations). Once the contrast is achieved, 

science observations will begin by taking 

long integrations to reveal targets of interest. 

During this time, onboard metrology systems 

and low-order and out-of-band wavefront 

sensors will work to maintain the wavefront 

error and high-contrast image. This entire 

process is executed autonomously by the 

onboard control system processor.

It is during the dark-hole acquisition 

process and science integrations that 

picometer-level wavefront error stability is 

required. If the wavefront error is drifting 

faster than the dark-hole wavefront control 

algorithm can keep up, the system will never 

achieve 10-10 contrast. Similarly, once the 

contrast is achieved, if the system drifts faster 

than the low-order or out-of-band wavefront 

sensors can keep up, then the contrast 

will degrade and long science integrations 

will not be possible. LUVOIR incorporates 

several techniques to achieve the necessary 

stability, discussed in more detail in the next 

section.

Parallel observations are also possible 

during exoplanet science observations, 

again assuming the secondary instrument 

operations do not impact the high-contrast 

performance. Given the picometer-level 

stability requirement, this may preclude even 

the actuation of filter wheels within the other 

instruments.

8.2.2 Design drivers

8.2.2.1 High-contrast imaging

While LUVOIR is intended to be a multipurpose, multi-user observatory, the desire 

to perform high contrast (10-10) direct 

imaging of habitable exoEarths around solartype stars with a coronagraph drives almost 

every aspect of the architecture. In addition 

to the need for coronagraph instruments 

capable of 10-10 contrast, it has been shown 

that achieving and maintaining such contrast 

requires wavefront stability of the end-toend optical system on the order of tens of 

picometers RMS over the spatial frequencies 

corresponding to the dark-hole region in the 

focal plane (Lyon & Clampin 2012; Shaklan 

et al. 2011). This stability is needed over 

time periods corresponding to the wavefront 

control bandwidth—on the order of tens 

of minutes for systems using light from 

the target star to generate the wavefront 

estimates. The approach to achieving highcontrast imaging on LUVOIR is three-fold:

High-contrast through ultra-stable 

systems: This approach attempts to design a 

system that is as stable as possible. Thermal 

stability is achieved using materials with 

near-zero coefficient of thermal expansion at 

the nominal operating temperature of 270 K: 

Corning ULE® glass for the mirror segment 

substrates and composite material for most 

of the structure (Eisenhower et al. 2015; Park 

et al. 2017). These materials are coupled with 

milli-Kelvin-level thermal sensing and control 

of the mirrors, structures, and interfaces. 

Material property characterization is critical to 

understanding the behavior of structures and 

interfaces and modeling their performance at 
LUVOIR The Large UV Optical Infrared Surveyor

8-14 The LUVOIR Interim Report

the picometer level. The choice of materials 

and the material properties work together 

to create a system that is both thermally 

stable and controllable at the observatory 

operating temperature. Ground-based 

metrology systems that can verify model 

predictions are currently being developed 

and have demonstrated the measurement of 

displacements of ~20 pm (Saif et al. 2017).

Dynamic stability is achieved with stiff 

mirrors and structures, passive isolation 

at the disturbance sources, and the noncontact VIPPS that actively isolates the 

payload from the spacecraft (Dewell et al. 

2017). The VIPPS “floats” the telescope and 

controls the payload attitude relative to the 

interface plane via six non-contact voicecoil 

actuators. The VIPPS effectively isolates any 

dynamic disturbances from the spacecraft 

attitude control system from transmitting to 

the payload and exciting resonances that 

contribute to wavefront instability. The VIPPS 

also provides fine pointing control of the 

payload during science observations (Dewell 

et al. 2017). Figure 8.7 shows a notional 

block diagram of the VIPPS control loops.

While the VIPPS provides ideal 

mechanical isolation, power and data 

signals must still be transmitted between the 

payload and spacecraft. Using a 120-v bus 

Figure 8.7. Block diagram of the VIPPS control system showing two separate position and attitude 

control loops. The payload provides a line-of-sight fine-guidance signal from the instrument suite, 

and commands the non-contact actuators to maintain a fixed pointing vector for the payload. 

The spacecraft attitude control actuators respond to commands from the non-contact position 

sensor to maintain an attitude relative to the payload. Dynamic disturbances from the attitude 

control actuators and flexible structures on the spacecraft are isolated from the payload via the 

non-contact interface, with the exception of power and data cables that must bridge the gap. The 

size of the VIPPS interface is exaggerated for clarity.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-15

for the electrical power system allows for 

fewer, smaller-gauge conductors to be used, 

minimizing the number of cables and cable 

stiffness that must bridge the non-contact 

gap. Similarly, by performing as much of 

the data-processing on the payload side as 

possible, the cabling requirements for data 

transfer between the payload and spacecraft 

are minimized. Work is being done to both 

determine the stiffness of cables that will be 

bridging the non-contact interface, as well 

as modeling those stiffness effects as part of 

the system-level integrated modeling effort.

High-contrast through wavefront 

control: This approach reduces the time 

period over which the system must be stable 

by increasing the temporal bandwidth of the 

wavefront control system. Figure 8.8 shows 

a block diagram of the pointing and wavefront 

control loops that are anticipated for LUVOIR. 

Pointing errors are sensed at the focal plane 

of the High Definition Imager instrument 

Figure 8.8. Pointing and wavefront control loops baselined for the LUVOIR-A architecture. 

Payload line-of-site pointing is sensed at the HDI focal plane via fast centroiding on selected 

guide stars. Pointing is then controlled by three actuators with different bandwidths: the fast 

steering mirror controls the highest bandwidth errors, the VIPPS controls lower bandwidth errors, 

and the spacecraft ACS controls the lowest. In addition to low-order wavefront error terms, the 

low-order wavefront sensor inside of the coronagraph also senses line-of-site pointing errors 

specific to the coronagraph channels and uses the deformable mirrors to correct slow drifts in 

both pointing and wavefront during exoplanet observations. A segment edge sensor and laser 

metrology system maintains phasing and alignment of the primary and secondary mirrors.
LUVOIR The Large UV Optical Infrared Surveyor

8-16 The LUVOIR Interim Report

and drive high-bandwidth corrections at the 

fast steering mirror, and lower bandwidth 

corrections at the VIPPS. Slow, low-order 

pointing and wavefront drifts are sensed 

internal to the coronagraph with a low-order 

wavefront sensor (LOWFS), similar to the one 

to be used on WFIRST (Shi et al. 2017) and 

corrected at the coronagraph deformable 

mirrors. The LOWFS system, however, is 

fundamentally limited in which wavefront 

terms are sensed, and how fast they can 

be sensed. At the same time, fast dynamic 

motions of the primary mirror segments 

will be measured using edge sensors and 

corrected with piezo-electric (PZT) actuators 

(Feinberg et al. 2016). A laser metrology 

system is also being evaluated for sensing 

and controlling secondary mirror position 

errors (Nissen et al. 2017). Closed-loop 

control will be achieved with both feedback 

and feedforward systems (Shi et al. 2017).

High-contrast through wavefront 

tolerance: This approach attempts to 

relax the fundamental stability requirement 

from tens of picometers to hundreds of 

picometers, or even nanometers, through new 

coronagraph architectures that are designed 

to be less sensitive to the wavefront error 

modes that are most prominently excited in 

the LUVOIR architecture (Jewell et al. 2017; 

Ruane et al. 2016; Zimmerman et al. 2016). 

Additionally, spatial filtering techniques are 

being explored that may enable additional 

orders-of-magnitude in contrast, further 

relaxing the stability requirement. While many 

of these approaches show promise, they 

are still very early in development and it is 

unclear how much the stability requirement 

may be relaxed.

The technologies and design principles 

described above represent an array of tools 

that will be applied to the LUVOIR concept 

to achieve the required stability for highcontrast imaging of exoEarths. The current 

state-of-the-art of these technologies, and 

their paths to development are described in 

more detail in Chapter 11.

It is also important to note that picometerlevel wavefront stability is only necessary 

during high-contrast exoplanet observations. 

It is not intended that LUVOIR will maintain 

picometer stability 100% of the time while 

on-orbit. During slews, stationkeeping 

maneuvers, and momentum dumps, the 

wavefront of the system will surely drift due 

to mechanical disturbances and changes 

in thermal loads, even with active control 

systems like the VIPPS, edge sensors, and 

thermal control always operating. However, 

most general astrophysics observations do 

not require the system to achieve its targeted 

wavefront stability before observations can 

begin. A more detailed concept of operations 

is provided in Section 8.2.1.4.

8.2.2.2 Thermal management

The thermal architecture of the observatory 

plays a critical role in enabling the broad 

wavelength range of LUVOIR’s science goals. 

The nominal 270 K operating temperature 

represents a balance between operating at 

a cold enough temperature to achieve low 

thermal backgrounds in the NIR science 

bands, while operating warm enough to help 

mitigate contamination concerns in the UV 

science bands. Evidence from JWST thermal 

testing shows that below 260 K, molecular 

species rapidly begin to deposit on surfaces, 

which would degrade UV throughput 

(see Appendix D). Thus, 270 K optimally 

maximizes NIR science while not significantly 

impacting UV science.

Separate from science considerations, 

270 K was also chosen for a number of engineering reasons. At warmer temperatures, 

damping of dynamic disturbances is increased. Also, the selection of materials that 

have both near-zero coefficient of thermal 

expansion and that are also well characterized is larger near room temperature. Finally, 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-17

tinues to be developed through investments 

elsewhere in NASA and by other government agencies, then LUVOIR could leverage these resources to extend its mission 

beyond the initial 5-year requirement. Thus, 

the reliability, lifetime, and consumables have 

been designed to meet a 5-year minimum 

lifetime for all mission elements, a 10-year 

goal lifetime for all serviceable or replaceable 

mission elements, and a 25-year goal lifetime for all non-serviceable/replaceable elements. An additional advantage to designing 

for serviceability can be realized much earlier 

in the mission lifetime: the modular nature of 

LUVOIR’s systems can facilitate testing and 

re-testing at multiple stages during integration.

The serviceable elements on the 

observatory include the four instrument 

modules themselves. Figure 8.9 shows a 

concept for how the instrument modules 

could be removed from the BSF and 

replaced. Key spacecraft subsystems are 

also designed to be replaceable, including the 

control moment gyroscopes, the avionics, 

and the solar array. The main elements of 

the observatory that have not been designed 

to be serviced include the optical telescope 

element and backplane support frame, the 

VIPPS and gimbal system, and the sunshield.

8.2.2.4 Launch Vehicle

As the capabilities of the SLS Block 2 and its 

user base are still evolving, so is the certainty 

of which of several fairing options will be 

developed: an 8.4-m × 19.1-m “short” fairing, 

an 8.4-m × 27.4-m “long” fairing, and a 10.0-

m × 27.4-m fairing. The LUVOIR team opted 

to be conservative in the fairing diameter but 

chose the 8.4-m “long” fairing as the “short” 

version does not provide enough height 

under the ogive to fit a > 7-m aperture. The 

lift capacity for the Block 2 configuration 

with the 8.4-m “long” fairing is 44.3 metric 

tons (NASA 2017). The deployable LUVOIR 

a warmer temperature was chosen to help 

reduce the complexity of fabrication, integration, and test campaigns. By fabricating 

components at a temperature near where 

they are to be operated, modeling errors can 

be reduced, as well as the complexity, number, and duration of thermal cycle tests.

While most of the payload maintains an 

operating temperature of 270 K, several 

optical benches and detectors operate 

at colder temperatures of 170 K and 100 

K, achieved through passive cooling. The 

thermal management system must work to 

simultaneously hold the optical telescope at 

270 K with 1-mK stability while enabling the 

passive cooling. A large sunshield, similar 

to JWST’s, provides a stable, cold-biased 

environment for the payload. Heaters are then 

used to precisely control the temperature 

throughout the payload.

8.2.2.3 Serviceability

Drawing on the legacy of HST, LUVOIR is being 

designed from the outset to be serviceable 

and upgradeable to enable a multi-decade 

science mission that is responsive to a 

changing scientific landscape. We note 

however, that while the Study Team is 

designing LUVOIR to be serviceable, we are 

not designing the infrastructure to actually 

execute the servicing. Nor is it required to 

service LUVOIR for its users to achieve all 

of the science presented in this report within 

the first five years of the mission.

Other programs within NASA are currently developing autonomous and tele-robotic servicing capabilities.2

 NASA’s Human 

Exploration and Operations Mission 

Directorate is also investigating the viability 

of using a Deep Space Gateway in cis-lunar 

space for the servicing of scientific assets, 

such as LUVOIR.3 If this infrastructure con2 For example, the NASA GSFC Satellite Servicing 

Projects Division (https://sspd.gsfc.nasa.gov).

3 See https://exoplanets.jpl.nasa.gov/exep/technology/in-space-assembly/
LUVOIR The Large UV Optical Infrared Surveyor

8-18 The LUVOIR Interim Report

architecture is designed to maximize the 

science capability that can be fit within these 

volume and mass constraints. Figure 8.10

shows the LUVOIR-A concept stowed in the 

8.4-m “long” fairing and Table 8.4 shows an 

allocated mass budget.

8.2.2.5 Integration and test

The integration and testing (I&T) of LUVOIR 

requires careful thought and planning 

to minimize both the programmatic and 

technical challenges a mission of this 

magnitude will present. We understand that 

a long and drawn out integration and test 

campaign will directly impact the total cost 

and schedule of the mission development. A 

lesson learned from the JWST program is that 

costs are generally driven by the expense of 

having a “marching army” to staff the project, 

including managers, scientists, engineers and 

technicians among others. Once the final I&T 

of the observatory is underway, more work is 

serialized rather than in parallel. When there is 

a problem in this phase of integration, a large 

contingent of the marching army may have 

to stand down while the problem is resolved. 

Figure 8.10. LUVOIR-A stowed in the SLS 

Block 2 8.4-m “Long” fairing.

Figure 8.9. Notional servicing sequence. Top 

Left: The payload is gimbaled to the 0° angle. 

Top Right: Servicing doors at either end of 

the BSF are opened. Bottom Left: Instrument 

modules are extracted. Bottom Right: The 

payload is ready to receive new instrument 

modules. In practice, only one instrument 

would be serviced at a time. While this model 

doesn’t have enough detail to show all of the 

mechanisms required, the study team has 

included rails, latches, grapple fixtures, and 

mate/demate fixtures in more detailed models, 

as well as the Master Equipment List and mass 

allocations.
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The LUVOIR Interim Report 8-19

This is distinctly different from earlier phases 

of a project where lower levels of assembly 

can be integrated and tested in parallel. A 

problem in one parallel path is rarely a reason 

to stand-down the majority of project staff in 

all paths. Thus, problems are generally less 

costly in terms of the marching army the 

earlier they occur. This is justification alone 

to do as much early prototyping and testing 

as possible. The less “new” information that 

is learned during Observatory I&T, the better.

At the time of preparing this interim 

report, the Study Team has only begun to 

think about the I&T flow for LUVOIR at the 

most basic levels: trying to determine which 

levels of assembly may be tested in parallel 

versus which must be done serially; which 

requirements must be verified by test versus 

which must be verified by analysis; and the 

logistics of transporting, assembling, and 

testing systems as large as LUVOIR. We will 

report here our initial thoughts on what will 

drive the LUVOIR I&T campaign, however a 

more detailed discussion will be presented in 

the final report, once a more complete picture 

of the LUVOIR architecture is available.

There are three primary challenges to the 

I&T of LUVOIR:

• the sheer size of the observatory, 

• the verification of picometer-level 

stability, and 

• the control of contamination impacting 

the far-UV science.

Overcoming these challenges will require 

a combination of leveraging processes 

and techniques that have been developed 

for missions such as HST and JWST and 

developing new methods and facilities 

specific to the LUVOIR architecture. We 

address each challenge separately below.

8.2.2.5.1 Observatory size

Like the International Space Station (ISS), 

LUVOIR-A will be a space system of 

extraordinary size and complexity. Unlike the 

ISS, however, LUVOIR will be assembled 

on the ground and launched as a single 

Table 8.4. Mass Budget for the LUVOIR-A Observatory. Current Best Estimate (CBE) values are 

a combination of grass-roots estimates based on instrument designs and top-down allocations 

based on mass fractions from JWST. Maximum Expected Values (MEV) include a 30% growth 

allowance that accounts for the early-stage nature of the designs. Maximum Permissible Values 

(MPV) include an additional 25% margin as required by NASA Goddard Space Flight Center 

GOLD Rules. We note that the OTE and BSF growth allowance factors are lower than would 

typically be applied as their designs are based heavily on flight-qualified prototypes. For the OTE, 

the primary mirror segment assembly is based on the Advanced Mirror System Demonstrator 

(Matthews et al. 2003), and the BSF is based on the JWST BSF design.

System / Subsystem Mass 

(kg)

Growth 

Allowance 

MEV Mass 

(kg)

GOLD Rules 

Margin

MPV Mass 

(kg)

Optical Telescope Element (OTE) 7699 19% 9505 25% 12673

Backplane Support Frame (BSF) 4436 26% 5995 25% 7993

ECLIPS 1200 30% 1714 25% 2286

HDI 1060 30% 1514 25% 2019

LUMOS 830 30% 1186 25% 1581

POLLUX 780 30% 1114 25% 1486

Spacecraft 7,483 30% 10690 25% 14253

Propellant 1,502 0% 1502 25% 2003

Total: 25,028 - 33,276 - 44294
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system. This presents a logistical challenge: 

how does one verify that the fully integrated 

system meets all performance requirements 

and is capable of surviving the extreme 

environments encountered during launch? 

Traditionally, NASA has adhered to a golden 

rule of “test as you fly, fly as you test,” 

subjecting fully integrated flight systems to 

a series of environmental, performance, and 

functional tests to ensure their viability onorbit. However, missions such as LUVOIR are 

so large and complex that it is impossible to 

exactly replicate the flight conditions in a test 

environment for the entire observatory. This 

necessitates the “test as you fly” paradigm 

to evolve into a process combining lowerlevel testing with using high-fidelity, validated 

models to verify the expected performance 

of the observatory. Projects such as JWST 

have had to obtain waivers for the “test as 

you fly” requirement: e.g., while the optical 

telescope and instruments (a.k.a. “OTIS”) 

underwent cryogenic optical performance 

testing, the test itself was not a full-aperture 

end-to-end optical test that verified image 

quality. Instead, the test confirmed the 

ability of the system actuators to respond to 

commands based on analysis of image data 

collected at the instrument focal planes. Most 

importantly, this test was the culmination of 

many subsystem tests and model validation 

efforts building on top of each other – all 

used to create a system-level model that 

is capable of accurately predicting on-orbit 

performance.

Similarly, once OTIS is integrated to 

the spacecraft and sunshield, the full 

observatory will not be tested at cryogenic 

temperatures in a thermal vacuum chamber. 

JWST will only see a portion of the tests 

that most space systems would normally 

see at this level, including mechanical 

testing followed by functional tests to 

verify assembly workmanship. Instead, 

the individual subsystems underwent 

rigorous performance testing at lower 

levels of assembly. Validated models, 

tracking of requirements, and verification of 

workmanship will be relied upon to ensure 

the fully integrated observatory will perform 

as expected on-orbit.

LUVOIR will require a similar I&T 

methodology for performance verification, 

though it is likely that the division between 

verification by test and verification by analysis 

will need to happen at even lower-levels 

of assembly. For example, even though 

LUVOIR-A’s 15-m diameter primary mirror 

would physically fit inside of the Johnson 

Space Center Chamber-A (where the JWST 

OTIS was tested, see Figure 8.11), it 

would not fit through the chamber door fully 

deployed. Once deployed, there would be 

very little room left for optical, mechanical, 

and thermal ground support equipment, all 

critical to executing a performance test. The 

Plum Brook test chamber at NASA’s Glenn 

Research Center (see Figure 8.11) is larger 

and would accommodate LUVOIR with some 

room for ground support equipment, but its 

remote location raises another challenge: 

transporting the observatory prior to launch. 

The stowed JWST OTIS barely fits in a C-5 

Galaxy aircraft—the largest cargo transport 

operated by the United States—and the 

fully integrated observatory must be barged 

to its final launch site in South America. 

Given that JWST has already stretched the 

available transport, integration, and testing 

infrastructure to the limit, LUVOIR will need 

to develop new options, including but not 

limited to:

• New integration and test facilities colocated with the final launch site. These 

facilities would allow components and 

subsystems that are assembled at 

remote locations to be transported by 

traditional means before being integrated 

and tested at the final launch site. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-21

• New transport vehicles, such as the 

Stratolaunch aircraft,4 or a modified 747 

that carries cargo outside its hull similar to 

the Shuttle Carrier Aircraft, or easy access 

to waterways may be necessary to move 

the largest components, including the 

fully integrated observatory. 

• A modular design of LUVOIR would 

enable the disassembly and reassembly 

of large portions of the system. Individual 

modules can undergo environmental 

and performance testing before being 

reassembled. Designing for serviceability 

is already a step in this direction, as at 

least the instruments and spacecraft 

subsystems are developed to be easily 

and repeatably inserted and removed.

This leads to the issue of qualifying the 

hardware for the launch environment. While 

it seems plausible, perhaps even advanta4 See http://www.stratolaunch.com/

geous, to argue that performance verification 

can be broken into smaller pieces, it is hard to 

make that same argument when it comes to 

verifying assembly workmanship in preparation for launch. Without subjecting the observatory to mechanical launch environments, it 

is hard to believe that a launch vehicle vendor 

will accept the risk of a catastrophic failure. 

At this point in time, the design of LUVOIR is 

not mature enough to know whether existing 

vibration and acoustic testing facilities will be 

able to accommodate it. Two of the largest 

vibration tables in the world are located at 

NASA Glenn Research Center’s Plum Brook 

Station5 and the European Space Agency’s 

European Space Research and Technology 

Centre (ESTEC)6. At this time, it is unclear if 

either has the capacity to shake something 

5 See https://www1.grc.nasa.gov/facilities/spf/

6 See http://www.esa.int/Our_Activities/Space_

Engineering_Technology/Test_centre/Hydraulic_shaker

Figure 8.11. Left: JWST emerges from the Johnson Chamber A test facility following cryogenic 

optical testing. It is clear from this photo that LUVOIR-A would not fit through the door fully 

deployed. If deployment inside the chamber were possible, the height of the chamber would limit 

the ability to perform optical testing. Right Top: The Plum Brook Station Space Power Facility at 

NASA’s Glenn Research Center could accommodate a fully deployed LUVOIR-A. Right Bottom: 

A 20-m solar sail being deployed inside of the Plum Brook Station chamber. Credit: NASA
LUVOIR The Large UV Optical Infrared Surveyor

8-22 The LUVOIR Interim Report

with as much mass and inertia as LUVOIR. It 

appears that both large acoustics chambers 

at Plum Brook Station and ESTEC would fit 

an observatory of LUVOIR’s size with some 

ground support equipment to protect the 

stowed observatory.7

Regardless of the approach taken, the 

LUVOIR I&T program will require careful 

attention to logistics to ensure a smooth flow. 

And while much of the hardware and ground 

support equipment that was developed for 

JWST won’t be directly useable by LUVOIR, 

the concepts and methodologies developed 

for JWST I&T will be. Key among them is the 

concept of using validated models at every 

level of assembly to verify requirements, 

especially those pertaining to ultra-stability.

8.2.2.5.2 Verifying picometer-level 

stability

Many of the challenging requirements 

for LUVOIR create similarly challenging 

requirements for the test facilities and ground 

support equipment, which may very well 

need to be more stable or sensitive than the 

flight hardware under test in order to verify 

system performance. LUVOIR’s ultra-stability 

requirements will demand micro-gravity-level 

vibration isolation and sub-milliKelvin-level 

thermal stability for at least some testing. 

While these levels of sensitivity are being 

demonstrated in lab facilities today over 

small scales (Saif et al. 2017), these levels 

are beyond the state-of-the-art for larger 

class facilities and may well be beyond what 

is physically possible for ground-based 

facilities. Verification by analysis may be 

the only way to demonstrate that LUVOIR 

is capable of achieving picometer level 

wavefront stability on-orbit.

7 See https://www1.grc.nasa.gov/facilities/

aapl/ and http://www.esa.int/Our_Activities/

Space_Engineering_Technology/Test_centre/

Large_European_Acoustic_Facility_LEAF

8.2.2.5.3 Controlling contamination 

for far-UV performance

The LUVOIR Study team recognizes that conducting science in the far-UV means that the 

hardware will be very sensitive to molecular 

contamination. At this time, our general plan 

is to use standard processes used on previous UV and X-ray instruments and systems. 

These include, but are not limited to, material controls, component bake-outs both on 

ground and on-orbit, and certifications. The 

270 K operating temperature was selected 

in part for contamination control reasons. It 

is slightly above the temperatures at which 

quartz crystal microbalance monitors have 

measured molecular buildup during JWST 

testing, and it is above the temperature at 

which known molecules of concern would 

stick to critical surfaces. Mirror heaters are 

also being sized to allow for occasional baking out of contaminants on-orbit, similar to 

the annealing done on flight detectors on 

HST. These bake-outs would be done infrequently, perhaps once a year, to minimize 

the impact on science operations. Finally, 

LUVOIR will use techniques developed for 

JWST to deal with particulates that are on 

the mirror substrate. For example, JWST 

has cleaned the entire primary mirror using 

a brush technique with excellent results on 

large particulates.

8.2.3 Design alternatives

While the architecture presented in Section 

8.2.1 represents one possible approach to a 

feasible 15-m-class LUVOIR mission, there 

exists a number of design alternatives that 

provide different approaches to achieving 

the ambitious science goals identified by the 

STDT. At the time of this writing, the Study 

Office has completed some of the trade 

studies identified below to develop the current 

LUVOIR-A architecture and is in the process 

of considering the others in an attempt to 

improve overall system performance. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-23

8.2.3.1 Coronagraph vs. starshade

There are two fundamental approaches to 

achieving the necessary starlight suppression 

for exoplanet science: a coronagraph, and 

a starshade (sometimes called an external 

occulter). Each has its own advantages and 

disadvantages.

A coronagraph works by rejecting onaxis starlight that has entered the telescope 

while allowing off-axis planet light to be 

imaged onto a focal plane. The starlight 

rejection is usually achieved through some 

combination of occultation, diffraction 

control, and destructive interference. 

Since a coronagraph is located inside the 

observatory payload, it allows for very nimble 

observations; as quickly as the telescope is 

able to slew from one target to the next, a 

new exoplanet system can be observed. The 

two primary drawbacks to a coronagraph 

instrument are the extraordinary wavefront 

stability required by the optical system to 

achieve 10-10 contrast, and limited throughput 

and bandpass that result from the complex 

optical systems that implement the starlight 

suppression. Coronagraphs are efficient at 

finding target exoplanets, but less efficient at 

characterizing them.

Conversely, starshades work by 

rejecting the on-axis starlight before it 

ever enters the telescope, removing the 

need for extreme diffraction and wavefront 

control from the observatory. Instead, the 

challenges of a starshade include the need 

to precisely deploy an extremely large (>70-

m diameter) structure and fly that structure in 

formation with the observatory at separation 

distances of tens to hundreds of thousands 

of kilometers. Starshades enable highthroughput, broadband observations of target 

stars making them effective at spectroscopic 

characterization of exoplanets. But the 

need to reposition the starshade over large 

distances for every telescope pointing vector 

means that starshades are inefficient at 

observing many systems and finding new 

targets. Figure 8.12 shows some of the 

technology challenges and demonstration 

efforts that are ongoing for starshades.

LUVOIR’s science case requires the 

ability to survey hundreds of potential 

planetary systems in order to guarantee 

enough exoEarths to statistically constrain 

the frequency of habitability. A coronagraph 

provides the necessary agility to perform 

such a survey. Ideally, once a target of 

interest is discovered, a starshade could 

be moved into place to enable efficient 

spectral characterization of the target. 

However, LUVOIR’s large aperture would 

require starshades in excess of 70 meters 

at very large separations, well beyond the 

starshade technologies that are currently 

being developed. For now, LUVOIR has 

decided to rely on a coronagraph instrument, 

equipped with a spectrograph, to perform the 

exoplanet discovery and characterization. 

However, the observatory is being designed 

to be starshade-compatible and a follow-on 

starshade rendezvous with LUVOIR may be 

a viable option; see Appendix E for more 

details.

8.2.3.2 Off-axis, unobscured vs. onaxis, obscured apertures

An off-axis, unobscured telescope design 

has the advantage of increasing throughput 

by removing the central obscuration, while 

also improving compatibility with several 

mask-based coronagraph architectures, 

providing substantial gains in coronagraph 

core throughput. However, an unobscured 

aperture would not leverage the approach 

used on the JWST for deploying the 

secondary and would require developing and 

demonstrating new packaging concepts for 

the observatory. Such repackaging is feasible 

given the large fairing volume provided by 

the SLS Block 2 vehicle, but likely constrains 
LUVOIR The Large UV Optical Infrared Surveyor

8-24 The LUVOIR Interim Report

the total aperture diameter to be smaller than 

would be possible with an on-axis design. 

It is possible the gains in coronagraph 

throughput exceed the reduction in aperture 

size in terms of exoplanet science yield, 

however the smaller aperture also negatively 

impacts collecting area and resolution for the 

general astrophysics program. An off-axis 

optical design also requires the secondary 

mirror to be nearly twice as far from the 

primary mirror than a comparable on-axis 

design in order to maintain the same anglesof-incidence across the pupil, a requirement 

driven by the need to mitigate polarization 

aberrations that can significantly reduce 

coronagraph performance.8

The specific geometry of the on-axis 

segmented pupil is related to this trade as well. 

The current LUVOIR-A baseline represents 

an extension of the JWST segment geometry 

with four additional rings of slightly smaller 

segments (1.15-m flat-to-flat vs. 1.32-m flatto-flat). The first optical design for LUVOIR 

required the central ring of segments to be 

removed to accommodate both a larger 

secondary mirror obscuration as well as the 

optical path between the secondary, tertiary, 

and fine-steering mirrors. The resulting large 

8 See https://exoplanets.nasa.gov/exep/technology/gap-lists/

Figure 8.12. Starshade technology development challenges (Figure courtesy NASA/ExEP8). 

Three main challenges remain for starshade technology development: validating the starlight 

suppression of the starshade design, precision deployment of a 70-m-class structure, and 

formation flying between the starshade and telescope.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-25

obscuration negatively impacts coronagraph 

performance. At the time of writing this 

report, the team has studied a number of 

new aperture geometries and optical designs 

to mitigate this effect. Although many of 

the figures in this interim report show the 

original aperture design with a large central 

obscuration, the team has since chosen a 

new aperture geometry and optical design 

that is more conducive to coronagraph 

performance. Figure 8.13 shows the old 

LUVOIR-A aperture compared to the new 

aperture.

8.2.3.3 Optical vs. radio 

communications

The conventional Ka-band system that is 

currently baselined on LUVOIR-A provides 

adequate margin for LUVOIR’s anticipated 

data volumes. However, an optical communication system would provide shorter downlink times, as well as additional room for data 

volume growth as new instruments may be 

deployed. Optical communication technology has a high Technology Readiness Level 

(TRL) after having been demonstrated on the 

Lunar Laser Communications Demonstration 

(Boroson et al. 2014). The existing limitation 

is with the number of available ground stations to ensure adequate coverage. Should 

there be investment in new optical communications ground systems, LUVOIR could 

easily adopt this solution. Thermal impacts 

of increased power consumption for the optical communication system also needs to 

be considered.

8.2.3.4 Disturbance isolation vs. 

disturbance reduction

The current LUVOIR baseline design uses 

several layers of both passive and active isolation to prevent disturbances from reaching 

the opto-mechanical payload. Passive isolators are used at the disturbance source (control moment gyroscopes, mechanisms, etc.) 

and a non-contact active isolation system 

separates the payload from the spacecraft. 

Initial analysis by the LUVOIR engineering 

team shows that this architecture contributes 

to a dynamically ultra-stable system capable 

of achieving high-contrast observations.

An alternative to isolating disturbance 

sources from the optical system is to remove 

those disturbances altogether. A primary 

Figure 8.13. A comparison between the initial LUVOIR-A aperture (left) and the new aperture 

(right). The new aperture achieves the same 15.0-m outer diameter while maximizing the inscribing 

diameter and minimizing the central obscuration. Both apertures have 120 segments, although 

the segment size increased from 1.15-m flat-to-flat (left) to 1.223-m flat-to-flat (right).
LUVOIR The Large UV Optical Infrared Surveyor

8-26 The LUVOIR Interim Report

source of dynamic disturbance comes from 

spinning mass used as part of the attitude 

control system, such as reaction wheel assemblies or control moment gyroscopes. 

Microthrusters are a relatively newer technology that may be used in place of these 

mechanisms to control and maintain the attitude of the system while introducing very little 

or no disturbance to the system (Chapman 

et al. 2011).

For the current LUVOIR design, the 

articulated nature of the payload necessitates 

the use of control moment gyroscopes; the 

force required to react against the gimbal 

mechanism as it slews the payload to a 

new pointing far exceeds the capabilities 

of microthrusters. However, a hybrid 

approach may be possible in which control 

moment gyroscopes or reaction wheels are 

used during slews of the payload and then 

stopped. Microthrusters would then take 

over and maintain the observatory attitude 

during an observation. It is unclear whether 

this approach is more efficient than the 

current architecture using a non-contact 

isolation system, but it does provide an 

alternative technology approach should 

the non-contact system’s development be 

delayed. 

8.2.3.5 Approaches for on-orbit 

metrology and wavefront control

As this and subsequent chapters describe, 

many tools are being applied to the challenge 

of achieving picometer-level stability on a 

system as large and complex as LUVOIR. 

Stable structures and materials, active and 

passive dynamic isolation, thermal sensing 

and control, wavefront sensing and control, 

and on-board metrology all contribute 

to the goal of ultra-stability. Within each 

technology there are several options for 

specific implementations. For example, 

metrology systems may consist of segment 

edge sensors, a laser truss, or a combination 

of both. Edge sensors themselves could be 

capacitive, inductive, or optical. Systemslevel modeling is being performed to 

determine the highest-performance, highestTRL configuration that is capable of achieving 

the metrology requirements.

The specific implementation of wavefront 

sensing and control is another area of study. 

Image-based techniques such as phase 

retrieval are high TRL after development for 

JWST but may not be sufficient for active 

control during coronagraph operations. Loworder and out-of-band wavefront sensing 

techniques are under development by 

WFIRST for maintaining wavefront stability 

as part of coronagraph instrument designs. 

All of these techniques suffer from the need 

to collect enough stellar photons to estimate 

the wavefront error, a process that can take 

many minutes to tens of minutes for the 

types of stars that LUVOIR will observe.

Chapter 11 goes into more detail on 

the specific technologies that enable the 

LUVOIR-A architecture as described here 

and in Chapter 9. There are many tools in the 

toolbox, of varying capability and technology 

readiness. At the time of writing this interim 

report, the Study Team is still evaluating the 

optimal combination of wavefront sensing, 

metrology, isolation, and control that will 

provide picometer-level stability to enable 

high-contrast imaging. 

8.3 Looking ahead

The LUVOIR study team has begun a 

second design iteration on the LUVOIR-A 

architecture, with the primary goal of further 

improving the exoplanet science yield. The 

effort focuses mostly on optimizing the 

segmented aperture geometry to be more 

compatible with coronagraph mask designs, 

specifically reducing the central obscuration 

size and maximizing the inscribing diameter 

of the aperture. In parallel with this effort, 

coronagraph design teams are continuing 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 8-27

to develop new masks and instrument 

architectures that continue to push the 

state-of-the-art on segmented-aperture 

coronagraphy to further improve the 

performance of LUVOIR.

The study team is also developing a 

structural-thermal-optical integrated model 

of the design that can be used to evaluate the 

thermal and dynamic stability of the system. 

This is critically important to demonstrating 

that the design and the incorporated 

technologies are capable of achieving the 

picometer-level wavefront stability required 

for the high-contrast imaging. The integrated 

models will also be used to inform technology 

development efforts and focus resources on 

the most promising technologies.

While completing an update to the 

LUVOIR-A architecture, the study team 

plans to develop a concept for LUVOIR-B: 

an 8-m observatory that is capable of being 

launched in a conventional 5-m diameter 

fairing. We expect that not all elements of 

the observatory will scale accordingly and 

that some level of redesign will be needed 

on LUVOIR-B. For example, while the size 

and mass will scale proportionally, the power 

consumed by the instruments most likely 

will not, requiring updates to the thermal 

management system. The LUVOIR-B design 

also gives the team an opportunity to explore 

other options for some of the trades outlined 

in Section 8.2.3, including an off-axis 

unobscured primary mirror option.

In Chapter 9, we describe in detail 

the payload of the LUVOIR-A design, 

including the optical telescope element, the 

backplane support frame, and the three USled instrument designs: ECLIPS, LUMOS, 

and HDI. Element-specific design drivers 

and design alternatives are also provided. 

Chapter 10 provides additional detail on 

the CNES-contributed POLLUX instrument 

design, and Chapter 11 provides greater 

detail on the technologies that enable or 

enhance the LUVOIR science mission, and 

what additional development is still needed.

References

Boroson, D. M., Robinson, B. S., Murphy, 

D. V., et al. (2014). SPIE Photonics 

West 2014-LASE Lasers Sources, 

SPIE, 8971, 89710S

Chapman, P., Colegrove, T., Ecale, E., & 

Girouart, B. (2011). 8th Int Conf Guid 

Navig Control Syst

Dewell, L. D., Tajdaran, K., Bell, R. M., et 

al. (2017). UV/Optical/IR Sp Telesc 

Instruments Innov Technol Concepts 

VIII, SPIE, 10398, 103980B

Eisenhower, M. J., Cohen, L. M., Feinberg, 

L. D., et al. (2015). UV/Optical/IR Sp 

Telesc Instruments Innov Technol 

Concepts VII, SPIE, 9602, 96020A

Feinberg, L., Rioux, N., Bolcar, M., et 

al. (2016). Space Telescopes and 

Instrumentation 2016: Optical, Infrared, 

and Millimeter Wave, SPIE, 9904, 

99040H

Jewell, J. B., Ruane, G., Shaklan, S. B., 

Mawet, D., & Redding, D. (2017). Tech 

Instrum Detect Exopl VIII, SPIE, 10400, 

104000H

Lyon, R. G., & Clampin, M. (2012). Opt Eng, 

51, 11002

Matthews, G., Barrett, D., Bolton, J., et al. 

(2003). Opt Manuf Test V, SPIE, 5180, 

169

NASA. (2017). Space Launch System (SLS) 

Mission Planner’s Guide Rev. F

Nissen, J. A., Azizi, A., Zhao, F., et al. 2017, 

UV/Optical/IR Sp Telesc Instruments 

Innov Technol Concepts VIII, SPIE, 

10398, 103980I

Park, S. C., Eisenhower, M. J., Bluth, M., 

et al. (2017). UV/Optical/IR Sp Telesc 

Instruments Innov Technol Concepts 

VIII, SPIE, 10398, 103980D
LUVOIR The Large UV Optical Infrared Surveyor

8-28 The LUVOIR Interim Report

Ruane, G., Jewell, J., Mawet, D., Pueyo, 

L., & Shaklan, S. (2016). Advances in 

Optical and Mechanical Technologies 

for Telescopes and Instrumentation II, 

SPIE, 9912, 99122L

Saif, B., Chaney, D., Greenfield, P., et al. 

(2017). Appl Opt, 56, 6457

Shaklan, S. B., Marchen, L., Krist, J., & 

Rud, M. (2011). Tech Instrum Detect 

Exopl V, SPIE, 8151, 815109

Shi, F., An, X., Balasubramanian, K., et al. 

(2017). Tech Instrum Detect Exopl VIII, 

SPIE, 10400, 13

Zimmerman, N. T., Riggs, A. J. E., Kasdin, 

N. J., & Vanderbei, R. J. (2016). JATIS, 

2, 11012
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-1

Chapter 9
LUVOIR The Large UV Optical Infrared Surveyor

9-2 The LUVOIR Interim Report

9 The LUVOIR telescope and instruments

LUVOIR is composed of two distinct 

elements: The Payload and the Spacecraft. 

The Payload is composed of several 

components, highlighted in Figure 9.1, 

which will be discussed further in this chapter:

• The optical telescope element (OTE)

• The backplane support frame (BSF)

• The science instruments

• Extreme Coronagraph for Living 

Planetary Systems (ECLIPS)

• LUVOIR Ultraviolet Multi-Object 

Spectrograph (LUMOS)

• High Definition Imager (HDI)

• POLLUX (discussed in detail in 

Chapter 10).

9.1 Optical telescope element 

(OTE)

9.1.1 OTE overview

The optical telescope element (OTE), 

highlighted in Figure 9.2, is the optical 

path every photon follows at the end of its 

long journey from the astronomical objects 

being observed to the instrument suite and 

detectors. The OTE must be designed to 

achieve all science objectives identified by 

the LUVOIR Study Team, and yet be capable 

enough to respond to an ever-changing 

scientific landscape.

Figure 9.1. Major elements of the LUVOIR payload.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-3

9.1.2 OTE design drivers

Some of the key drivers that shape the 

implementation of the OTE include:

• High-contrast imaging

• Science Instrument fields-of-view

• Wavelength range

• Wavefront error

• Thermal management

• Launch vehicle mass-to-orbit capability

• Launch vehicle volume

• Contamination control

As discussed in Chapter 8, the wavefront 

error stability needed to achieve the highcontrast exoplanet science is a critical 

requirement that drives the OTE design. The 

material choices, thermal control system, 

segment-level architecture, segment phasing 

concept, and secondary mirror support 

structure design have been driven by the 

need to maintain wavefront stability at the 

picometer level (Feinberg et al. 2017).

Unlike the science instruments and 

significant portions of the spacecraft, the 

OTE is not being designed for modularity nor 

potential serviceability and as such, it must 

be reliable and robust.

9.1.3 OTE design implementation

The breadth of the science case described 

in earlier sections implies a challenging set of 

high-level capabilities for the OTE: sensitivity, 

resolution, stability, and wavelength range. It 

must be agile enough to repoint quickly and 

maximize observation time, and the whole 

system must be flexible enough to adapt to 

ever-evolving science objectives. Combining 

these capabilities with constraints such as 

mass, volume, and technical maturity, the 

Study Team derived the OTE performance 

specifications, shown in Table 9.1.

9.1.3.1 Optical design 

The LUVOIR-A OTE is designed as a threemirror anastigmat system, with a fourth 

fast steering mirror located at the real exit 

pupil of the OTE. The advantages of this 

system include a wide field-of-view that can 

be accessed by a number of instruments, 

Figure 9.2. The LUVOIR Payload with the optical telescope element highlighted. The small cage 

shown inside of the backplane support frame volume is the tertiary mirror assembly.
LUVOIR The Large UV Optical Infrared Surveyor

9-4 The LUVOIR Interim Report

with spherical, coma, and astigmatism 

aberrations corrected. The inclusion of the 

fast steering mirror also allows for ultrafine pointing stability to be achieved by all 

of the instruments and relaxes some of the 

requirements on the Vibration Isolation and 

Precision Pointing System (VIPPS) to point 

the massive science payload. 

The biggest disadvantage of a threemirror anastigmat system is the two 

additional reflections that are needed (tertiary 

mirror, fast steering mirror) before light enters 

any instrument. This throughput reduction 

is compensated by both the collecting area 

of the telescope and the high reflectivity of 

the coating at UV wavelengths. The two 

extra reflections also present two additional 

surfaces that can be contaminated and 

further degrade the throughput in the UV 

science bands. That being said, eliminating 

the aberration correction offered by the 

tertiary mirror, and the pointing control of the 

fast steering mirror before light is directed 

to each of the instruments may result in 

significantly more complex instruments, 

each potentially requiring their own 

corrective optics and fast steering mirror 

system. The Study Team has weighed the 

throughput gains achieved by changing the 

OTE design against the resulting complexity 

of the individual science instrument designs 

and has decided to maintain the three-mirror 

anastigmat design of the OTE. 

Figure 9.3 shows a ray trace of the current OTE system, and Figure 9.4 shows a 

wavefront error map over the telescope’s 

field-of-view, with each instrument’s field-ofview superimposed. Figure 9.5 shows an 

example wavefront error allocation for the 

OTE+ECLIPS instrument system. Note that 

even with the ~25 nm RMS design residual 

for the OTE, the total system wavefront error 

Table 9.1. Performance specifications for the LUVOIR-A OTE, derived from science capabilities.

Specification Value Mapped Capabilities

Aperture Diameter 15 meters Sensitivity, Resolution, 

Flexibility

Field-of-View 15 arcmin × 8 arcmin Flexibility, Agility

Wavelength Range* 100 nm – 2.5 mm Sensitivity, Flexibility

Reflectivity

> 60% at 105 nm

> 90% at 115 nm

Average ~ 90% between 400 and 850 nm

Average > 95% between 850 nm and 2.5 mm

Sensitivity, Flexibility, Agility

Static Wavefront Error† < 38 nm RMS Resolution, Stability

Pointing Stability‡ ± 0.3 mas 1-s per axis over an observation Resolution, Stability

Object Tracking 60 mas / sec Flexibility, Agility

Slew Rate

Required: Repoint anywhere in anti-sun 

hemisphere in 45 minutes

Goal: Repoint anywhere in anti-sun hemisphere 

in 30 minutes

Agility

*The blue end of the wavelength range is limited by the coating performance, while the red end is limited by the telescope thermal background; longer wavelengths can be observed for sufficiently bright objects. †The static wavefront 

error is the end-to-end optical performance, including that of the instrument. To achieve an end-to-end 38 nm RMS 

wavefront error, the contribution from the OTE itself would necessarily need to be lower. ‡The pointing stability here 

is the requirement at the OTE focal plane and is achieved by a tiered approach incorporating the fast steering mirror, 

VIPPS, and spacecraft attitude control system. +Object tracking and slew rates levy requirements on the VIPPS and 

attitude control system, however we include them here for a complete picture of the OTE’s capabilities.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-5

still achieves diffraction-limited performance 

with margin at the system-level. All four 

mirrors are coated with a protected, “enhanced” Al+LiF coating to enable observations at wavelengths as short as 100 nm. A 

thin overcoat of MgF2 or AlF3 reduces the hygroscopic sensitivity of the LiF layer (Quijada 

et al. 2014).

For this particular optical design, a 

sizeable central hole in the primary mirror was 

needed to pass the ray bundle through to the 

aft-optics system. The minimum achievable 

Figure 9.3. Ray-trace of the LUVOIR OTE optical design. First-order optical parameters are 

shown in the inset.

Figure 9.4. The RMS wavefront error over the 15 x 8 arcmin OTE field-of-view. With the exception 

of the LUMOS far-UV imager, each instrument’s field-of-view has a better-than-diffraction-limited 

wavefront from the telescope. The LUMOS far-UV imager uses its internal optics to correct the 

wavefront error from the telescope.
LUVOIR The Large UV Optical Infrared Surveyor

9-6 The LUVOIR Interim Report

clearance hole was 2.7 m in diameter; the 

secondary mirror obscuration is entirely 

contained within this diameter. To optimally 

pack the mirror segments around this hole 

and minimize unused collecting area, while 

also maintaining the outer diameter of the 

primary mirror in the 15-meter range, a 

segment size of 1.15 m flat-to-flat was 

selected and arranged in five concentric 

rings, for a total of 120 segments. Figure 

9.6 shows an early design for the LUVOIR-A 

aperture, with annotated diameters.

As with the number of reflections in the 

OTE, the central obscuration does little to 

degrade the general astrophysics science 

because the obscuration is balanced by the 

sheer size of the primary mirror. However, 

we have found that the obscuration 

substantially degrades the performance of 

the coronagraph instrument, specifically 

with respect to the “core throughput,” or 

the percentage of planet photons that are 

focused into the central core of the planet 

point-spread function. Evidence indicates 

that the ratio of the circumscribing diameter 

of the obscuration to the inscribing diameter 

of the primary mirror should be on the order 

of 10–15% to maximize the performance of 

the on-axis telescope / coronagraph system. 

Additionally, most coronagraph masks, when 

Figure 9.5. Top-level wavefront error budget showing static and dynamic wavefront error 

allocations. In this example, the ECLIPS instrument is show for a reference. Yellow boxes indicate 

top-down allocations; teal boxes indicate bottom-up roll-ups. The static wavefront error roll-up is 

based on optical design residuals and engineering judgment on alignment and fabrication errors. 

In this example, system level stability is shown to be 40 pm RMS, however additional analysis is 

necessary to confirm the underlying assumptions. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-7

optimized, will circularize the outer diameter 

of the aperture, discarding any portion of the 

aperture outside of the red-dashed circle in 

Figure 9.6.

Variations on the aperture shown in 

Figure 9.6 that address these issues have 

been explored and the telescope design has 

been adjusted. Figure 9.7 shows the new 

LUVOIR-A aperture geometry, compared to 

the old one. The optical design of the OTE was 

also updated to accommodate the reduction 

in the central obscuration, although it still 

maintains a three-mirror anastigmat form. At 

the time of preparing this report, however, 

detailed opto-mechanical models of the new 

aperture have not yet been completed, so 

the figures throughout this report still reflect 

the old aperture design shown in Figure 9.6.

These types of design evolutions are 

currently morphing LUVOIR-A into a more 

efficient system that better balances the 

general astrophysics science and the search 

for habitable planets.

9.1.3.2 Mechanical design

While the design and implementation of the 

OTE draws heavily from the James Webb 

Space Telescope (JWST) in many ways, the 

size of LUVOIR compared to other space 

telescopes such as the Hubble Space 

Telescope (HST) and the JWST is immense, 

as shown in Figure 9.8.

The mechanical layout initially focused 

on maximizing the primary mirror area while 

minimizing deployment complexity while 

being constrained by the Space Launch 

Figure 9.6. The initial LUVOIR entrance pupil 

aperture. There are 120 segments, each 1.15 

m flat-to-flat, with 6 mm gaps. Inscribed 

and circumscribed diameters are shown for 

reference.

Figure 9.7. A comparison between the initial LUVOIR-A aperture (left) and the new aperture (right). 

The new aperture achieves the same 15.0-m outer diameter while maximizing the inscribing 

diameter and minimizing the central obscuration. Both apertures have 120 segments, although 

the segment size increased from 1.15-m flat-to-flat (left) to 1.223-m flat-to-flat (right).
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9-8 The LUVOIR Interim Report

System fairing volume. In order to fit in the 

fairing, a JWST-like wing-fold deployment has 

been adapted to a dual-wing-fold on each 

side of the primary mirror as shown in Figure 

9.9. This allows JWST-like deployment 

mechanisms, hinges, and latches to be used 

on LUVOIR. The dual-wing-fold geometry 

also allows the primary mirror to better 

conform to the circular diameter of the fairing, 

allowing more volume in the center of the 

fairing for the instrument stack. Similarly, the 

JWST single-hinge secondary mirror support 

structure (SMSS) deployment was adapted 

for LUVOIR, with two additional hinge points 

on the top beam of the SMSS, as shown in 

Figure 9.10. Finally, the aft-optics system 

was made to be deployable using a nestingtelescope structure to accommodate launchvehicle envelope constraints.

Figure 9.8. The LUVOIR-A primary mirror compared to the HST/WFIRST and JWST primary 

mirrors.

Figure 9.9. Left: JWST “folded” each wing of 

the primary mirror along one axis in order to fit 

the observatory into the launch vehicle fairing 

(only a single wing is shown folded here). Right: 

LUVOIR builds upon this concept by using two 

“folds” per wing to fit a much larger primary 

mirror into the (larger) launch vehicle fairing. 

JWST photos courtesy NASA/GSFC.

Figure 9.10. Left: JWST’s secondary mirror 

fully deployed. Center-Left: JWST’s secondary 

mirror stowed so that it can fit into the launch 

vehicle. Right: LUVOIR uses a similar technique 

to stow the much longer secondary mirror 

support structure for launch. JWST photos 

courtesy NASA/GSFC.
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The LUVOIR Interim Report 9-9

The design of the primary mirror segment 

assembly architecture also leverages the 

JWST design, shown in Figure 9.11. A 

single, stiff mirror segment is mounted on 

a hexapod for 6 degree-of-freedom rigid 

body positioning of the segment. A whiffle 

structure and delta frame interfaces the mirror 

segment substrate to the hexapod actuators 

and to the primary mirror backplane support 

structure.

The LUVOIR segment assembly architecture departs from that of JWST in the materials, optimized for the stability requirements 

and the operating temperature. Instead of 

beryllium segments, LUVOIR has baselined 

Corning’s Ultra-Low Expansion (ULE®) glass 

for the mirror substrate, although other low 

coefficient of thermal expansion (CTE) materials such as Zerodur may also be used. 

Additionally, LUVOIR’s delta frame and whiffle will be zero-CTE composite material. 

JWST required radius-of-curvature actuators on each segment because the mirrors could not hold their shape in a 1-g environment during integration and test, and 

therefore needed built-in correctability. For 

LUVOIR, further detailed analysis is required, 

but it is believed that the stiffness of the ULE 

LUVOIR’s optical telescope element uses many design features that were 

developed for the James Webb Space Telescope such as segmented mirrors, 

deployable sections of the primary mirror, mirror actuation, and a deployable 

secondary mirror.

Figure 9.11. Left: JWST primary mirror segment assembly, viewed from the back (Credit NASA/

GSFC). Right: LUVOIR's primary mirror segment assembly concept, showing many shared 

components. While JWST uses a radius-of-curvature actuator, LUVOIR does not. However, 

LUVOIR uses a heater plate to control the segment temperature and co-locates the mirror 

segment control electronics with each segment.
LUVOIR The Large UV Optical Infrared Surveyor

9-10 The LUVOIR Interim Report

mirrors, with their closed back design, will 

be substantially stiffer than JWST’s openbacked beryllium mirrors, and therefore 

able to hold their prescribed shape in 1-g. 

Studies currently being conducted with mirror vendors will help to demonstrate if these 

assumptions are accurate. For now, LUVOIR 

is not baselining the use of a radius-of-curvature actuator.

Another critical new component to 

the segment assembly architecture is the 

edge sensor and piezoelectric transducer 

(PZT) actuator control system. Each mirror 

segment is fitted with one edge sensor 

per edge (i.e., two edge sensors total per 

shared edge) for a total of 622 edge sensors 

across the LUVOIR aperture. While the 

current design baselines capacitive-based 

edge sensors, optical or inductive sensors 

could also be used if sufficiently mature 

and capable. Each sensor measures local 

gap, shear, and dihedral angle between 

the segment edges. Incorporating these 

three measurements from all 622 sensors 

allows for a global solution to be found for 

the six degree-of-freedom position of each 

segment. The positions of each segment are 

then fed back to the hexapod fine-stage PZT 

actuators to control the segment position 

with picometer resolution (Saif et al. 2017). A 

discussion regarding both edge sensor and 

PZT technology development can be found 

in Chapter 11.

9.1.3.3 Thermal design

High-contrast imaging necessitates that 

the mirrors of the OTE be extremely stable. 

In order to eliminate the effects of thermal 

expansion and contraction, an ultra-stable 

thermal environment must be created. 

As mentioned in Chapter 8, the nominal 

operating temperature of LUVOIR has been 

set to 270 K. In another departure from 

JWST, the individual segment, secondary, 

and tertiary mirror assemblies incorporate 

an active thermal control system. A heater 

plate immediately behind the glass substrate 

radiatively heats the mirrors to 270 K±1 mK. 

The primary mirror backplane and secondary 

mirror support structures are also actively 

heated at key control points in order to 

maintain the global thermal stability of the 

OTE (Eisenhower et al. 2015; Park et al. 

2017).

The nominal 270 K temperature has 

several advantages over JWST. First, by 

fabricating components at a temperature 

near where they are to be operated, 

modeling error can be reduced, as well as the 

complexity, number, and duration of thermal 

cycle tests. Second, performance testing of 

LUVOIR will be completed at a much warmer 

temperature than JWST which should ease 

the qualification campaign since it won’t 

involve cryogenic tests.

9.1.3.4 Electrical design

The payload electrical architecture is shown 

in Figure 9.12. Each of the 120 mirror 

segment control electronics boxes are 

located within the OTE at each segment 

location, as illustrated in Figure 9.11. A 

block diagram of the mirror segment control 

electronics is shown in Figure 9.13. Due 

to the large number of these boxes (120×, 

plus one for the secondary mirror assembly), 

it is desirable to use low power designs 

and techniques for the individual boards to 

LUVOIR’s nominal operating temperature of 270 K provides for several 

advantages over JWST including the manufacturing of the primary mirror 

segments as well as the qualification testing of the OTE.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-11

Figure 9.12. The LUVOIR electrical system architecture. Primary data interfaces include 1553, 

low-voltage differential signaling (LVDS), SpaceWire (SpW), and RS422. Most systems receive 

28-v power from the power distribution unit, with the exception of the mirror segment piezo 

actuators, which require 120-v power. “(A/B)” denotes electrical boxes that are board-level 

redundant; “(2x)” denotes boxes that are box-level redundant. Cross-strapping is not shown for 

clarity.

minimize the multiplying effect on the overall 

power demand. Circuit board size and mass 

will be minimized for the same reason. The 

high data rate of the edge sensor readout 

exceeds the capability of the 1553 bus, so 

a Low-Voltage Differential Signaling (LVDS) 

Figure 9.13. Block diagram of the mirror segment electrical system. PCB: printed circuit board, 

LVDT: linear variable differential transformer (for actuator position measurement).
LUVOIR The Large UV Optical Infrared Surveyor

9-12 The LUVOIR Interim Report

interface and data router is employed to 

transport the data to the Control System 

Processor, located inside the coronagraph 

instrument.

9.2 Backplane support frame 

(BSF)

9.2.1 BSF overview

The backplane support frame (BSF), 

highlighted in Figure 9.14, is the primary 

mechanical interface between the instrument 

suite and the OTE. It must be robust enough to 

support both the large primary mirror as well 

as the entire suite of science instruments and 

their related support hardware. Additionally, 

it must provide modular interfaces for the 

science instruments to allow for serviceability.

9.2.2 BSF design drivers

Some of the key drivers that shape the 

implementation of the BSF include:

• Provide needed stiffness to the OTE 

during ground integration and test and 

on-orbit

• Provide modular interfaces for all 

science instruments and related 

hardware

• Provide housing for the payload 

main electronics box and the power 

distribution unit

• Provides launch supports for the stowed 

primary and secondary mirrors

• Provide an interface between the 

Payload and the Spacecraft

• Launch vehicle mass-to-orbit capability

• Launch vehicle volume

9.2.3 BSF design implementation

9.2.3.1 Optical design

The BSF is primarily a mechanical structure, 

so there is no optical design related to it. 

Even so, the BSF serves as the primary 

metering structure between the OTE and 

the science instruments. Thus, optical 

alignment tolerances set requirements on 

the mechanical and thermal design of the 

BSF and instrument interfaces.

Figure 9.14. The LUVOIR Payload with the backplane support frame is highlighted.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-13

9.2.3.2 Mechanical design

Like the combination of BSF and the 

integrated science instrument module (ISIM) 

structure on the JWST, LUVOIR’s BSF would 

be a bonded composite, precision optical 

metering structure. The composite material 

provides a very stiff yet relatively lightweight 

and thermally stable structure.

LUVOIR is being designed with modularity 

in mind, which would enable the telescope 

to be serviceable should the technology 

exist to do so and should NASA decide to 

extend the life of the telescope beyond its 

initial core mission. While serviceability is 

not necessary to meet any of the science 

cases described in this report, it may be an 

attractive way to enhance or extend the life 

of a flagship observatory. However, because 

it is not required to meet the science goals, 

the servicing mission itself is outside the 

scope of this study and this report. 

At this time, the design has two access 

doors, shown in Figure 9.15, which can 

open away from the science instrument bays 

LUVOIR’s BSF is an evolution and combination of design and features from 

both the Hubble Space Telescope and the James Webb Space Telescope.

Figure 9.15. Key features of the BSF. Not shown are instrument radiator panels that would be 

mounted to either side of the BSF, for passively cooling specific components to 270 K or 170 K. 

An additional radiator, used to cool 100 K components, would be mounted near the top of the 

BSF.
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9-14 The LUVOIR Interim Report

providing access to the science instruments 

themselves. The instruments would be 

attached to the BSF at the “instrument 

interface bulkhead,” also shown in Figure 

9.15, via a guiderail and latching system 

similar to those used on HST. Figure 9.16

shows a possible servicing concept for 

LUVOIR.

Despite LUVOIR’s many similarities with 

the JWST, this modularity is a deviation from 

the JWST architecture. During the design 

phase of JWST, modular interfaces were 

considered for the science instruments and 

ISIM structure, with designs that were an 

evolution of the Hubble latches. However, 

the modular interface concept on JWST was 

discarded for three primary reasons:

1. JWST was not designed to be serviceable,

2. The ISIM element integration and 

test plan—at the time—had only one 

installation of the flight instruments 

planned which didn’t justify the level of 

effort for this highly complex interface, 

and 

3. The initial architecture for the ISIM element 

was very asymmetric which didn’t lend 

itself to easy access for modularity and 

adding it was non-trivial.

That being said, there is nothing inherent 

in a composite structure design that would 

prevent a modular interface from being 

implemented as long as it was designed to 

be modular from the start. Trying to retrofit 

a modular interface to an existing design 

would be very problematic.

9.2.3.3 Thermal design

The BSF must maintain alignment of the 

instrument modules with the OTE, and 

therefore must be both structurally and 

thermally stable. The individual composite 

beams of the structure will be instrumented 

with thermal sensors and heaters and 

wrapped in thermal blanketing, with a goal 

of maintaining a temperature of 270 K ± 1 K.

The external surface of the BSF structure 

will also carry radiators for the payload 

Figure 9.16. LUVOIR’s science instrument 

interfaces will be modular by design. Left: The 

Hubble Space Telescope was designed to 

be serviceable. Here, two of the doors to the 

science instruments are open and astronauts 

are working to replace an instrument (Photo 

credit: NASA). Right: LUVOIR’s BSF is designed 

to be modular in nature, using similar interfaces 

as those on the Hubble Space Telescope. 

While not necessary to achieve the baseline 

science described in this report, this allows for 

the possibility of servicing in the future should 

the necessary infrastructure be available.

Design modularity is a key feature of LUVOIR and is expected to make science 

instrument integration less complex. Modularity also enables the possibility of 

servicing LUVOIR, a feature that isn’t necessary to achieve the science cases 

described in this paper. Rather, servicing allows for future upgrades which 

could enable science cases that haven’t been thought of today.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-15

electronics. Portions of either side of the 

BSF will hold radiators to help maintain 

the nominal 270 K temperature of most of 

the Science Instrument systems, as well 

as radiators that will passively cool certain 

detectors and optical sub-benches to ~170 

K. Finally, a small radiator mounted to the 

top of the BSF will be used to passively cool 

specific detectors to ~100 K.

9.2.3.4 Electrical design

The BSF holds both the payload power 

distribution unit and the payload main 

electronics box. The power distribution 

unit receives 120-v power provided by the 

spacecraft and distributes it to the various 

payload systems. The 120-v high-voltage is 

passed directly to the PZT actuators on the 

mirror segments. The power distribution unit 

also steps the 120-v power down to 28-v 

for the rest of the instruments and payload 

systems. The payload main electronics box 

serves several functions:

• Routes data from the individual storage 

within each Science Instrument to 

the spacecraft solid state recorder for 

buffering prior to downlink,

Figure 9.17. The LUVOIR Payload with the science instruments highlighted.

• Controls heaters and temperature 

sensors throughout the BSF and OTE,

• Controls all deployment mechanisms on 

the payload, and

• Provides telemetry and housekeeping 

data to the spacecraft command & data 

handler.

Figure 9.12 shows the payload electrical 

system architecture and the interfaces 

between the main electronics box, power 

distribution unit, and the rest of the payload 

systems.

9.3 Science instruments

9.3.1 Overview

Several science instruments have been 

baselined for LUVOIR by the Study Team. 

They will all be housed in the BSF, shown 

in the area highlighted in Figure 9.17. Three 

of the instruments are being studied by 

the LUVOIR Study Team, while the fourth, 

POLLUX, is being studied by a European 

consortium led by the Centre National 

d’Etudes Spatiales (CNES). The three USstudied instruments are described in this 

section, while more details on POLLUX and its 
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9-16 The LUVOIR Interim Report

science objectives can be found in Chapter 

10.

The Extreme Coronagraph for Living 

Planetary Systems (ECLIPS) is intended to 

survey sun-like stars in the local neighborhood 

and search for exoplanets within an annular 

region around the star defined by the inner 

and outer working angles (IWA and OWA 

respectively). It will directly image exoplanets 

via high-contrast imaging and characterize 

the atmospheres of those planets via 

spectroscopy of reflected light. Its emphasis 

is on the search for biosignatures on earthlike planets within the habitable zone, though 

all planets will receive some degree of 

characterization (Pueyo et al. 2017).

The LUVOIR Ultraviolet Multi-Object 

Spectrograph (LUMOS) is designed to support 

the UV science requirements of LUVOIR, 

from exoplanet host star characterization to 

tomography of circumgalactic halos to water 

plumes on outer solar system satellites. 

LUMOS offers multi-object spectroscopy 

across the UV bandpass, with multiple 

resolution modes to support different science 

goals (France et al. 2017).

The High Definition Imager (HDI) 

instrument is the primary astronomical 

imaging instrument for observations in 

the near UV through the near IR. It will be 

used to characterize stellar populations 

to rigorously test star formation theories; 

explore outer planet atmospheres; discover 

and characterize distant objects in the solar 

system; reveal the impact of the epoch of 

reionization on galaxy formation; visualize 

the evolution of galaxies; map dark matter; 

measure cosmological parameters with wellcalibrated distance indicators out to the 

distance of the Coma Cluster; and detect 

100s of exoEarths via astrometry.

A more detailed discussion of the current 

iterations of the science instruments follows. 

Table 9.2summarizes the general capabilities 

of the LUVOIR science instrument suite. It 

should be noted that the designs continue 

to evolve as the architecture for LUVOIR-A 

evolves.

Table 9.2. Science instrument capability summary.

Instrument Channel

Wavelength 

Range

Field-ofViewAngular 

Resolution

Spectral 

Resolving 

Power

Inner 

Working 

Angle 

(max.)

Outer 

Working 

Angle 

(min.)

nm arcmin mas/pix λ/D λ/D

ECLIPS Total 200 2,000

UV 200 525 0.006 Ø 1.4 7 3.5 64

Vis 515 1,030 0.016 Ø 3.5 140 3.5 64

NIR 1,000 2,000 0.030 Ø 6.9 70, 200 2 64

LUMOS Total 100 400

Far-UV 

MOS 100 200 3 × 1.6 ~25 500 – 60,000

Near-UV 

MOS 200 400 1.3 × 1.6 ~25 40,000

Far-UV 

Imager 100 200 2 × 2 ~12.5

HDI Total 200 2,500

UVIS 200 ~1,000 2 × 3 2.75 ~600–700

NIR ~1,000 2,500 2 × 3 8.25 ~600–700
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9.3.2 Extreme Coronagraph for 

Living Planetary Systems (ECLIPS)

9.3.2.1 ECLIPS overview and science 

goals

The scientific goals of the coronagraph 

instrument are commensurate with the 

ambitious philosophy underlying LUVOIR-A, 

and are organized around two key science 

themes:

(1) measuring the occurrence rate of 

biomarkers in the atmospheres of rocky 

planets orbiting in the habitable zone of 

their host stars, and 

(2) studying the diversity of exoplanet 

systems.

The former science theme is significantly 

more stressing on the instrument and drives 

the trades we studied to design ECLIPS. Any 

mission aimed at measuring the occurrence 

rate of biomarkers in the atmosphere of 

nearby habitable-zone rocky planets, ought 

to first be capable of detecting a statistically 

significant ensemble of exoEarth candidates 

(Stark et al. 2015). The detectability of 

exoplanets in long coronagraph exposures 

depends on both the level of contrast achieved 

in the high-contrast region or “dark hole” of 

the focal plane, and on the throughput of 

the coronagraph at the apparent separation 

of the planets, expressed using the Inner 

and Outer Working Angle (IWA and OWA, 

respectively) scalar metrics.

The characterization of identified 

exoEarth candidates is equally as important 

Figure 9.18. Simulations of the spectra from an Earth-like planet around a sun-like star at 10 pc. 

The orange line depicts a simulated Archean Earth spectrum, the blue line depicts a simulated 

modern-Earth spectrum, and the points simulate data collected by LUVOIR-A with ECLIPS. 

Credit: LUVOIR Tools.
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9-18 The LUVOIR Interim Report

as their detection. Figure 9.18 shows a 

simulated spectrum of a mature earth along 

with a 2-Gyrs-old Archean earth generated 

with the LUVOIR STDT online exoplanet 

spectrum simulation tool.1 This example 

illustrates the most salient characteristics of 

the atmosphere of earth analogs which we 

seek to characterize with great precision 

using ECLIPS, and translate into the following 

three requirements on the instrument:

(1) Continuous spectral coverage from 200 

nm to 2.0 mm in order to capture the 

spectral features associated with carbon 

and oxygen-based molecules, which 

help discriminate the various atmospheric 

compositions,

(2) Spectral resolution of R = 140 in the 

visible, and 

(3) Spectral resolutions of R = 70 & 200 in 

the near-IR. 

Spectroscopy of faint exoEarths beyond 1.6 mm will be limited to the closest 

and brightest targets due to the thermal 

background from the 270 K telescope. 

Nevertheless, redder spectral coverage will 

be invaluable to studying the details of the 

atmospheres of our nearest neighbors, as 

well as characterizing larger planets.

9.3.2.2 ECLIPS design drivers

ECLIPS is a complex instrument, as captured 

in Figure 9.19 and Figure 9.20. At the top 

1 See https://asd.gsfc.nasa.gov/luvoir/tools/

level this complexity stems from several 

design drivers:

• The need to have continuous spectral 

coverage from 200 nm to 2.0 μm,

• The need to minimize the angle of 

incidence at all reflective surfaces to 

mitigate polarization aberrations,

• Packaging constraints due to instrument 

volume constraints, and

• Ultra-stable wavefront errors to 

enable long-duration high-contrast 

observations.

9.3.2.3 ECLIPS design implementation

There is currently no flight heritage for a 

coronagraph such as the one proposed for 

LUVOIR. There are several ground-based 

systems that use coronagraphs, however 

their overall requirements are far less 

stringent than those imposed on ECLIPS, 

and their contrast ratios are several orders of 

magnitude less. The WFIRST Coronagraph 

Instrument (CGI) is the most similar 

coronagraph to ECLIPS but it is still under 

development.

We expect that CGI development will 

help to advance the state-of-the-art in 

terms of how to successfully implement a 

coronagraph instrument on a space telescope, despite WFIRST and LUVOIR being 

very different observatories. The experience 

gained from integrating, testing, and operating a high-contrast coronagraph on-orbit 

will be invaluable. Some of the most critical 

pieces of information that will be learned by 

ECLIPS will build on advances from the development of WFIRST’s 

Coronagraph Instrument (CGI).

WFIRST’s CGI will mature high-contrast technologies and provide an endto-end system-level demonstration of advanced starlight suppression in the 

presence of actual observatory disturbances as well as background sources 

such as exozodiacal light.
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The LUVOIR Interim Report 9-19

the operation of WFIRST CGI will be how to 

post-process high-contrast data with positively verifiable targets. Ground data processing has advanced significantly in the 

past 10 years but operates on large-signal 

data sources. The WFIRST CGI speckle field 

will be low signal and relatively uncorrelated. 

Performing transit spectroscopy with HST is 

an analogous example: this was a new field 

of data analysis, and the experience from 

working with Hubble data is now feeding directly into JWST. Similarly, working with real 

high-contrast data from WFIRST will inform 

the design of LUVOIR and ECLIPS.

In order to accommodate the variety 

of high reflectivity coatings and detector 

technologies that span ECLIPS’s 

wavelength range, the instrument is split 

into three channels that cover the following 

bandpasses: UV (200 to 525 nm), optical 

(515 nm to 1030 nm), and NIR (1.0 μm to 2.0 

μm). These bands are achieved using a 

series of dichroics at the entrance of the 

instrument. The small overlap in wavelength 

coverage between each channel is used to 

calibrate detector gains between bands. 

Each channel is equipped with:

• Two deformable mirrors (DMs) for 

wavefront control,

• A suite of coronagraph apodizing, 

occulting, and Lyot stop masks

• A Zernike wavefront sensor to be used 

as a low-order or out-of-band wavefront 

sensor (LOWFS, or OBWFS), 

• A spectral filter wheel to select the 

instantaneous science bandpass for the 

channel, and

• Separate science imagers and 

spectrographs.

While the three channels can operate in 

parallel, each channel can only observe in one 

Figure 9.19. Side view of the ECLIPS instrument showing the three spectral channels in a layered 

configuration. Light enters from the telescope in the lower right corner and a pair of crossed fold 

mirrors directs the light up to the instrument.
LUVOIR The Large UV Optical Infrared Surveyor

9-20 The LUVOIR Interim Report

bandpass at a time. Figure 9.19 shows a 

side view of the three spectral channels in 

their layered configuration.

Technology development for segmented-aperture coronagraphs is ongoing, and 

there are many promising candidates including shaped/apodized pupil Lyot coronagraphs, vector vortex coronagraphs, 

phase-induced amplitude apodization coronagraphs, and nulling coronagraphs. The 

landscape of coronagraph design may look 

very different at the time that an observatory such as LUVOIR is actually built, and the 

Study Team recommends that these candidates be re-evaluated then to maximize the 

possible science yield. At the time of this 

writing, however, the Study Team has chosen to design the ECLIPS instrument around 

the apodized pupil Lyot coronagraph (APLC) 

and vector vortex coronagraph architectures. 

Models suggest the APLC architecture currently delivers very good performance with 

LUVOIR-A’s segmented, obscured aperture, 

and the masks and implementation directly 

leverage the significant investments in the 

WFIRST coronagraph instrument technology development effort (Eldorado Riggs et 

al. 2017; Mazoyer et al. 2016; N’Diaye et al. 

2014; Zimmerman et al. 2016). Vector vortex 

coronagraphs provide advantages for distant 

unresolved stars. More details about each 

coronagraphs’ strengths and weakness are 

available in Chapter 11.

As the name suggests, APLCs work by 

placing an apodizing mask in the pupil plane 

to correct for the segment and obscuration 

geometry of the telescope. The design of 

these masks is a complex optimization 

problem. Once the geometry of the focal 

plane mask and Lyot stop are set, a linear 

optimization is used to generate binary 

apodizing masks. Because it is difficult to 

design apodizing masks that cover the 

entire possible habitable zone with sufficient 

throughput, ECLIPS is designed with a suite 

of focal plane masks with varying inner and 

outer working angles. The masks will be 

chosen as a function of the distance to each 

observed host star.

To make the design robust to stellar 

angular size and pointing errors, the focal 

plane masks are designed that the inner edge 

of the dark hole is set to be smaller than the 

outer edge of the focal plane mask (N’Diaye 

et al. 2015). And to make the design robust 

to pupil misalignments, an input pupil with 

larger segment gaps and secondary struts 

than the actual telescope design is assumed. 

This baseline approach is conservative as 

it allows for large pupil shear between the 

plane of the apodizer and the primary mirror. 

Based on calculations using preliminary 

geometries similar to the LUVOIR-A primary 

mirror geometry, we predict that relaxing this 

constraint once the system stability is better 

understood will increase the throughput by 

about a factor of two.

9.3.2.3.1 Optical design

In the current design of the ECLIPS 

instruments, three mirrors are used to 

pick off the light from the OTE focal plane, 

collimate it, and re-image the telescope pupil 

onto the first DM in each channel. Once the 

light is collimated by these pre-optics, it is 

separated into each channel via a series 

of dichroic beamsplitters. The mirrors in 

each channel are coated according to their 

bandpass to provide the best reflectivity: the 

pre-optics and UV channel mirrors are coated 

in aluminum, the visible channel mirrors are 

coated in silver, and the NIR channel mirrors 

are coated in gold. To minimize polarization 

aberration effects within the instrument, all 

90° fold mirrors are used in crossed-pairs, 

such that one mirror compensates the 

polarization effects of the other.
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The LUVOIR Interim Report 9-21

Figure 9.20 shows a top-down view of 

the UV channel opto-mechanical layout; the 

visible and NIR channels are very similar in 

design. Light enters from the telescope in 

the upper right corner and is relayed to the 

pair of DMs, with a pupil image being formed 

at the first DM in each channel. A pair of relay 

mirrors re-image the pupil from the first DM 

onto the apodizing mask. A second pair of 

relay mirrors re-images the pupil again onto 

the Lyot stop, and the coronagraph focal 

plane mask is located at the intermediate 

focus. After passing through the Lyot stop, 

the optical bandpass is further reduced 

to 10–20% by spectral filters for science 

observations, or 2% for wavefront sensing.

Depending on the channel, after passing 

through the spectral filters, the light can be 

sent to one of several back-end instruments. 

In the UV channel, the only back-end 

instrument is an imaging camera. In the 

visible channel, a flip-in mirror can be used to 

direct the light to either an imaging camera 

or an R=140 integral field spectrograph (IFS). 

In the NIR channel, a flip-in mirror can be 

used to direct the light to either an R=70 IFS, 

or an R=200 slit spectrograph. These backend instruments were chosen to maximize 

the science capabilities of ECLIPS.

The visible and NIR IFSs are similar to 

those installed on ground-based instruments 

(Hinkley et al. 2011; Macintosh et al. 2008; 

Wildi et al. 2009) and the one envisioned 

for WFIRST (Demers et al. 2015). A highresolution (R~1500) fiber-fed spectrograph 

(Mawet et al. 2017) was also considered. 

Figure 9.20. This top-down view of the ECLIPS instrument shows the UV channel beam path. 

Light enters from the telescope in the upper right, and the UV dichroic directs the 200-525 nm 

light into the channel; longer wavelengths pass to the other channels below. After reflecting from 

the two deformable mirrors, two image relay mirrors re-image the pupil from the first DM onto the 

apodizing mask. A second pair of relay mirrors re-image that pupil onto the Lyot mask. A focal 

plane mask is located at the intermediate focus. A final set of imaging mirrors form an image at 

the science camera. The visible channel is similar, except a flip-in mirror directs light after the Lyot 

stop to either an imaging camera or an integral field spectrograph. The NIR channel is also similar, 

except after the Lyot stop a flip-in mirror directs light either to an integral field spectrograph, or a 

high-resolution slit spectrograph.
LUVOIR The Large UV Optical Infrared Surveyor

9-22 The LUVOIR Interim Report

While this solution is less mature, it was 

thought that it would better use detector 

real estate and could potentially yield higher 

resolution spectra for the brighter planets/

most nearby systems. Unfortunately, at this 

time the fiber-fed spectrograph has been 

found to significantly reduce throughput 

and increase serial characterization times of 

targets thus reducing efficiency and exoEarth 

yields.

In addition to the primary science beam 

path, each channel also supports a low-order 

/out-of-band wavefront sensor (LOWFS/ 

OBWFS). In LOWFS mode, the focal plane 

mask doubles as a Zernike wavefront sensing 

mask. The core of the point-spread function 

that is rejected by the focal plane mask is 

reflected into the LOWFS camera. A divot in 

the center of the focal plane mask provides 

the phase reference for the Zernike wavefront 

sensor (Shi et al. 2015). In OBWFS mode, 

the system operates much the same way, 

except instead of reflecting only the central 

core of the point-spread function, the mask 

reflects the entire point-spread function. 

Note, that using the OBWFS in a channel 

precludes science observations in that same 

channel. Thus, it is intended that at any given 

time, one of the three channels operates as 

an OBWFS for the entire instrument while 

science observations are being performed in 

the other two channels.

9.3.2.3.2 Mechanical design

ECLIPS is a very dense instrument, requiring 

a large number of optical elements, mechanisms, and detectors to be packaged in a 

relatively small volume. A central reference 

bench serves as the primary metering structure of the instrument. This reference bench 

holds the pre-optics that pick-off the light 

from the OTE focal plane and direct it to 

the individual channels. Two optical benches, aligned perpendicularly to the reference 

bench, hold the three instrument channels. 

One optical bench holds the UV and visible 

channels on either face; the second optical 

bench holds the NIR channel. Figure 9.19

shows the two optical benches holding the 

three channels.

9.3.2.3.3 Thermal design

The entire instrument is held at 270 K; the 

exact required thermal stability of the optical 

benches and components is still being 

analyzed but is expected to be on the order 

of ±10 mK or better. Within the NIR channel, 

a sub-bench holding all elements after the 

Lyot stop is passively cooled to 170 K to help 

reduce the thermal background. The UV and 

visible channel EMCCD detectors are also 

passively cooled to 170 K; the NIR channel 

H4RG detectors are passively cooled to 100 

K. As a contamination control approach, all 

heaters are also sized to allow for infrequent 

bake-outs of the optical elements on-orbit, 

similar to the annealing approach used on 

HST detectors.

9.3.2.3.4 Electrical design

The ECLIPS main electronics box also 

includes the control system processor (CSP). 

The CSP is the central brain for all control 

systems on LUVOIR, including the primary 

mirror segment edge-sensor / PZT control 

system discussed in Section 9.1.3.2, and 

any other metrology that may be included 

(see Figure 8.8 for a control system block 

diagram that incorporates the CSP). The CSP 

is responsible for autonomously executing the 

wavefront control algorithms required to dig 

the high-contrast dark hole. The CSP uses 

a hybrid architecture consisting of a central 

processing unit to handle overall algorithm 

execution and data I/O, and a battery of 

field programmable gate arrays to perform 

dedicated, computationally intensive tasks 

such as matrix inversions or fast Fourier 

transforms.
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The LUVOIR Interim Report 9-23

9.3.2.3.5 Detectors

As with the coronagraph architectures, the 

technology development of detectors for 

exoplanet science is proceeding at a rapid 

pace. By the time LUVOIR is to be developed, 

it is likely that detectors with better sensitivity, 

less noise, and higher radiation tolerance 

will exist. However, to benchmark system 

performance and science yields now, the 

Study Team has selected a suite of detectors 

that exist today and only require a moderate 

amount of engineering development to be 

capable of achieving the science objectives 

described in earlier chapters. Chapter 

11 provides more detail on other detector 

technologies that may prove superior with 

sufficient technology development. The 

detector technologies are specific to each 

channel and our baseline choices rely heavily 

on technologies that have been used on 

previous missions and/or are planned in the 

WFIRST baseline instruments. 

In the UV and visible channels, electron 

multiplying CCD (EMCCD) devices will be 

used, with the UV channel devices being 

d-doped for better shortwave performance. 

Current devices are being developed and 

tested for the WFIRST coronagraph mission 

with 1k × 1k, 13 μm pixels. These 1k × 1k 

devices will be sufficient for the UV and 

visible channel imagers, however to cover 

the field-of-view and spectral resolution of 

the visible channel IFS, a 4k × 4k device 

will be required. In photon-counting mode, 

these devices produce <1 e– read noise, and 

1×10-4 e–/pixel/second dark current (Nemati 

et al. 2016). 

The baseline technology for the NIR 

channel is the Teledyne HAWAII 4RG (H4RG) 

HgCdTe detector. Current devices have 

a format of 4k × 4k with 10 mm pixels and 

are three-side buttable for building larger 

arrays. H4RGs are the baseline detector for 

the WFIRST wide-field instrument and are 

an evolution of H2RG detectors used on the 

JWST NIRCam instrument. Current devices 

have a median read noise of ~5 e–/pix, and 

a median dark current: 2 x 10-3 e–/pix/sec. 

It is believed that the read noise can be 

further reduced to 1–2 e–/pix with additional 

optimization of the readout circuit.

9.3.2.3.6 Deformable mirrors

Our choice of DM technology is based on 

the need for high-density devices to achieve 

large outer-working angles. For instance, 

achieving high contrast for the outer region of 

the habitable zone around the most nearby 

stars requires an outer-working angle of 

~48 λ/D. Providing wavefront control over a 

dark hole of that size requires ~96 actuators 

across the pupil. Imaging of outer giant 

planets drives the actuator count higher, to 

128 actuators across the pupil to achieve a 

~64 λ/D outer working angle. In order to keep 

the optical design compact, we baselined a 

128 ×128 actuator micro-electro-mechanical 

systems (MEMS) DM device. A secondary 

technical driver is the fact that such compact 

DMs can achieve small Fresnel numbers, 

and thus are more amenable to DM-based 

correction of amplitude errors. While this 

technology will not be matured by WFIRST 

at the component level, there are avenues to 

do so using smaller satellites (Cahoy et al. 

2014).

9.3.2.3.7 Element select 

mechanisms

Each of the three channels is equipped with 

a series of element select mechanisms that 

accommodate the various apodizing, focal 

plane, and Lyot masks, as well as spectral 

filters. The number of elements in these 

mechanisms is driven by two considerations. 

First, each channel can only operate over an 

instantaneous 10–20% spectral bandpass 

at a time, due to limitations on wavefront 

control techniques and mask designs, 
LUVOIR The Large UV Optical Infrared Surveyor

9-24 The LUVOIR Interim Report

requiring between six and eight spectral 

filters per channel. Each of those 10–20% 

bands must further be reduced to three 2% 

bands for collecting the wavefront control 

images. With additional neutral density filters, 

each spectral channel has between 29 and 

37 filters alone. The second consideration is 

that each combination of spectral bandpass, 

IWA, and OWA, requires a specific set of 

apodizing, focal plane, and Lyot stop masks. 

Each channel has between 8 and 11 mask 

combinations to enable different optimized 

high-contrast regions on the focal plane.

9.3.3 LUVOIR Ultraviolet MultiObject Spectrograph (LUMOS)

9.3.3.1 LUMOS overview and science 

goals

A ubiquitous theme that has emerged in the 

science definition phase has been the study 

of gas in the cosmos, its relationship to (and 

evolution with) star and galaxy formation, 

and how this gas is transferred from one 

site to another. Understanding the flow of 

matter and energy from the intergalactic 

medium to the circumgalactic media, and 

ultimately into galaxies where it can serve as 

a reservoir for future generations of star and 

planet formation, is essentially a challenge 

in characterizing the ionic, atomic, and 

molecular gas at each phase in this cycle. 

LUVOIR will be capable of characterizing 

the composition and temperature of this 

material in unprecedented scope and detail; 

on scales as large as the cosmic web and as 

small as the atmospheres of planets around 

other stars. The common denominator 

for this science case is that the strongest 

emission and absorption lines in the gases 

to be studied—therefore the highest 

information content for understanding the 

physical conditions in these objects—reside 

at ultraviolet wavelengths, roughly 100–400 

nm. The LUVOIR Ultra-Violet Multi-Object 

Spectrograph (LUMOS) instrument is 

designed to make revolutionary observational 

contributions to all of the disciplines that 

call for high-resolution spectroscopy, multiobject spectroscopy, and imaging in the 

ultraviolet bandpass.

9.3.3.2 LUMOS design drivers

Like the other systems on LUVOIR, the 

LUMOS instrument design is constrained 

by launch vehicle mass-to-orbit capability 

and fairing volume. But the most significant 

design driver for LUMOS is throughput 

at far-UV wavelengths. Although recent 

developments in mirror coating prescription 

and processes have improved performance 

at short wavelengths, the reflectivity in the 

far-UV is still only a fraction of what it is for 

most metallic coatings in the visible and NIR. 

Figure 9.21. Effective area of the LUMOS far-UV (left) and near-UV (right) multi-object 

spectrograph modes.
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The LUVOIR Interim Report 9-25

Thus, the optical design must be carefully 

optimized to reduce the number of surfaces 

in the system, and contamination must be 

minimized to ensure as many photons as 

possible make it through the system to the 

detector.

9.3.3.3 LUMOS design 

implementation

LUMOS is a highly multiplexed ultraviolet 

spectrograph, with medium and lowresolution multi-object imaging spectroscopy 

and far-UV imaging modes. LUMOS 

can be thought of as an analog to the 

successful HST Space Telescope Imaging 

Spectrograph (STIS) instrument, with two 

orders-of-magnitude higher efficiency, multiobject capability, and a wide-field multiband imaging channel (France et al. 2016). 

Coupling the relatively high instrumental 

throughput with a factor of ~40× gain in 

collecting area over HST ([15 m / 2.4m]2 = 

40), LUMOS can reach to limiting fluxes of 

order 100–1000 times fainter than currently 

possible with the HST Cosmic Origins 

Spectrograph (COS) and STIS instruments. 

Figure 9.21 shows predicted effective area 

curves for the multi-object spectrograph 

Figure 9.22. Side view of LUMOS showing key components. Items with a red arrow correspond 

to elements in the multi-object spectrograph channel; items with a blue arrow correspond to 

elements in the imaging channel. Note that the far-UV imaging channel detector is hidden behind 

the bulkhead of fixed gratings. The scanning calibration mirror is retractable to move it out of the 

beam path.
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9-26 The LUVOIR Interim Report

modes. Figure 9.22 shows the layout of the 

LUMOS instrument.

9.3.3.3.1 Optical design

9.3.3.3.1.1 Multi-object imaging 

spectrograph

Figure 9.23 shows the light path through the 

multi-object spectrograph (MOS) channel. 

Light from the OTE enters from the upper right. 

The entrance aperture for the spectrograph 

is a 3 x 2 grid of microshutter arrays (Li et 

al. 2014). The microshutter arrays build on 

the design of the Near Infrared Spectrograph 

instrument on JWST (Kutyrev et al. 2008), 

with 6 individual tiles of 420 × 840 shutters 

with individual shutter dimensions of 100 

μm × 200 μm. The microshutter array grid 

defines the field-of-view for multi-object 

spectroscopy: 3' × 1.6' for the far-UV modes 

and 1.3' × 1.6' for the near-UV mode. In 

both modes, light passing through the 

microshutter array is directed onto a series of 

six fixed gratings via a fixed convex biconic 

optic and a second aberration-correcting 

toroidal steering mirror. Piston, tip, and tilt 

control of second mirror selects which of the 

six gratings is illuminated at any given time.

Of the six fixed gratings, five are used 

for far-UV (100–200 nm) spectroscopy, and 

include medium, low, and very low spectral 

resolution options. The sixth grating provides 

a medium-resolution near-UV (200-400 nm) 

option. Table 9.3 summarizes the spectral 

resolution, bandpass, angular resolution, 

and field-of-view for each mode of the multiobject spectrograph. All of the far-UV MOS 

modes are focused onto a 2 × 2 array of 

large-format microchannel plate detectors. 

The near-UV MOS mode is focused onto a 3 

× 7 array of d-doped CMOS devices, similar 

to those used in the HDI UVIS channel (see 

Section 9.3.4.3.5).

9.3.3.3.1.2 Far-UV imaging

The majority of the LUVOIR imaging science 

is addressed through the HDI instrument 

(200 nm–2.5 mm, see Section 9.3.4), and 

LUMOS will provide a complimentary farUV imaging capability from 100–200 nm. 

The LUMOS far-UV imaging aperture is 

physically offset from the MOS microshutter 

array aperture, and light from the OTE 

enters this channel through an unobstructed 

open aperture. Figure 9.24 shows the light 

path through the imaging channel. Two 

aberration-correcting optics direct the light 

Figure 9.23. Ray path through the multi-object 

imaging spectrograph channel of LUMOS. Light 

enters from the telescope at the upper right 

corner and passes through the microshutter 

array (teal box), which is located at the OTE 

focal plane. Light is then directed to the grating 

selection mirror, which is actuated in piston, 

tip, and tilt to direct the light at one of six, fixed 

gratings. The bottom five gratings are used for 

far-UV observations and direct light toward the 

far-UV detector array, which is also actuated in 

tip and tilt depending on the selected grating. 

The top grating is for near-UV observations 

and directs light to the near-UV detector via an 

additional relay mirror.
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The LUVOIR Interim Report 9-27

through two identical reflective filter wheel 

assemblies that serve to define the imaging 

bandpass in this mode. A neutral-density 

filter wheel is also inserted between the two 

filter wheels to accommodate far-UV bright 

object protection, target acquisition for 

the MOS channel, and safe imaging of the 

target field through the microshutter array for 

shutter selection. The images are recorded 

on a single 200 mm × 200 mm microchannel 

plate detector. Table 9.3 summarizes the 

bandpass and angular resolution achieved 

by the FUV imaging channel.

9.3.3.3.1.3 Cross-over mode

LUMOS performs bright object protection 

by pre-imaging the target field through 

the microshutter array. LUMOS contains 

a ‘cross-over mode’ that uses the grating 

selection mirror to direct light to the first 

filter wheel assembly in the imaging channel, 

through a neutral density filter wheel, and 

to the far-UV imaging detector. In this 

way, target brightness is quantified prior 

to spectral imaging acquisition with the 

MOS (imaging target acquisitions can also 

be made through the neutral density filter 

for bright-object-protection in the far-UV 

imaging mode). Imaging the spectroscopic 

target field through the microshutter array 

has a second benefit: it enables autonomous 

microshutter selection and fine-guidance 

adjustments to acquire the target through 

the selected shutter/slit. Operationally, target 

acquisition is achieved as following:

1. The payload slews to the target field 

using the VIPPS, gimbal system, and/

or spacecraft attitude control system, as 

needed.

2. The target field is imaged through the 

microshutter array (with all shutters open) 

in cross-over mode with the darkest 

neutral density filter in place for bright 

object protection. 

3. As needed, the neutral density filter is 

reduced until adequate signal is achieved.

4. User-supplied celestial coordinates for 

the target objects are referenced to local 

detector coordinates, and the instrument 

autonomously identifies those targets in 

the imaged field.

Figure 9.24. Ray path through the LUMOS 

imaging channel. In this view, the imaging 

channel is one layer “below” the MOS channel – 

thus the imaging channel is offset from the MOS 

channel in the telescope field-of-view. Light 

enters from the telescope at the upper right and 

is reflected from a fixed mirror (hidden behind 

the microshutter array). The light then reflects 

from another fixed mirror to two reflective filters 

that define the optical bandpass. Two filters 

are used in series to improve the out-of-band 

rejection of the filters. A neutral density filter 

can be placed via a filter wheel between these 

two reflective filters for bright object protection. 

After the second reflective filter, light is directed 

to the far-UV imaging detector. The location of 

the detector, hidden behind a bulkhead in this 

view, is indicated by the dashed box.
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9-28 The LUVOIR Interim Report

5. Fine pointing adjustments are 

autonomously computed to align the 

target objects with microshutters. VIPPS 

and fast-steering mirror commands are 

generated to achieve the fine pointing 

adjustments.

6. Unused shutters are closed, and the 

instrument switches to the appropriate 

MOS-mode configuration to begin 

science observations.

9.3.3.3.2 Mechanical design

LUMOS is essentially two instruments in the 

same housing, and in very close proximity. 

The mechanical design uses a composite 

truss structure to hold the optical elements 

in place without obstructing the beam paths 

of neighboring channels.

9.3.3.3.3 Thermal design

Contamination control is a primary concern 

for the LUMOS instrument, thus it is designed 

to operate at a slightly warmer temperature 

(280 K) than the surrounding structure and 

instruments (270 K). Thus, contaminants will 

be more likely to stick to the colder surfaces, 

and not the warmer LUMOS mirrors. 

Additionally, heaters on all mirrors, gratings, 

and detectors are sized to allow for infrequent 

on-orbit bake-outs, to help evaporate any 

accumulated water vapor from the optics.

9.3.3.3.4 Electrical design

In addition to the standard mechanism and 

detector control electronics, a dedicated 

electronics box is included to control the 

microshutter array. A high-voltage power 

supply is also included to drive both the 

calibration lamp sources, as well as the 

microchannel plate detectors.

9.3.3.3.5 Detectors

The far-UV MOS and imager focal planes 

consist of large-format microchannel plate 

detectors, which leverage a long heritage 

of reliable performance on multiple NASA 

space and sub-orbital missions. The far-UV 

imaging channel detector is a single 200 mm 

× 200 mm microchannel plate with 20 μm 

Table 9.3. The instrument parameter goals for each LUMOS mode. The target value is shown on 

top, the average value at the center of the bandpass or field-of-view (FoV) is shown second in 

parentheses, and the average value over 80% of the bandpass or FoV is shown third in parentheses 

and italics. This last value demonstrates that LUMOS achieves the spectral and spatial resolution 

goals across the majority of its spectral and spatial detector area.

Instrument 

Parameter

G120M G150M G180M G155L G145LL G300M FUV 

Imaging

Spectral 

Resolving 

Power

30,000

(42,000)

(30,300)

30,000

(54,500)

(37,750)

30,000

(63,200)

(40,750)

8,000

(16,000)

(11,550)

500

(500)

30,000

(40,600)

(28,000)

N/A

Optimized 

Spectral 

Bandpass

100-140 nm

(92.5-147.4 

nm)

130-170 

nm

(123.4 

-176.6 nm)

160-200 

nm

(153.4-

206.6 nm)

100-200 

nm

(92.0-

208.2 nm)

100-200 

nm

200-400 

nm

100-200 

nm

Angular 

Resolution

50 mas

(11 mas)

(17 mas)

50 mas

(15 mas)

(19.5 mas)

50 mas

(17 mas)

(24 mas)

50 mas

(15 mas)

(27.5 mas)

100 mas

(32 mas)

50 mas

(8 mas)

(26 mas)

25 mas

(12.6 mas)

(12.6 mas)

Temporal 

Resolution

1 msec 1 msec 1 msec 1 msec 1 msec 1 sec 1 msec

Field-of-View 2’ × 2’

(3’ × 1.6’)

2’ × 2’

(3’ × 1.6’)

2’ × 2’

(3’ × 1.6’)

2’ × 2’

(3’ × 1.6’)

2’ × 2’

(3’ × 1.6’)

2’ × 2’

(1.3’ × 1.6’)

2’ × 2’

(2’ × 2’)
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The LUVOIR Interim Report 9-29

spatial resolution and employing a crossstrip anode readout system (Ertley et al. 

2016; Vallerga et al. 2014), which enhance 

detector lifetime. The imaging microchannel 

plate will have an open-face design with an 

opaque CsI photocathode that has high 

flight heritage on the HST Cosmic Origins 

Spectrograph instrument and numerous 

rocket missions (Hoadley et al. 2016).

The far-UV MOS focal plane consists 

of a 2 × 2 array of 200 mm × 200 mm 

microchannel plates, with a 12 mm gap 

between the tiles. These microchannel plates 

again use a cross-strip anode readout and 

have 20 μm spatial resolution. As the MOS 

will disperse spectra across this tiled array, 

each side of the array can be tailored to the 

specific wavelengths that will fall there. The 

blue side of the array will use the same openfaced CsI photocathodes as the imaging 

channel, while the red side of the array will 

use sealed-tube bialkali photocathodes 

(Ertley et al. 2016). The near-UV MOS focal 

plane consists of a 3 × 7 array of d-doped 

CMOS detectors with 8,192 × 8,192, 6.5-

μm pixels per tile.

9.3.3.3.6 Coatings

LUMOS optics will use the same protected 

“enhanced” LiF coatings as the LUVOIR OTE 

using a high-temperature Al+LiF deposition 

technique (Quijada et al. 2014). These 

coatings have demonstrated > 85% normalincidence reflectivity between 103 and 130 

nm. An additional thin protective layer of 

MgF2 or AlF3 is added above the LiF layer to 

reduce the hygroscopic sensitivity of the LiF 

without degrading the far-UV performance of 

the coating.

9.3.4 High-Definition Imager (HDI)

9.3.4.1 HDI overview and science 

goals

The LUVOIR observatory will revolutionize 

the study of the formation and evolution 

of planets, stars, and galaxies through a 

combination of very high sensitivity, high 

angular resolution, and a highly stable and 

well-calibrated point spread function. A key 

instrumental capability for LUVOIR is the 

High Definition Imager (HDI) instrument – the 

primary astronomical imaging instrument for 

observations in the near-UV through the NIR. 

Among the key science cases for HDI are:

• Understand the detailed physics of cosmic 

reionization by measuring the ionizing 

radiation escape fraction in galaxies as 

a function of time and environment. This 

is accomplished via the detection and 

measurement of Lyman continuum flux 

for z ≥ 2 galaxies. 

• Study the mechanisms of cosmic 

reionization by measuring variations, 

as a function of position on the sky, of 

the ultra-faint end of the z ~ 7 galaxy 

luminosity function (down to absolute AB 

magnitude of -12). A drop in the ultrafaint galaxy number density is predicted. 

• Reconstruct detailed and accurate star 

formation histories in many galactic 

environments not reachable with other 

The LUMOS instrument is designed to make revolutionary observational 

contributions to all of the disciplines that call for high-resolution spectroscopy, 

multi-object spectroscopy, and imaging in the ultraviolet bandpass. LUMOS 

will reach limiting fluxes on the order of 100 – 1000 times fainter than currently 

possible with the HST Cosmic Origins Spectrograph and Space Telescope 

Imaging Spectrograph instruments.
LUVOIR The Large UV Optical Infrared Surveyor

9-30 The LUVOIR Interim Report

facilities by directly detecting stars below 

the main sequence turn off in all major 

types of galaxies. This requires reaching 

out to distances of ~10 Mpc.

• Map the growth of substructure and the 

evolution galaxy morphology in the era 

where cosmic star formation peaks (2 < 

z < 4) by observing small-scale structure 

within z > 2 galaxies, down to spatial 

scales of 100 pc, in the rest-frame UV and 

visible. In particular, probe the distribution 

and properties of sub-galactic stellar 

systems (<100,000 solar masses) over 

the redshift range 2 < z ≤ 10.

• Constrain the distribution and properties 

of dark matter by measuring proper 

motions of stars in Local Group galaxies 

and by measuring proper motions of 

galaxies out to nearest groups and 

clusters within 15 Mpc of Milky Way.

• Search nearby stars for exoplanets 

via their induced astrometric wobble 

signature on their host stars to identify 

systems with Earth-mass planets in 

habitable zone regions. This is a potential 

companion program to the main LUVOIR 

coronagraphic exoplanet survey. 

• Measure the long-term global atmospheric 

and interior dynamics of the gas and ice 

giant planets in the outer Solar System. 

On smaller scales, monitor the changes 

in surface features and exospheres of 

small, airless bodies in the Solar System 

due to volcanic or geyser activity. 

These scientific investigations all call 

for an instrument that can instantaneously 

observe a field-of-view that spans 6 square 

arcminutes and provides pixel sampling that 

takes full advantage of the angular resolution 

provided by the telescope. The diverse 

nature of the above science cases (and these 

are just a few of many), also demand an 

instrument with an ample range of spectral 

elements including standard broad, medium, 

and narrow band filters as well as several 

dispersing elements to enable low-resolution 

slitless spectroscopy.

9.3.4.2 HDI design drivers

The HDI design is driven by several 

considerations:

• The need to enable a wide-field-of-view 

with Nyquist sampling at the focal plane 

implies a very large number of detector 

elements and pixels,

• The need for HDI to serve as the primary 

fine-guidance sensor for the observatory 

places special requirements on the 

detector performance and instrument 

reliability,

• Supporting image-based wavefront 

sensing for observatory commissioning 

and periodic wavefront maintenance 

routines, and

• The astrometric science objectives 

require on-board calibration systems 

to achieve the required pixel calibration 

accuracies.

9.3.4.3 HDI design implementation

9.3.4.3.1 Optical design

The optical design provides a 2 x 3 arcminute 

field-of-view with two channels—an 

ultraviolet-visible (UVIS) channel covering the 

range 200 nm– ~1000 nm and a near-infrared 

(NIR) channel covering the range ~1000 

nm–2500 nm. The respective focal plane 

detector arrays provide Nyquist sampled 

images at 400 nm (2.75 mas/pixel) for UVIS 

imaging and at 1200 nm (8.25 mas/pixel) for 

NIR imaging. The choice to achieve Nyquist 

sampling is motivated by a desire to maximize 

information extraction from the images and 

to enable the best possible image quality 

when HDI is operating in parallel with other 

instruments (some of which cannot tolerate 

dithering maneuvers by the observatory 

pointing system). In addition, the optical 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-31

design of the HDI instrument gives the UVIS 

and NIR channels nearly identical fields-ofview and allows for the option to perform 

simultaneous observations in both channels. 

A model of the HDI instrument layout is 

shown in Figure 9.25.

The UVIS channel contains a filter 

select mechanism consisting of 4 wheels, 

with each wheel capable of holding 13 

elements. The design allows for 41 science 

spectral elements, 4 clear slots (needed to 

allow access to each set of filters on each 

wheel), 1 dark slot, 4 defocus lenses, and 

2 Dispersed Hartman sensor elements. The 

NIR channel filter select mechanism consists 

of 3 wheels with 10 elements per wheel. 

The current design allows for 26 science 

spectral elements, 3 clear slots, and 1 dark 

slot. For both the UVIS and NIR channel, the 

filters will be selected based on current user 

preferences, experience with the imagers 

currently on board HST, and imagers that will 

be on JWST.

The channel select mechanism is a single 

wheel with 6 positions and controls the path 

of the light from the relay optics to the HDI 

detectors. The six positions are:

1. Clear – sends all light from the telescope 

to the NIR channel. This position also 

allows light from the internal calibration 

system to be sent into the UVIS channel.

2. Full Reflective – send all light from the 

telescope into the UVIS channel. The 

Figure 9.25. Rendering of the HDI instrument. Blue lines trace the UVIS channel ray-path; red 

lines trace the NIR channel ray-path. Light enters from the telescope at the upper left. For a sense 

of scale, the UVIS filter wheel assembly is ~1 m in diameter.
LUVOIR The Large UV Optical Infrared Surveyor

9-32 The LUVOIR Interim Report

backside of this mirror is also fully 

reflective, allowing light from the internal 

calibration system to be sent into the NIR 

channel.

3. 50/50 Beam Splitter – broadband beam 

splitter for simultaneous UVIS and 

NIR imaging, albeit with each channel 

receiving half the light.

4. Dichroic Beam Splitter – sends light 

from the telescope to both the UVIS 

and NIR channels simultaneously but 

over a restricted range of wavelengths 

in each channel, allowing nearly 100% 

transmission in each of these windows.

5. Optimized UV mirror – send all light from 

the telescope in a defined UV passband 

into the UVIS channel. Can supersede 

the need to use a transmissive UV filters 

for some wavelength ranges of interest in 

the 200–390 nm range.

6. A second optimized UV mirror for UVIS 

only imaging.

9.3.4.3.2 Mechanical design

The mechanical design of HDI is straightforward. A single optical bench holds all of instrument opto-mechanical components and 

serves as the primary interface between the 

instrument and the BSF.

9.3.4.3.3 Thermal design

The bulk of the instrument is held at 270 K, 

however a passively cooled shroud around 

the NIR channel maintains the NIR optics at 

170 K to reduce thermal background. The 

visible focal plane array is also passively 

cooled to 170 K, while the NIR focal plane 

array is passively cooled to 100 K.

9.3.4.3.4 Electrical design

The HDI electrical system is designed to 

accommodate the very large focal plane 

read outs. The form factor and placement 

of the detector front-end electronics in 

each channel are chosen so that the ASICs 

(application-specific integrated circuits) are 

as close as possible to the detector arrays 

to minimize harness length. The front-end 

electronics also contain the necessary 

hardware to accommodate the pixel 

processing algorithms like frame co-adding 

and sampling-up-the-ramp readouts.

The large focal planes imply large data 

volumes. For normal science operations, 

assuming a 200 s average integration time 

consisting of 40 co-added frames at 5 

second intervals, the data volume from both 

channels combined is ~144 Mbps (assuming 

1.6:1 lossless compression). HDI includes 

enough memory to hold data from two days’ 

worth of continuous observation, or about 

25 Tbits. With an expected two downlinks 

per day, this provides ample margin for data 

storage within the instrument.

9.3.4.3.5 Detectors

The UVIS channel includes a 2.7 Gigapixel 

imaging array comprised of forty (40) 8k × 8k 

detectors arranged in a 5 × 8 pattern. The 

UVIS detectors are currently envisioned to 

be CMOS-based devices with 5 μm pixels. 

The 8k × 8k format for each sensor included 

in the current design has not yet been 

produced in flight-qualified scientific systems 

but is within a realistic technology trajectory 

from current devices. The assumed read 

noise is 2.5 e—/pixel and the assumed dark 

current is 0.002 e—/sec/pixel, consistent 

with current state of the art devices. A final 

detector decision, of course, would not need 

to be made for several years allowing ample 

time for detector development.

The NIR detectors are envisioned to be 

a 4 × 5 array of HgCdTe-based devises with 

10 μm pixels. The 4k × 4k format for each 

sensor included in the current design is TRL 

5, due to the development work on H4RG 

detectors for the WFIRST mission (Content 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-33

et al. 2013). We are assuming 2.5 e— per 

pixel readout noise and dark current levels 

of ~0.002 e—/sec/pixel. Current state-ofthe-art read noise values are closer to 5 e—, 

however it is believed a 2× improvement can 

be achieved with modest improvements to 

the read-out electronics. More discussion 

regarding the technology development of 

these detectors can be found in Chapter 11.

9.3.4.3.6 Special instrument modes

In addition to simultaneous UVIS and NIR 

imaging, HDI provides three special modes 

of operation. HDI’s precision astrometry 

mode enables a range of exciting science 

not feasible on existing telescopes including 

astrometric detection of exoplanets and 

measuring proper motions of extragalactic 

sources in the Local Group and beyond. To 

achieve the astrometric accuracy required 

for these applications (better than ±1microarcsecond), we need to calibrate the position 

of every pixel in the UVIS detector array. Such 

a metrology calibration system is needed 

if one wishes to measure galaxy proper 

motions out at the 10–15 Mpc distance 

scale or to detect the stellar wobble induced 

by Earth-mass exoplanets orbiting their host 

main sequence stars. An all-fiber metrology 

system (Crouzier et al. 2016) is included in 

HDI to allow for the calibration of the UVIS 

focal plane pixel geometry to a precision of 

10-4 pixels. We will investigate the astrometry 

error budget more thoroughly in future.

A second special mode of HDI is to 

function as LUVOIR’s primary fine-guidance 

sensor. Both the UVIS and NIR focal plane 

have the capability of defining small regionsof-interest around bright foreground stars. 

These regions-of-interest can be read-out 

at high speeds (up to 500 Hz) to provide a 

pointing signal to the fast steering mirror and 

VIPPS. This can be done without interrupting 

regular science operations.

Finally, the HDI focal plane will serve a 

function similar to that of the Near Infrared 

Camera (NIRCam) on JWST and provide 

defocused image data for phase retrieval and 

wavefront sensing (Dean et al. 2006). This 

dataset will be used during commissioning 

of the observatory to align and phase the 

primary mirror segments after deployment, 

as well as during routine maintenance of the 

wavefront as needed. Six elements in the 

UVIS filter wheel are dedicated to supporting 

this mode of operation: 4 weak lenses for 

generating defocused point-spread function 

(PSF) data, and 2 dispersed Hartmann 

sensors (DHS) for performing coarse phasing 

of the primary mirror segments.

9.4 Alternate instruments and 

design implementations

The instrument suite discussed in this 

chapter is representative of what a set of firstgeneration instruments might look like but 

is by no means the only set of instruments 

LUVOIR could support. The instrument 

capabilities presented here are necessary 

to achieve the science objectives discussed 

earlier in this report, but even that discussion 

is not exhaustive of all of the science one 

could imagine doing with a facility as capable 

as LUVOIR. In the Appendices of this report, 

you’ll find over 20 additional science cases 

The High Definition Imager (HDI) instrument will use Gigapixel arrays and serve 

as the primary astronomical imaging instrument for observations in the near 

UV through the near IR.
LUVOIR The Large UV Optical Infrared Surveyor

9-34 The LUVOIR Interim Report

(Appendix A), as well as additional instrument 

ideas (Appendix E) for the LUVOIR mission 

concept.

It should also be noted that the specific 

details of the instruments presented in this 

Chapter are not final. For each instrument, 

there exist different design implementations 

that may provide similar, or even expanded, 

science capability. For example, extending 

the wavelength range of LUMOS would 

afford high-resolution multi-object imaging 

spectroscopy in the visible regime. Different 

concepts for the ECLIPS back-end 

spectrometers could also be considered 

to support different types of exoplanet 

observations. The Study Team continues to 

explore several options to expand the science 

capability of the proposed instrument suite.

The payload described here in Chapter 

9 only represents a point design for the 

LUVOIR-A concept. It is an architecture that 

closes on all of the driving requirements, 

achieves the compelling science objectives 

laid out in this report, and is technologically 

and programmatically feasible and 

executable. Design optimizations and 

alternative design concepts will certainly be 

explored by future teams, should LUVOIR be 

prioritized for implementation.

References

Cahoy, K., Marinan, A., Kerr, C., Cheng, K., 

& Jamil, S. (2014). Sp Telesc Instrum 

2012 Opt Infrared, Millim Wave, SPIE, 

8442, 84424F

Content, D. A., Armani, N. V., Baker, C. L., 

et al. (2013). UV/Optical/IR Sp Telesc 

Instruments Innov Technol Concepts VI, 

SPIE, 8860, 88600F

Crouzier, A., Malbet, F., Henault, F., et al. 

(2016). A&A, A108, 1

Dean, B. H., Aronstein, D. L., Smith, J. S., 

Shiri, R., & Acton, D. S. (2006). Space 

Telescopes and Instrumentation I: 

Optical, Infrared, and Millimeter, SPIE, 

6265, 626511

Demers, R. T., Dekens, F., Calvet, R., et al. 

(2015). Tech Instrum Detect Exopl VII, 

SPIE, 9605, 960502

Eisenhower, M. J., Cohen, L. M., Feinberg, 

L. D., et al. (2015). UV/Optical/IR Space 

Telescopes: Innovative Technologies 

and Concepts VII, SPIE, 9602, 96020A

Eldorado Riggs, A. J., Zimmerman, N. T., 

Nemati, B., & Krist, J. E. (2017). Tech 

Instrum Detect Exopl VIII, SPIE, 10400, 

10400O

Ertley, C. D., Siegmund, O. H. W., Jelinsky, 

S. R., et al. (2016). High Energy, Opt 

Infrared Detect Astron VII, SPIE, 9915, 

99152G

Feinberg, L. D., Bolcar, M. R., Knight, J. S., 

& Redding, D. C. (2017). UV/Optical/

IR Sp Telesc Instruments Innov Technol 

Concepts VIII, SPIE, 10398, 103980E

France, K., Fleming, B., & Hoadley, K. 

(2016). JATIS, 2, 990506

France, K., Fleming, B., West, G., et al. 

(2017). UV, X-Ray, Gamma Ray Sp 

Instrum Astron XX, SPIE, 10397, 

1039713

Hinkley, S., Oppenheimer, B. R., 

Zimmerman, N., et al. (2011). PASP, 

123, 74

Hoadley, K., France, K., Kruczek, N., et al. 

(2016). Sp Telesc Instrum 2016 Ultrav 

to Gamma Ray, SPIE, 9905, 99052V

Kutyrev, A. S., Collins, N., Chambers, J., 

et al. (2008). Sp Telesc Instrum 2008 

Opt Infrared, Millimeter, SPIE, 7010, 

70103D

Li, M. J., Brown, A. D., Kutyrev, A. S., et al. 

(2014). MOEMS Miniaturized Syst IX, 

SPIE, 7594, 75940N
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 9-35

Macintosh, B. A., Graham, J. R., Palmer, D. 

W., et al. (2008). Adapt Opt Syst, SPIE, 

7015, 701518

Mawet, D., Ruane, G., Xuan, W., et al. 

(2017). ApJ, 838, 92

Mazoyer, J., Pueyo, L., N’Diaye, M., et 

al. (2016). Space Telescopes and 

Instrumentation 2016: Optical, Infrared, 

and Millimeter Wave, SPIE, 9904, 

99044T

N’Diaye, M., Pueyo, L., & Soummer, R. 

(2015). ApJ, 799, 225

N’Diaye, M., Pueyo, L., Soummer, R., & 

Carlotti, A. (2014). Sp Telesc Instrum 

2014 Opt Infrared, Millim Wave, SPIE, 

9143, 91434R

Nemati, B., Effinger, R., Demers, R., et 

al. (2016). High Energy, Opt Infrared 

Detect Astron VII, SPIE, 9915, 99150M

Park, S. C., Eisenhower, M. J., Bluth, M., 

et al. (2017). UV/Optical/IR Sp Telesc 

Instruments Innov Technol Concepts 

VIII, SPIE, 10398

Pueyo, L., Zimmerman, N., Bolcar, M., et 

al. (2017). UV/Optical/IR Sp Telesc 

Instruments Innov Technol Concepts 

VIII, SPIE, 10398, 103980F

Quijada, M. A., Del Hoyo, J., & Rice, 

S. (2014). Space Telescopes and 

Instrumentation 2014: Ultraviolet to 

Gamma Ray, SPIE, 9144, 91444G

Saif, B., Chaney, D., Greenfield, P., et al. 

(2017). Appl Opt, 56, 6457

Shi, F., Balasubramanian, K., Bartos, R. 

D., et al. (2015). Techniques and 

Instrumentation for Detection of 

Exoplanets VII, SPIE, 9605, 960509

Stark, C. C., Roberge, A., Mandell, A., et al. 

(2015). ApJ, 808, 1

Vallerga, J., Raffanti, R., Cooney, M., et al. 

(2014). Sp Telesc Instrum 2014 Ultrav 

to Gamma Ray, SPIE, 9144, 91443J

Wildi, F., Mouillet, D., Beuzit, J.-L., et al. 

(2009). Tech Instrum Detect Exopl IV, 

SPIE, 7440, 74400Q

Zimmerman, N. T., N’Diaye, M., St. Laurent, 

K. E., et al. (2016). Sp Telesc Instrum 

2016 Opt Infrared, Millim Wave, SPIE, 

9904, 99041Y
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 10-1

Chapter 10
LUVOIR The Large UV Optical Infrared Surveyor

10-2 The LUVOIR Interim Report

10 POLLUX: European Study of a UV Spectropolarimeter

Study leads and contact information

Coralie Neiner Jean-Claude Bouret

LESIA, Paris Observatory Laboratoire d’Astrophysique de Marseille

5 place Jules Janssen 38, rue Frédéric Joliot-Curie

92190 Meudon, France 13388 Marseille cedex 13, France

coralie.neiner@obspm.fr jean-claude.bouret@lam.fr

Phone: +33145077785 Phone: +33491056902

Participating institutes

The Phase 0 study of POLLUX is led by LAM 

and LESIA with the support of CNES (France), 

and developed by a consortium of European 

scientists. The consortium consists of 154 

scientists and engineers from 67 institutes in 

13 European countries. The main institutes 

participating in the present document are: 

Armagh Observatory (Northern Ireland), 

Bulgarian Academy of Sciences (Bulgaria), 

CEA (France), Center for Astrobiology 

(Spain), Complutense University of Madrid 

(Spain), CNES (France), CSL (Belgium), 

DESY (Germany), Exeter University (UK), 

Geneva University (Switzerland), GEPI 

(France), IAP (France), IMPS (Germany), 

IPAG (France), IRAP (France), LAM (France), 

LATMOS (France), LESIA (France), MPS 

(Germany), Royal Observatory Edinburgh 

(UK), Space Research Institute (Austria), 

Stockholm University (Sweden), Strasbourg 

Observatory (France), Trinity College (Ireland), 

TU Delft (The Netherlands), UK Astronomy 

Technology Center (UK), University of 

Edinburgh (UK), University of Hamburg 

(Germany), University of Leicester (UK), 

Catholic University of Leuven (Belgium), 

University of Liège (Belgium), University of 

Pisa (Italy), University of Warwick (UK).

10.1 Executive summary

The POLLUX instrument concept study 

started in January 2017. POLLUX is a highresolution spectropolarimeter operating at 

UV wavelengths, designed for the 15-meter 

LUVOIR-A architecture. The instrument will 

operate over a broad spectral range (97 

to 390 nm), at high spectral resolution (R 

≥ 120,000). These capabilities will permit 

resolution of narrow UV emission and 

absorption lines, allowing scientists to follow 

the baryon cycle over cosmic time, from 

galaxies forming stars out of interstellar gas 

and grains, and stars forming planets, to the 

various forms of feedback into the interstellar 

and intergalactic medium (ISM and IGM), 

and active galactic nuclei (AGN).

The most innovative characteristic of 

POLLUX is its unique spectropolarimetric capability that will enable detection of 

the polarized light reflected from exoplanets 

or from their circumplanetary material and 

moons, and characterization of the magnetospheres of stars and planets and their 

interactions. The magnetospheric properties 

of planets in the solar system will be accessible at exquisite levels of detail, while the influence of magnetic fields on the Galactic scale 

and in the IGM will be measured. UV circular 

and linear polarization will provide a full picture of magnetic field properties and impact 

for a variety of media and objects, from AGN 

jets to all types of stars. POLLUX will probe 

the physics of accretion disks around young 

stars, white dwarfs, and supermassive black 

holes in AGNs, and constrain the properties, 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 10-3

especially sphericity, of stellar ejecta and explosions.

Since the parameter space opened by 

POLLUX is essentially uncharted territory, its 

potential for groundbreaking discoveries is 

tremendous. It will also neatly complement 

and enrich some of the cases advanced for 

LUMOS, the multi-object UV spectrograph 

for LUVOIR. In this chapter we outline a 

selection of key science topics driving the 

POLLUX design. We also introduce the 

current instrument concept and identify 

technological challenges that we will address 

in the coming years toward the advanced 

design and construction of POLLUX.

10.2 Science overview

10.2.1 Stellar magnetic fields across 

the HR diagram

From the ground in the visible domain, 

we can obtain sophisticated tomographic 

mapping of structures on the surfaces of 

stars and their magnetic fields. However, to 

understand the formation and evolution of 

stars and their accompanying planets it is 

necessary to also explore their circumstellar 

environments. POLLUX will provide a 

powerful, high-resolution, full-Stokes (IQUV) 

spectropolarimetric capability across the 

ultraviolet domain (97–390 nm) to uniquely 

trace these structures. When combined 

with the immense light-gathering power of 

LUVOIR, it will deliver unprecedented views 

of the disks, winds, chromospheres and 

magnetospheres around a broad range of 

stars, as highlighted below (also see Figure 

10.1).

Magnetic fields play a significant role in 

stellar evolution, while also being a key factor 

in both planet and star formation. We need 

POLLUX spectropolarimetry to identify and 

characterize the B-fields across all stellar 

masses, to address central questions such 

as:

• How do B-fields develop in the pre-mainsequence (PMS) phase? POLLUX will 

characterize the strong B-fields arising at 

the sheared interface between star and 

disk in the PMS phase (via MgII, FeII, and 

CII UV lines from the interface region), 

and will trace the flows via polarization 

measurements of the nearby continuum.

• How does the stellar dynamo build-up 

and evolve? As stellar rotation decouples 

from the young planetary disk, the B-field 

is predicted to get stronger and more 

complex (Emeriau-Viard & Brun 2017). 

POLLUX will investigate propagation 

of magnetic energy through the stellar 

atmosphere into the uppermost coronal 

layers and stellar wind. 

• How do stellar dynamos form and evolve 

in cool stars? What is their impact on 

planet formation and evolution? POLLUX 

will reveal the magnetically mediated 

interaction between young cool stars 

and their planetary disks during dynamo 

formation and stabilization when planets 

and planetary atmospheres form.

• A small but significant fraction of massive 

stars have strong B-fields (~10%; e.g., 

Fossati et al. 2015). However, very weak 

Figure 10.1. Schematic view of the different 

features and regions traced by observations at 

different wavelengths around a hot star. Credit: 

S. Cnudde.
LUVOIR The Large UV Optical Infrared Surveyor

10-4 The LUVOIR Interim Report

fields could be ubiquitous in massive 

stars. POLLUX will uniquely detect subGauss B-fields in massive stars, providing 

a much-needed understanding of weak 

fields, field-decay mechanisms, and their 

impact on stellar evolution.

• POLLUX will enable studies of stellar 

B-fields beyond the Milky Way for the first 

time. These will open-up observations in 

the metal-poor Magellanic Clouds (MCs), 

probing the evolutionary processes linked 

to B-fields at metallicities equivalent to 

those at the peak of star-formation in the 

Universe.

POLLUX will also enable breakthroughs 

in other areas of contemporary stellar 

astrophysics, including:

• Jet formation: The mechanism driving 

the powerful jets from the interaction 

between disks around PMS stars 

and their magnetospheres is poorly 

constrained; POLLUX will transform our 

understanding of this dramatic part of 

star formation.

• Winds & outflows: A large uncertainty 

in evolutionary models of massive stars 

is the link between rotation and mass 

lost via their winds, which limits our 

understanding of the progenitors of 

g-ray bursts, pair-instability SNe, and 

gravitational-wave sources. POLLUX 

observations in the Galaxy and MCs will 

map the density contrasts with stellar 

latitude required to calibrate these 

models.

• Novae & SNe: Both classical novae 

and SNe remnants provide excellent 

opportunities to learn about the 

production and lifecycle of dust. 

POLLUX spectropolarimetry is required 

to identify sites in the ejecta where dust 

condenses—allowing us to also infer the 

effects of kicks imparted by core-collapse 

SNe—and the shapes/sizes of the grains 

involved.

• White dwarfs (WDs): A minority of WDs 

are strongly (>1MG) magnetic (Ferrario 

et al. 2015), but little is known of the 

incidence of weaker fields. POLLUX 

UV spectropolarimetry will explore this 

regime for the first time to investigate the 

ubiquity of B-fields in WDs, as well as the 

processes generating their fields (e.g., 

amplification of fossil fields and/or binary 

mergers?)

A 100-hour POLLUX observing 

program would be transformational in our 

understanding of B-fields in stellar evolution. 

For instance, we could include UV monitoring 

of two low-mass sources (PMS T Tauri, 

evolved T Tauri) to directly characterize their 

B-fields and magnetic-energy transport, 

combined with observations of a first sample 

of tens of Galactic massive stars to assess 

the presence/strength of weak B-fields 

10.2.2 What are the characteristics 

of exoplanet atmospheres and how 

do planets interact with the host 

stars?

The characterization of exoplanets is key to 

our understanding of planets, including those 

in the Solar System. POLLUX’s unique, simultaneous, high-resolution and polarimetric 

capabilities in the UV are essential to unveiling the origins of the huge range of chemical 

and physical properties found in exoplanetary atmospheres (e.g., Sing et al. 2016), 

and to understand the interaction between 

planets and their host stars (e.g., Cuntz et 

al. 2000).

The line and continuum polarization 

state of starlight that is reflected by a planet 

is sensitive to the optical properties of the 

planetary atmosphere and surface and 

depends on the star-planet-observer phase 

angle (Figure 10.2). Atmospheric gases are 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 10-5

efficient scatterers at UV wavelengths and 

deviations from the expected polarization 

across the UV would reveal the presence 

and microphysical properties of aerosol and/

or cloud particles. This information is crucial 

for getting insight into a planet’s climate. 

It would also strongly complement transit 

observations. POLLUX can significantly 

detect polarization signatures for close-in 

gas giants (and brown dwarfs) orbiting stars 

out to distances of 70 pc, which currently 

comprises over a dozen targets, two of them 

transiting.

Star-planet interactions (SPI) could generate detectable signatures in exoplanetary 

systems. Searching for SPI has developed 

into a very active field (e.g., Shkolnik et 

al. 2003; Kashyap et al. 2008; Miller et al. 

2015). Evidence of SPI has been observed in 

UV stellar emission lines (e.g., NV; France et 

al. 2016). POLLUX will be capable of studying the intensity of UV stellar emission lines 

forming in a wide range of temperatures as 

a function of planetary orbital phase, uniquely combining this information with measurements of the magnetic field derived from the 

Stokes V profiles of these lines. This will allow for the first time identification of the stellar regions mostly affected by SPI and hence 

to understand their origin.

Figure 10.2. Top panels 1 & 2: Schematic of a planetary system in which the unpolarized stellar 

light becomes polarized through reflection by a planetary atmosphere. Bottom panel 3: Degree of 

polarization (in %) as a function of planetary orbital phase, labeled as in panel 1, at 300 nm. The 

different lines indicate different atmospheric cloud coverages, where zero corresponds to the 

cloudless condition. POLLUX will measure the degree of polarization as a function of wavelength 

and planetary orbital phase. Credit: L. Rossi
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10-6 The LUVOIR Interim Report

At UV wavelengths, a hot Jupiter orbiting 

a Sun-like star with a two-day orbital period 

presents a maximum polarization of ≈30% 

(Figure 10.3), leading to a maximum 

polarization signal of the order of 7  x  10-5, 

when observed unresolved from its star. 

With POLLUX mounted on a 15-m LUVOIR 

telescope and a binning size of 50 Å, the 

signal-to-noise ratio (S/N) necessary to 

detect such a signal can be obtained in less 

than 3 hours of exposure time for systems 

as distant as 70 pc (Figure 10.3). Measuring 

and modeling the variation of the polarization 

signal as a function of the planet’s orbital 

phase requires at least eight measurements 

spread across the orbit. A POLLUX 100-hour 

observing program would allow the detection 

and characterization of the UV polarimetric 

signature for over a dozen targets.

10.2.3 The various phases of the ISM 

and extragalactic IGM

Matter in the interstellar space is distributed 

in diverse, but well-defined phases that 

consist of (1) the hot, ionized (T~106-7K) ISM 

that emits soft X-rays, (2) the warm neutral or 

ionized medium (6000 < T < 104 K), and (3) 

the cold (T~10–200 K) neutral medium and 

molecular star-forming clouds. Boundaries 

between the different phases play a 

fundamental role in the cooling of the gas 

and by consequence in Galactic evolution. 

However, it is not yet clear how these different 

phases trade matter and entropy. We need 

to address the multi-phase aspect of the ISM 

and the influence of different factors such 

as magnetic field, thermal/pressure (non-)

equilibrium, metallicity, shocks, etc.

Gaining fundamental insights into the 

processes that influence the different 

phases requires the wealth of interstellar 

absorption features appearing in the UV 

spectra of hot stars, and requires observing 

them at R ≥ 120,000 to measure individual 

clouds and disentangle the contributions 

from the different phases. POLLUX will, for 

the first time, give access to all phases at 

the required high spectral resolution. It will 

enable simultaneous study of tracers of hotgas (OVI, CIV), warm gas (OI, NI, and many 

singly-ionized species), as well as cold-HI gas 

like CI, and of the molecular phases through 

H2 and CO lines. POLLUX will resolve the 

velocity profiles of the different H2 rotational 

levels, yielding temperature and turbulence, 

and information on the formation of H2 and 

its role as a coolant, e.g., in turbulence 

dissipation.

ISM studies also need to be generalized 

to external galaxies. POLLUX will enable 

tomography of chemical abundances in 

several hundred nearby galaxies using neutral 

gas absorption lines toward individual stars 

across the entire (stellar) body of galaxies or 

Figure 10.3. Average S/N in the near-UV 

obtained with 3 hours of POLLUX exposure time 

as a function of the distance to a target Sunlike star (line colour) and size of the telescope 

primary mirror. The red dashed horizontal line 

indicates the maximum near-UV polarization 

signal of a hot-Jupiter with a 2-day orbital 

period. The black dotted vertical line indicates 

the size of the LUVOIR-A primary mirror.
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toward background quasars. Abundance 

variations across and between galaxies 

of different types and metallicities will 

document the dispersal/mixing spatial/time 

scales of newly produced elements and their 

nucleosynthetic origin, the role of infalling 

and outflowing gas in the metallicity buildup, and the dust production/composition 

through depletion patterns. Furthermore, 

cooling rates, physical conditions, and the 

molecular gas content can be determined 

spatially in the HI reservoir (with implications 

for the regulation of star formation and for 

galaxy evolution at large) and in neutral shells 

of HII regions (with implications for the starformation process itself).

A huge step forward in the knowledge 

of the Galactic magnetic field in terms of 

sensitivity, sky coverage and statistics has 

been made from recent dust polarization 

measurements. However, they tell us nothing 

about the distance of the magnetic field and 

its distribution in the different components 

or phases. The POLLUX spectropolarimeter 

will provide this information through its ability 

to detect linear polarization in UV absorption 

lines through Ground State Alignment (GSA, 

Yan & Lazarian 2012). It will open up access 

to the magnetic field 3-D, by measuring its 

orientation in different clouds, on limited 

distances along sight lines and on small 

scales (Figure 10.4). In a 100-hour program, 

observing several hundreds of hot stars with 

a S/N of 500 to measure linear polarization 

in optically thin absorption lines at a level of 

a few % will help understand the interplay of 

magnetic field and the different ISM phases. 

By discerning magnetic fields in gas with 

different dynamical properties, POLLUX will 

allow the first study of interstellar magnetic 

turbulence (Zhang & Yan 2017).

10.2.4 Beyond the unresolvable 

regions of active galactic nuclei: 

revealing accretion disk physics, 

dust composition and magnetic field 

strength with UV polarimetry

POLLUX will offer unique insight into the still 

largely unknown physics of Active Galactic 

Nuclei (AGNs), which are believed to arise 

from accretion of matter by supermassive 

black holes in the central regions of galaxies. 

Some key signatures of accretion disks can 

be revealed only in polarized light, and with 

higher contrast at ultraviolet wavelengths. 

Specifically, high-resolution UV polarimetry 

will provide geometric, chemical and 

thermodynamic measurements of accretion 

disks in unprecedented detail. By probing 

the ubiquitous magnetic fields expected to 

align small, non-spherical dust grains on 

scales from the accretion disk out to the 

extended dust torus, POLLUX will be able to 

reveal the mechanisms structuring the multiscale AGN medium. The key information 

encoded into the polarized light will allow 

Figure 10.4. Synthetic polarization (%) map 

of the simulated diffuse ISM (1 pc2). Contour 

color: percentage of polarization in the SII 

1250 A absorption line. Orientation of the bars: 

polarization direction. The mean magnetic 

field (3 μG) is oriented along the x-axis. This 

figure shows that the direction of polarization 

correctly indicates the magnetic field direction 

and that the expected polarization is mostly 

above 5%, within POLLUX capabilities.
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determinations of the mineralogy, structure 

and alignment of the smallest dust grains, 

together with line-of-sight magnetic-field 

strengths. Measurements of magnetic field 

strengths will also provide constraints on the 

structure of magneto-hydrodynamic winds 

in nearby, broad absorption-lines systems 

accessible to the instrument. 

On larger scales, UV polarimetric studies 

of young star-forming regions will provide 

unprecedented insight into the enigmatic 

relation between onset of star formation and 

triggering of nuclear activity. Once activated, 

an AGN is expected to feed vast amounts of 

energy back into the interstellar medium of 

its host galaxy through radiation and shocks. 

This feedback can decrease or increase 

the star formation activity of the host; our 

understanding of the physical conditions in 

this interaction will be greatly improved with 

UV polarization observations from POLLUX. 

In addition to the extended environment 

of AGNs, POLLUX will provide insight into the 

nature and lifetime of particles in relativistic 

jets, which are other key factors to fully 

understand AGN feedback on star formation 

in galaxies. By focusing on bright, low-redshift 

galaxies, it will be possible to obtain, for the 

first time, high-resolution spectra providing 

striking details on the structure and physics 

of AGNs. Complemented by the optical 

and infrared instruments on board LUVOIR, 

POLLUX will constitute a groundbreaking 

Figure 10.5. Effective area of POLLUX (in black; assuming 135 m2 for LUVOIR with a RC telescope), 

compared to those of previous space-based UV spectrographs, as indicated. Only 4 AGN were 

observed with WUPPE. A total of 117 were observed with different polarimetric instruments on 

board HST, out of which only 35 were in the UV with HST/FOS (for which an effective area similar 

to that of HST/COS was adopted). POLLUX will enable observations of hundreds of AGN over a 

wide UV spectral domain, and with much greater spectral resolution than ever achieved.
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means of assessing the important role played 

by AGNs on galaxy evolution (Figure 10.5).

10.2.5 Testing fundamental physics 

and cosmology using absorption lines 

towards quasars

Physics and cosmology as we know them 

today have been remarkably successful 

in reproducing most of the available 

observations with only a small number of 

parameters. However, it also requires that 

96% of mass-energy content of the Universe 

is in mysterious forms (dark energy and dark 

matter) that have never been seen in the 

laboratory. This shows that our canonical 

theories of gravitation and particle physics 

may be incomplete, if not incorrect. Improving 

the sensitivity of current observational 

constraints is therefore of utmost importance, 

irrespective of whether it is consistent with 

the current standard physics—in which case 

it will reject other scenarios—or whether it 

will instead favor new physics.

Absorption-line systems produced by 

intervening gas in the spectra of background 

sources provide original sensitive probes of 

fundamental physics and cosmology. The 

high UV spectral resolution of POLLUX on 

LUVOIR will open a unique window on such 

probes, in particular (1) the measurement of 

the primordial abundance of deuterium, (2) 

the stability of fundamental constants over 

time and space and (3) the redshift evolution 

of the cosmic microwave background (CMB) 

temperature. The D/H ratio can be estimated 

from the DI and HI Lyman series lines. Any 

change in the proton-to-electron mass ratio 

(μ=m

p

/me) translates into a relative wavelength 

change of the H2 Lyman and Werner lines. 

Finally, the CMB radiation excites the CO 

molecules so that the relative population in 

different rotational levels (measured through 

the electronic bands in the UV) is an excellent 

thermometer for the CMB temperature.

We remark that, while these are 

independent probes, they are intimately 

related by the underlying physics. For 

example, models involving varying scalarphoton couplings (e.g., Avgoustidis et al. 

2014) also affect the temperature-redshift 

relation so that constraining this relation 

is complementary to a search for varying 

fundamental constants. The Big Bang 

nucleosynthesis calculations of the D/H 

ratio are also dependent on the fundamental 

constants and can be altered if new physics 

is at play (e.g., Olive et al. 2012).

The baseline specifications of LUVOIR/

POLLUX are S/N~100 per resolution 

element in 1h for Fλ=10-14 erg s-1 cm-2 Å-1. 

This means that for known quasars with 

low-z H2 absorbers as observed by HST/

COS (Muzahid et al. 2015), we can reach a 

precision of a Δμ/μ ~ a few 10-7 in 20–30h 

(Figure 10.6). This is about an order of 

magnitude better than the current best limits 

(around 5x10-6 with UVES on the Very Large 

Telescope), in less observing time. Similarly, 

the achieved precision on TCMB scales directly 

with the S/N ratio (0.1 K at S/N~100 and 

R~100,000 when current limits are ΔT~1K). 

The same is again true with the D/H ratio, 

although a very high resolution is less critical 

Figure 10.6. Expected error on Δμ/μ as a 

function of signal-to noise ratio (red band, lower 

x-axis) and minimum wavelength (dashed line, 

absorber’s rest-frame, upper x-axis) covered 

by a R=120,000 spectrum.
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10-10 The LUVOIR Interim Report

since the corresponding lines are typically 

thermally broadened by at least 10 km s-1.

10.2.6 Solar system: Surfaces, dust 

scattering, and auroral emissions

UV observations uniquely probe the surfaces 

of telluric bodies of the solar system. They 

diagnose their volcanic and plume activity, 

their interaction with the solar wind, and their 

composition relevant to the space weather 

and exobiology/habitability fields. Very few 

UV polarimetric observations were obtained 

with WUPPE (e.g., Fox et al. 1997), revealing 

in particular the Io surface as spatially 

covered by 25% SO2 frost with polarization 

variations associated to different volcanic 

regions. POLLUX will primarily characterize 

volcanism and/or plume activity of icy moons 

from polarized solar continuum reflected light 

and spectral UV albedo. Its high sensitivity 

is necessary to track any organic and ice 

composition of the crust of comets and 

Kuiper Belt objects from their UV spectra.

The giant planets’ UV aurorae are mainly 

radiated from H and H2 atmospheric species, 

collisionally excited by accelerated charged 

particles precipitating along the auroral 

magnetic field lines. Aurorae thus directly 

probe complex interactions between the 

ionosphere, the magnetosphere, the moons, 

and the solar wind. Precipitation of auroral 

particles is additionally a major source of 

atmospheric heating, whose knowledge is 

needed to assess the energy budget, the 

dynamics, and the chemical balance of the 

atmosphere. The narrow FOV of POLLUX 

Figure 10.7. Left: auroral emissions observed on Jupiter by HST. The Ly-alpha emission spectrum 

in the polar auroral region was obtained with STIS. Its strong asymmetry can be reproduced with 

a wind shear reaching 4-8 km/s. Credit: Chaufray et al. (2010). Right: Io observed by Galileo. The 

linear polarisation of the surface measured by WUPPE between 220–320 nm presents complex 

variations with wavelength and provide information on the volcanic activity on the surface of Io. 

Credit: Galileo Project / JPL / NASA / Fox et al. (1997)
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The LUVOIR Interim Report 10-11

will measure the bright complex aurorae 

of Jupiter and Saturn, the fainter ones of 

Uranus, and catch those of Neptune, only 

seen by Voyager 2 (Lamy et al., 2017). The 

high spectral resolution will be used to finely 

map the energy of precipitating electrons 

from partial spectral absorption of H2 by 

hydrocarbons (Ménager et al. 2010, Gustin 

et al., 2017) and the thermospheric wind 

shear from the H Ly-α line (Figure 10.7, left 

panels; Chaufray et al. 2010).

The asymmetric profile of the H Ly-α line 

in the Jovian auroral region (Figure 10.7, 

left panels) should be resolved by POLLUX. 

For an exposure time of 10 minutes, and 

considering the brightness of the wings of 

the Ly-α line measured by HST/STIS, the 

two wings should produce 68±8 cts/px and 

34±6 cts/px respectively, implying a velocity 

larger than 4 km/s, which can be measured 

precisely by POLLUX.

The albedo of a region of 100 km2 can 

be derived with an accuracy of 0.1% with 

a spectral resolution of 1 nm and a 5-min 

exposure near 320 nm. The linear polarization 

of Io’s surface varies between 1 and -10% 

between 220 and 320 nm; near 320 nm, the 

linear polarized signal observed by POLLUX 

for a binning size of 1 nm in 5 minutes should 

have a S/N~10, which is better than WUPPE 

(Figure 10.7, right panels).

10.3 Design drivers

The science goals described in the previous 

section lead to the technical requirements 

for POLLUX presented in Table 10.1, with 

comments in the last column.

10.4 Design overview & 

implementation

The baseline configuration of POLLUX that 

we will now present allows fulfillment of all the 

Table 10.1. High-level requirements

Parameter Requirement Goal Reasons for requirement

Wavelength range 97–390 nm 90–650 nm 97 nm to reach Lyγ line.

390 nm to reach CN line at 388 nm in 

comets

Spectral resolving 

power

120,000 200,000 Resolve line profiles for ISM, solar 

system, and cosmology science cases

Spectral length of 

the order

6 nm ≥6 nm To avoid having broad spectral lines 

spread over multiple orders

Polarization mode Circular + linear (= IQUV)

Polarization 

precision 

10-6 Detect polarization of hot Jupiters

Aperture size 0.03" 0.01" Avoid contamination by background 

stars in Local Group galaxies

Observing modes spectropolarimetry and pure spectroscopy

Radial velocity 

stability

Absolute = 1 km/s and relative = 1/10 pixel Avoid spurious polarization signature

Flux stability 0.1% Probe flux and polarization correlation 

in WDs

Limiting Magnitude V=17 To reach individual stars in MCs

Calibration Dark, bias, flat-field, 

polarization, and 

wavelength calibration

+ Flux calibration
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requirements for the instrument performance. 

Most of the technologies required for a 

complete implementation are at TRLs 

compatible with a Phase 0 study. We did 

not find fundamental restrictions or physical 

limitations preventing its implementation. We 

will discuss what research and development 

is needed to allow us to realize the baseline 

configuration by the time of LUVOIR 

implementation.

10.4.1 Baseline optical architecture 

and specifications

In the baseline configuration of POLLUX, 

we adopted the telescope parameters 

provided by the LUVOIR study. POLLUX 

is a spectropolarimeter working in three 

channels. For practical reasons we refer to 

these as NUV (19–390 nm), MUV (118.5–195 

nm), FUV (97–124.5 nm). Each is equipped 

with its own dedicated polarimeter followed 

by a high-resolution spectrograph. The 

spectra are recorded on δ-doped EMCCD 

detectors. MUV+NUV channels are recorded 

simultaneously, while the FUV is recorded 

separately. POLLUX can be operated in pure 

spectroscopy mode or in spectropolarimetric 

mode. POLLUX can be fed by the light 

coming from the telescope or from sources 

in the calibration unit. We anticipate that 

POLLUX can operate with a 270 K housing, 

in line with the requirement of LUVOIR.

We now describe the major assumptions 

that we adopted to design the optical 

architecture of this configuration. They are 

illustrated on Figure 10.8:

• The instrument entrance is a pinhole, 

rather than a slit, for simpler aberration 

correction. 

• The working spectral range is 300 nm. It 

is split into three channels: far ultraviolet 

(FUV), medium ultraviolet (MUV) and near 

ultraviolet (NUV). This allows POLLUX to 

Figure 10.8. POLLUX instrument baseline architecture schematic diagram
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achieve high spectral resolving power 

with feasible values of the detector length, 

the camera optics field of view, and the 

overall size of the instrument. It also 

allows to use dedicated optical elements, 

coatings, detectors, and polarimeter for 

each band, to obtain gains in efficiency. 

• The FUV and MUV boundaries are set 

relative to the Lyman-α line. The lower 

limit for the MUV band is set at Lyman-α 

minus roughly 3 nm, that is 118.5 nm, 

while the upper one for the FUV is 

Lyman-α + roughly 3 nm, i.e., 124.5 nm.

• The shortest wavelength for the FUV 

strongly depends on the main telescope 

throughput and may be reconsidered in 

the future.

• The MUV and NUV channel are separated 

by means of a dichroic splitter. Such 

splitters can have a high efficiency (e.g., 

a mean reflectance of 61% over 140–170 

nm, most of the MUV band, and a mean 

transmittance of 83% over 180–275 

nm, most of the NUV band; see http://

www.galex.caltech.edu/researcher/

techdoc-ch1.html). A dichroic splitter 

allows the instrument to work in two 

bands simultaneously and use the full 

aperture, thus achieving high resolving 

power with relatively small collimator 

focal length. In the present design, we 

set the MUV/NUV boundary at 195 nm, 

to have a maximum of one full octave in 

each channel (here the NUV). 

• Currently there are no dichroic splitters 

operating in the FUV below the Ly-α 

line and there is no evidence that such 

an element will become possible in the 

future. We have decided to use a flip 

mirror to feed the FUV channel. The flip 

mirror is located immediately before the 

dichroic splitter. 

• The splitters are placed as close to 

the focal point as possible in order to 

decrease their size and the size of the 

polarimeters. Currently the flip mirror is 

located 20 mm away from the focus and 

the distance from focus to dichroic is 35 

mm. 

• In each channel, the beam is collimated by 

an ordinary off-axis parabolic (OAP) mirror. 

The off-axis shift and the corresponding 

ray deviation angle are chosen in such 

a way that the distance between the 

entrance pinhole and the echelle grating 

is large enough to place the polarimeter 

and corresponding mechanical parts. 

The MUV and NUV mirrors have identical 

geometry, though they may have different 

coatings and have slightly different 

operation mode due to the difference in 

each polarimeter’s design. 

• Echelle grating works in a quasi-Littrow 

mounting. The exact values of the 

groove frequency and the blaze angle are 

computed to obtain the target dispersion 

and subsequently the spectral resolving 

power. 

• The cross-disperser in each channel 

operates also as a camera mirror, so 

it is a concave reflection grating. This 

approach allows minimization of the 

number of optical components and 

increases the throughput. In order to 

correct the aberrations, the crossdisperser’s surface is a freeform and has 

a complex pattern of grooves formed by 

holographic recording.

• Adopted coatings on the optical elements 

of POLLUX are those used for the 

telescope, except for the polarimeters. In 

the future, they will be optimized for each 

element of each channel (see Section 

10.5).

• Polarimeters are located immediately 

after the splitters in each channel to 

avoid instrumental polarization by the 

spectrograph elements. The polarimeters 

are retractable in the MUV and NUV to 

allow the pure spectroscopic mode. In 
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the FUV only the modulator is retractable. 

The analyzer is kept in the optical path to 

direct the beam towards the collimator. 

• Change of the optical path caused by 

removing the polarimeter from the beam 

is compensated by translating the OAP 

mirror for the three channels. 

• The polarimeter design was optimized 

for each channel accounting for the 

technological feasibility (see Section 

10.4.2). The polarimeters should have 

minimal size in order to decrease their 

influence on the image quality. Firstly, 

transparent plates introduce some 

aberrations. Secondly, due to polarization 

ray splitting, the collimator may operate in 

an unusual mode and have considerable 

aberrations. Thirdly, the shorter the 

optical path inside the polarimeter, the 

smaller the difference between the 

spectropolarimetric and the pure spectral 

observation modes.

It is necessary to switch beams in POLLUX 

in order to feed the detectors with light 

coming from the telescope, or from sources 

in the calibration unit. Furthermore, in order 

to compensate the optical path difference 

and maintain the same beam position and 

the angle of incidence at the echelle when 

switching from the spectropolarimetric 

mode to the spectroscopic mode (done by 

removing the polarimeters from the optical 

train), it is necessary to change the collimator 

mirror (see #6 in Figure 10.8). Due to the 

focal length change, the collimated beam 

and therefore the theoretical resolution 

limit also change. On the other hand, the 

pinhole projection size also changes, so the 

resolution values found while accounting for 

the aberrations should be re-scaled.

The optical design is optimized for the 

conditions described above. In order to 

account for possible misalignments due to 

switching from the pure spectroscopic mode 

to the spectropolarimetric one, the target 

spectral resolving power was set to 130,000 

(i.e., 110% of the requirement).

10.4.2 Polarimeters

Below 120 nm, MgF2 is opaque. Above 

this wavelength, both the birefringence and 

transparency of MgF2 recover quickly. In 

order to optimize the throughput in particular 

below 150 nm, we explored modulation 

based on reflection rather than transmission 

for the FUV and MUV modulators.

Figure 10.9. An example of a schematic optical scheme, here for the FUV channel, including a 

zoom on the FUV polarimeter unit.
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• The FUV polarimeter has a three-mirror 

modulator with SiC mirrors and high 

incidence Brewster angle (close to 80 

degrees) to record polarization. Only the 

reflected P beam can be recovered with 

this technique, hence we cannot use twobeam polarimetry to reduce systematics.

• The MUV polarimeter (see Figure 10.10) 

has a three-mirror modulator coated with 

Al+LiF. The three mirrors rotate as a whole 

around the optical axis of the instrument. 

The first and third mirrors work at an 

incidence angle close to 47 degrees and 

the second mirror at the complementary 

of twice this angle. The choice of three 

mirrors ensures that the output beam is 

in the same axis as the entrance beam. 

A Wollaston prism made of MgF2 is the 

current baseline solution for the analyzer, 

but other options will be studied. 

• The NUV polarimeter is completely 

adapted from the ARAGO design 

(Pertenais et al. 2016), that is a birefringent 

modulator made of three pairs of MgF2 

plates, and a Wollaston prism of MgF2 

for the analyzer. However, thanks to its 

reduced operational range with respect to 

ARAGO, it will be simplified for POLLUX.

10.4.3 Mechanical architecture

The mechanical layout is based on the optical 

design discussed above (cf. Figure 10.11). 

It is intended to illustrate a feasible structural 

design concept including the necessary 

mechanisms, which fit within the available 

instrument volume. 

The major points of this layout can be 

summarized as:

• The central support structure of POLLUX 

consists of a central chassis, which 

provides mechanical interfaces for 

the MUV, FUV, NUV arms, calibration 

unit, polarimetry and beam switching 

mechanisms. This will also provide the 

mechanical interface to the telescope but 

has not been represented at this time.

• MUV, FUV, NUV arms are generically 

similar optical benches constructed of 

rectangular or triangular box sections. 

They provide flat mounting interfaces 

for the collimator, disperser, camera and 

detector subassemblies. The benches 

are connected to the central support via 

box section brackets with angled bolted 

flanges (Figure 10.12).

• The collimator exchange mechanism 

is implemented by linear slides using 

Figure 10.10. Optical schemes for the polarimeter units of the MUV (left) and NUV (right) channels.
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recirculating ball bearing carriages. 

Two slides are used to better control tilt 

error and for robustness. The slides are 

positioned via a leadscrew driven by a 

stepper motor. 

• The calibration insert mechanism rotates 

a fold mirror into the beam. The mirror 

is mounted on a radius arm attached 

directly to the motor shaft. To achieve the 

required repeatability, the mirror position 

should be compliantly mounted and the 

position defined kinematically against a 

stop. 

• The polarimetry insert mechanisms for 

the three channels are implemented 

by linear slides using recirculating ball 

bearing carriages. Two slides are used to 

better control tilt error and for robustness. 

The slides are positioned via a leadscrew 

driven by a stepper motor. 

• The polarimetry rotation mechanisms 

are implemented by worm and gear 

Figure 10.12. Rendering of POLLUX within the allocated volume (left); Opto-mechanical layout 

(center); Beam switching and polarimetry mechanisms (right).

Figure 10.11. Optical system of POLLUX arranged inside the dedicated volume.
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mechanism driven by a stepper motor. 

The axis is defined by a pre-loaded 

angular contact bearing pair. For the FUV 

and MUV, the polarimeters are rotated 

to four positions, while for the NUV the 

modulator is rotated to six positions.

10.4.4 Detectors

The detectors of POLLUX are based on 

the technology of surface processing of 

thinned, back-side illuminated EMCCDs 

(e.g., “δ-doping”). These have now become 

competitive with micro-channel plates 

(MCPs) in the FUV to NUV range. They 

combine the linearity of CCDs with photoncounting ability, which is a key capability 

enabling detection of faint UV signals. 

Furthermore, these detectors now deliver 

high quantum efficiency (e.g., in-band QE 

> 60%, see http://www.mdpi.com/1424-

8220/16/6/927), thus offering the possibility 

to reach very high signal-to-noise ratios.

Recent developments show that visibleblindness can be achieved with proper 

treatment (e.g., anti-reflection coatings). 

Detectors with 13 mm pixels will be used for 

POLLUX. They may be passively cooled to 

~ 120K (to reduce dark current level etc.). In 

the FUV channel, the detector active area is 

203 mm x 19 mm, while for MUV and NUV, 

the active areas are 131 mm x 19 mm and 

131 mm x 24 mm, respectively.

10.4.5 Operation modes

POLLUX is designed to work in two science 

modes, a pure spectroscopic mode and 

spectropolarimetric mode.

• In pure spectroscopy mode, one 

measurement produces either one 

spectrum in the FUV, or two spectra, 

one in the MUV and one in the NUV. It is 

expected that in a majority of cases, two 

measurements (one in the FUV and one 

in the MUV+NUV) will be obtained for a 

target. 

• In spectropolarimetric mode, one fullStokes polarimetric measurement requires four spectra in the FUV, or ten 

spectra in the MUV+NUV channels (six 

spectra in the NUV and four spectra in 

the MUV. Here again, we anticipate that 

in most cases two measurements (one in 

the FUV and one in the MUV+NUV) will 

be obtained for a target.

When switching to the spectroscopic 

mode, the full polarimetric unit for MUV and 

NUV, or the polarimetric modulator for the 

FUV, is removed from the optical train. The 

optical path length and the beam position are 

then maintained by translating the collimator 

mirrors (see Section 10.4.1). 

In addition to the science modes, 

POLLUX can work in calibration mode. This 

consists of inserting one of the calibration 

lamps in the light path, blocking the stellar 

light, to acquire calibration images.

The calibration unit (CU, Figure 10.13) 

groups the necessary light sources, including 

a cold redundancy. In order to minimize 

the impact of the instrumental polarization 

Figure 10.13. Schematic rendering of a 

possible calibration unit for POLLUX.
LUVOIR The Large UV Optical Infrared Surveyor

10-18 The LUVOIR Interim Report

on the final calibration, the light from the 

calibration sources will be injected before 

the polarimeters of each channel as early 

as possible in the optical chain, i.e., at the 

level of the flip-mirror mechanism directing 

the light towards the FUV or MUV+NUV 

channels.

The injection of the calibration beams 

requires an additional mechanism inside 

the CU, for the selection of the calibration 

sources. A wheel, bearing the calibration 

lamps, is placed facing a mirror assembly that 

is used to redirect the light from the lamps to 

POLLUX while reproducing the f-number of 

the LUVOIR telescope. This way, we avoid 

any systematic effect resulting from different 

illumination patterns between the scientific 

and calibration beams within the instrument. 

Such wheel mechanisms are usually used as 

filter wheels inside optical instruments and 

therefore they have a long flight heritage.

The main calibration purposes for 

POLLUX internal calibration sources are 

wavelength calibration and flat fielding. 

The optical stimulus for the wavelength 

calibration is a Pt/Ne Hollow Cathode Lamp 

(HCL), covering the entire wavelength range 

of the MUV+NUV channel. The flat-field (FF) 

source is used to calibrate the pixel-to-pixel 

response variations and to monitor the blaze 

function and/or the evolution of the relative 

spectral response function (which will be 

tied to celestial calibrators at sparser time 

intervals). The optical stimulus for the FF is 

a deuterium arc lamp. Given that the FF will 

not be acquired with every observation, the 

impact on the overall mission efficiency is 

marginal. A power supply box will be placed 

next to the CU and will include a high voltage 

supply, which is required for the HCL.

The error matrix of polarization will be 

calibrated on the ground. Any possible 

evolution or aging will be monitored in flight 

by regular observations of a set of celestial 

calibrators, used as reproducibility sources.

We have currently not identified 

calibration sources covering the full FUV 

channel. Consequently, in this channel, the 

ground-based and onboard calibrations will 

be completed with observations of celestial 

standards, essentially white dwarfs.

Standard calibration images will be 

collected once per day for each detector 

through a fixed sequence: 10 flat-field 

images, 5 bias images, and 1 dark image. 

In addition, wavelength calibration should 

be obtained. In pure spectroscopy mode: 

2 wavelength calibration images will be 

downloaded at each new pointing of the 

telescope, one once the telescope is pointed 

and before the science acquisition start, 

and the other after the science acquisition 

is finished and the telescope moves away. 

In spectropolarimetric mode: 1 wavelength 

calibration image would be obtained not 

only before and after the acquisition but 

also between each spectropolarimetric 

measurement.

10.4.6 Performance evaluation

The overall efficiency of POLLUX was 

computed under the following set of 

assumptions:

• The pick-off mirror is assumed to be 

covered with the same broadband coating 

as the telescope mirrors (Al+MgF2+SiC).

• The dichroic is taken to be identical to 

that used in GALEX, but its efficiency 

curves are shifted by 17.1 nm to the red. 

• The coating of the flip mirror is singlelayer SiC. The coatings of the collimators 

and the cross-dispersers are single layer 

SiC in the FUV, and Al+LiF+AlF3 in the 

MUV and NUV.

• The 3-mirror modulator and analyzer 

of the polarimeter in the FUV are SiC. 

For the MUV, the 3-mirror modulator is 

coated with Al+LiF, while the analyzer is 

MgF2. For the NUV, both the plates and 

analyzers are MgF2. 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 10-19

• For each channel, the echelle gratings 

work under pure Littrow mounting. They 

are etched into Si substrate, and their 

profiles are not fully optimized at this 

stage of the study. The echelle coating 

is taken to be Al+LiF for all the channels. 

• Efficiency of the cross-disperser grooves 

is that of an ideal blazed profile in Al 

multiplied by the coating reflectivity.

• The detector quantum efficiency assumes 

an uncoated δ-doped EMCCD. In the 

future it will be optimized for each of the 

channels separately.

The total efficiency (Figure 10.14) is the 

product of efficiencies of all the elements. The 

result was also converted into the effective 

area (adopting a geometrical coefficient 

A

geom

=135 x 104 cm2 for LUVOIR).

In the final design of POLLUX, we expect 

that using specifically tailored coatings for 

each channel will improve the throughput 

of POLLUX. We also intend to investigate 

alternative designs reducing the number of 

reflections. However, the current baseline 

already shows that POLLUX is feasible and 

its science goals reachable.

10.4.7 Future developments for 

POLLUX

In 2018, we will continue to improve the 

optical and mechanical design. We will also 

study the thermal architecture, the thermomechanical stability, the main electronic 

hardware and software, the data telemetry, 

the power and mass budget, the AIT/AIV 

model philosophy, the contamination and 

cleanliness issues, and the radiation impact.

10.5 Engineering and technology 

challenges

Table 10.2 presents the current TRLs 

and heritage of the main components of 

POLLUX. Here we present the development 

plan we have prepared for key elements in 

Table 10.2. Two of the elements have low 

TRLs, namely the cross-dispersers and 

polarimeters:

Cross-dispersers. Each of the crossdispersers of the present design is a 

concave freeform holographic grating. This 

is a novel type of optical element having high 

aberration correction capabilities. For the 

cross-dispersers, computations show that 

a sufficient aberration correction is possible 

only if the spectral components at the crossdisperser’s surface are separated, although 

it leads to the grating’s aperture increase.

A R&D program has been set up in 

collaboration with Jobin-Yvon HORIBA 

(France) to study the feasibility of the required 

holographic recording geometry parameters, 

and develop and test prototypes. According 

to current technology, no showstoppers 

have been identified.

Examples of clear apertures and 

asphericities of the cross-dispersers used 

in the present design are shown in Figure 

10.15. 

Polarimeters. An existing R&D program 

funded by CNES started four years ago on 

the development of new concepts for UV 

polarimeters. It was extended for three more 

years in the fall of 2017. In this frame, a first 

prototype of the concept proposed for the 

NUV channel has been built and tested in the 

visible, and is now being tested in the UV. 

The prototype was optimized for the ARAGO 

project and will be specifically tailored for 

POLLUX in 2018.

Furthermore, the concept with three 

mirrors proposed for FUV and MUV will 

be further studied and prototypes will be 

developed and tested in the framework of 

the CNES R&D. A PhD student has already 

been hired to work on this specific point. 

The performance of the polarimeters 

strongly depends on the properties of 

the material and coatings to be used. 

Measurements to characterize them 
LUVOIR The Large UV Optical Infrared Surveyor

10-20 The LUVOIR Interim Report

Figure 10.14. Throughput for POLLUX and effective area. For the MUV and NUV, they have been 

computed with and without the polarimeters. See text for underlying assumptions.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 10-21

(e.g., refractive index) will be performed in 

collaboration with the Max-Planck institute in 

Göttingen and the Metrology Light Source of 

PTB in Berlin (Germany).

Other elements of POLLUX have higher 

TRLs but require further development and 

tailoring:

Detectors. The technology of δ-doped 

EMCCDs is not fully mature yet. More 

R&D is required to further demonstrate the 

dynamic range and how low spurious noise 

is, in a realistic, end-to-end environment 

for spectroscopy. We need to assess how 

the detectors behave around and below 

Lyman-α for instance. Implementation of 

antireflection coatings, or of metal-dielectric 

stacks as visible rejection filters must be 

improved. A last point will be to demonstrate 

feasibility of detector wafers large enough to 

accommodate our needs (typical detector 

size is 15k x 2k for POLLUX). This size 

appears achievable in the coming years 

(communication from Dr. Shouleh Nikzad, 

JPL). In case tiling of detectors will be needed, 

tests of the impact on the science cases will 

Table 10.2. TRL and heritage of main POLLUX components

Element Est. TRL Status Comment

Flip-Mirror mechanism 9 Exist Similar mechanism has already flown 

(e.g., HST)

Dichroic 5 To be tailored Heritage from GALEX and ARAGO 

study

FUV polarimeter 2 Concept defined New concept

MUV polarimeter 2 Concept defined New concept - Analyzer exists and has 

been tested

NUV polarimeter 4 Proven concept Based on HINODE and CLASP 

heritage, and ARAGO study 

FUV, MUV and NUV 

collimator translation

9 Exist Similar mechanism has already flown 

(e.g., CHEMCAM)

FUV, MUV, NUV collimator 8 To be tailored OAP mirror 

FUV echelle grating 4 To be studied Manufacturer identified. Size exceeds 

what is presently doable. Prospects are 

encouraging 

MUV and NUV echelle 

grating

7 To be tailored Manufacturer identified. Similar gratings 

flew on sound rockets

FUV Cross-disperser 2 Concept defined To be studied. On-going contact with 

Jobin-Yvon Horiba. 

MUV, NUV Crossdisperser2 Concept defined On-going contact with Jobin-Yvon 

Horiba. Preliminary studies are 

encouraging 

FUV, MUV, NUV detectors 6 To be studied 

and tailored

δ-doped EMCCDs; will rely on 

heritage from recent or soon to be 

flown missions (e.g., CHESS, LIDOS, 

FIREball-2)

FUV coating 5 To be studied Choice will depend on the cut-off 

wavelength of the telescope. 

MUV, NUV coatings 5 To be studied Several options are identified and will 

be compared

Calibration lamps 9 Exist Heritage from SCIAMACHY, IUE, HST...
LUVOIR The Large UV Optical Infrared Surveyor

10-22 The LUVOIR Interim Report

be done. These are specific studies that will 

be led by Leicester University (UK). 

Echelle gratings. In the current design, 

the MUV and NUV echelle gratings have 

almost identical sizes and blazing angles. The 

groove frequencies differ approximately by a 

factor of two. Both the groove frequencies 

and the angles are non-standard. For the 

FUV echelle grating, the groove frequency 

and blazing angle are even more unusual. 

We have no evidence that such a grating has 

been ever produced before for the UV domain. 

For the three channels however, feasibility is 

in the range of today’s technological limits 

(communication from Dr. Randall McEntaffer, 

Penn State University).

Open issues include the accuracy of the 

groove profiles and the coatings optimization 

for each of the channels. Another high 

technological risk is the clear aperture of the 

gratings. Today’s process tools are limited 

to 200 mm diameter wafers (~140 x 140 

mm clear aperture), so at least one of the 

dimensions on each of these gratings is 

a challenge (e.g., McEntaffer et al. 2013). 

These points will be addressed in the 

framework of a R&D plan between Penn 

State and University of Colorado, and the 

POLLUX consortium.

Coatings. A CNES R&D program to 

obtain and select coatings with excellent 

UV and FUV efficiency, particularly in the 

90 nm to 390 nm range, started in October 

2017. The goal is to reach above 90% for 

as much as possible of this spectral range, 

high-uniformity coatings (< 1%), over a large 

spectral range with low polarization (< 1%) 

for high-contrast imaging, and pre-launch 

stability of ultra-thin coatings. 

Materials with the best properties are 

MgF2, LiF, and AlF3. Single and double layers 

of these materials have been investigated, 

e.g., Al/LiF, Al/AlF3, Al/LiF/MgF2, Al/LiF/AlF3, 

and combinations with different process 

conditions. At the short wavelength edge, 

presently a top coating with AlF3 gives 

the best reflectance results, as has been 

published most recently by Del Hoya and 

Quijada (NASA GSFC). A reflectance of more 

than 30% above 100 nm and more than 90% 

from 120 nm to 130 nm has been reported 

(see Figure 10.16).

Members of our consortium have 

investigated in the past protective coatings 

of MgF2, LiF, and AlF3 for mirrors at 120 nm 

and studied also their environmental stability 

and durability. To optimize these coatings at 

the short wavelength cutoff it is necessary 

to investigate further the dependence on 

Figure 10.15. MUV cross-disperser. (Left) Footprint diagram. (Right) Asphericity map in microns.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 10-23

coating thickness and substrate temperature 

during the deposition process. We propose 

to investigate protected aluminum coatings 

in collaboration between MPS (Göttingen, 

Germany) and OptiXfab (Jena, Germany). 

Experimental studies with mirror samples will 

start in 2018.

Dichroic. The dichroic filter used for 

beam separation of the MUV and the NUV 

is based on the GALEX heritage. It provides 

a mean reflectance of 61% over the 140–

170 nm band and a mean transmittance 

of 83% over the 180–275 nm band. The 

reflectance was extrapolated down to 

118.5 nm, the transmittance up to 390 

nm, for the computation of the efficiency 

calculations presented in Section 10.4.6. A 

study by REOSC performed for ARAGO has 

demonstrated feasibility down to 119nm. Our 

development plan is to tailor a dichroic beam 

splitter to the MUV and NUV, and further 

increase the efficiency in both channels of 

POLLUX. This will be done in partnership 

with the REOSC Company.

Figure 10.16. Reflectance of Al protected with AlF3 coatings. Credit: Quijada et al. (2017).

References

Avgoustidis et al. 2014, Journ. Of 

Cosmology and Astroparticle Phys., 06, 

062

Ben-Jaffel et al. 1993, Geophys. Res. Lett., 

20, 747

Chaufray et al. 2010, J. Geophys. Res., 

115, E05002 

Clarke et al. 1980, Astrophys. J., 240, 696

Cuntz, M. et al. 2000, ApJ, 533, L151

Emeriau-Viard & Brun, 2017, ApJ 846, 8

Ferrario, L. et al. 2015, SSRv, 191, 111

Fossati, L. et al. 2015, ApJ, 812, L35

Fox et al. 1997, Astron. J., 113, 1152

France, K. et al. 2016, ApJ, 820, 89

Gustin et al. 2017, Icarus, 284, 264

Kashyap et al. 2008, ApJ, 687, 1339 

Lamy et al. 2012, Geophys. Res. Lett., 39, 

L07105

Ménager et al. 2010, A&A, 509, A56

McEntaffer et al. 2013, Exp. Astron., 36, 

389

Miller et al. 2015, ApJ, 799, 163

Muzahid et al. 2015, MNRAS 448, 2840

Olive et al. 2012, MNRAS 426, 1427
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10-24 The LUVOIR Interim Report

Pertenais et al. 2016, SPIE, 9905

Quijada et al., 2017, SPIE, 10372

Shkolnik et al. 2003, ApJ, 597, 1092 

Sing et al. 2016, Nature, 529, 59
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-1

Chapter 11
LUVOIR The Large UV Optical Infrared Surveyor

11-2 The LUVOIR Interim Report

11 LUVOIR technology development

11.1 Introduction

The engineering concept behind LUVOIR 

combines JWST-leveraged telescope design, high-contrast imaging technologies 

developed and matured by the WFIRST mission, and innovations from the aerospace and 

commercial sectors. This chapter describes 

how the critical technological elements of 

LUVOIR are driven by its major scientific 

goals and outlines a near-term technology 

development roadmap. It is distilled from the 

efforts of the LUVOIR Technology Working 

Group, a group of subject matter experts in 

telescopes, structures, mirrors, actuators, 

wavefront sensing and control, coronagraphy, astronomical instruments, optical devices, mechanisms, detectors, spacecraft 

attitude control, vibration isolation, and other 

relevant disciplines. This group is working 

in concert with the LUVOIR Study Team to 

identify technologies needed to implement 

a successful LUVOIR mission. Preliminary 

assessments were reported in (Bolcar 2017) 

to provide early identification and prioritization of key LUVOIR technologies. This chapter provides an updated evaluation of these 

technologies, capturing progress made since 

that initial report.

LUVOIR’s extraordinary capabilities will 

be driven, first and foremost, by its large 

aperture. As described earlier, LUVOIR will 

be launched with several elements stowed 

or in a folded configuration. These will deploy 

on orbit, and be locked into a solid, stable 

structure. Image-based wavefront sensing 

and control techniques will phase its optical 

elements to achieve diffraction-limited 

optical performance. The technologies for 

this process are derived directly from JWST 

and will be proven in space by that mission 

(Dean et al. 2006; Redding et al. 2003; 

Feinberg et al. 2008; Acton et al. 2012). 

Extension of JWST wavefront sensing and 

control technologies to the shorter LUVOIR 

wavelengths is a matter of engineering, with 

the exception of coronagraph control and 

ultra-stability aspects discussed later in this 

chapter. LUVOIR will be larger than JWST, 

but its individual structures and mechanisms 

scale directly from space qualified JWST 

structures and mechanisms. Indeed, they 

may be easier to fabricate and test, since 

LUVOIR will not be cryogenic, and represent 

engineering evolution of existing designs that 

do not require new invention.

The technologies discussed in this chapter are critical to the science objectives of the 

LUVOIR mission. Some are currently immature, without demonstration by previous flight 

missions. In NASA parlance, they have a low 

Technology Readiness Level (TRL), from TRL 

2 to TRL 4 (see Table 11.1 for definition of 

TRLs). All will need to be matured, through 

analysis and simulation, then by hardware 

breadboards and testbeds, then by building 

engineering models and prototypes for system or subsystem level testing in representative environments, to establish TRL 6 by the 

mission Preliminary Design Review (PDR). 

Recommendations for how and when they 

can be matured are noted throughout the 

chapter.

There is overlap within the major technology areas: between different coronagraph 

design families, with differing performance 

and sensitivity to disturbances; and between 

the various active optics and instrument 

technologies. There is a wide range of maturity as well. This provides LUVOIR scientists and engineers with options—and an 

architectural challenge. What combination of 

approaches at the system level best balances cost, risk, and complexity, while providing the required performance to achieve the 

stated science cases? The answer to this 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-3

Table 11.1. Technology Readiness Levels, and when they need to be achieved for critical LUVOIR 

technologies (Hirshorn et al. 2007).

TRL # Maturity Criteria Needed Prior To...

9 Actual system “flight proven” through successful operations on-orbit

8 Actual system completed and “flight qualified” through test and 

demonstrations (ground or flight)

7 System prototype demonstration in a target/space environment

6 System/subsystem model or prototype demonstration in a relevant 

environment (ground or space)

Project Preliminary Design 

Review (PDR)

5 Component and/or breadboard validation in relevant environment

4 Component and/or breadboard validation in laboratory environment Project Phase A

3 Analytical and experimental critical function and/or characteristic 

proof-of-concept

2 Technology concept and/or application formulated

1 Basic principles observed and reported

question will be a moving target, changing 

as progress is made in the various technologies. The initial approach is to prefer less 

complex solutions, while anticipating needs 

that may arise as understanding of the most 

challenging requirements improves.

The path to TRL 6 is based on performance testing at the component, subsystem, and system levels that will guide design 

choices, and ground the overall LUVOIR 

architecture in demonstrated performance. 

Computer modeling and analysis is an essential part of this process, and model verification at each step of technology demonstration is required to validate the tools that 

predict system performance. These tools will 

quantify the key architectural trades and provide an essential guide for LUVOIR development at every project phase, as they have for 

other observatories in the past.

We begin the discussion by reviewing 

coronagraph technologies, as they are 

the source of requirements for other 

technologies—such as the need for ultrastable optics and high-performance 

detectors. Ultra-stability is the second topic 

of discussion—this includes mirror fabrication 

and coatings, as well as active optics 

technologies. Then, instrument technologies, 

especially detectors for all of the LUVOIR 

instruments, as well as devices such as 

microshutter arrays for spectrographs, are 

discussed.

11.2 Coronagraph technology: High 

contrast imaging with an obscured 

aperture

LUVOIR’s groundbreaking exoplanet observations will be accomplished by high contrast direct imaging, using a coronagraph to 

block the light from a star, in order to detect and characterize the much fainter light 

reflected from planets orbiting the star. The 

optical layout of a canonical coronagraph is 

sketched in Figure 11.1. It is an optical instrument that focuses light captured by the 

telescope onto an occulting mask. The mask 

blocks the star light at the very center of the 

field but passes light from objects even a 

very small angle off-center—like planets orbiting that star. After the mask, the beam is 

re-collimated and passed through a pupil 

stop, to remove starlight diffracted by the 

occulting mask. Finally, the beam is re-focused to form a starlight-suppressed image 

of the exoplanet system. This image can be 
LUVOIR The Large UV Optical Infrared Surveyor

11-4 The LUVOIR Interim Report

produced either directly on a detector for 

broadband imaging, or on a lenslet array at 

the entrance of an integral field spectrograph 

for wavelength-resolved imaging.

11.2.1 Coronagraph architectures

Coronagraph designs from the late 2000’s 

were optimized for circular (or elliptical) 

apertures that do not have any obscurations 

in the pupil. This will not be the case 

for LUVOIR’s on-axis telescope, with its 

secondary mirror and support struts, plus 

gaps between mirror segments introducing 

discontinuities in the optical beam. The 

diffraction caused by these features results 

in a broadened, azimuthally structured pointspread function. To achieve high suppression 

of the central starlight, these effects must 

be countered by other elements in the 

coronagraph, such as:

• an apodizing mask to filter the obscuring 

pupil features

• a focal plane occulting mask with a 

complex transmission characteristic

• deformable mirrors (DMs): small mirrors 

that can be precisely shaped to remap 

the telescope pupil geometry.

The coronagraph research community is 

actively exploring how to optimally combine 

these design elements (Mazoyer et al. 2017).

The key design objectives for a 

coronagraph are the deep suppression of 

the on-axis starlight, and the preservation of 

the planet light immediately adjacent to the 

center of the field, in the “dark hole” detection 

region. Starlight suppression is quantified in 

terms of contrast: the ratio of the intensity of 

the faintest detectable planet to that of the 

central star. The inner dimension of the dark 

hole is the inner working angle (IWA): the 

angle which quantifies how close a planet 

can be to the star and still be detected. 

IWA is commonly expressed in aperturenormalized terms, as a multiple of l/D: 

the ratio of the wavelength of light divided 

by the aperture diameter, which is about 

the diffraction-limited half-width of an ideal 

Figure 11.1. Coronagraph elements, showing apodized pupil Lyot coronagraph and vector 

vortex coronagraph features. The telescope is pointed at a target star, whose light is passed to 

the coronagraph, through an apodized pupil mask, and then focused on a focal plane mask. The 

mask suppresses the star image. The rest of the light, including the off-axis planet light, passes by 

the mask. Downstream, a Lyot stop suppresses residual diffracted starlight, and then the planet 

light is refocused onto a focal plane array detector or fed into an integral field spectrograph. 

Two deformable mirrors (DMs) are used to establish high contrast. Out-of-band and low-order 

wavefront sensors (OBWFS, LOWFS) stabilize wavefront and intensity variations to preserve 

contrast. OAP: off-axis parabola—a typical reflective mirror used to relay the light from pupil 

plane to image plane throughout the system.

!"#$% '!#$% &$%

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The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-5

point-spread function. Other key design 

metrics are bandpass—typically 10 to 20% 

of the center wavelength—and throughput, 

the percentage of the planet light captured 

by the telescope that makes it through to 

the core of the point-spread function imaged 

on the detector. Together, these metrics 

determine how quickly an observation can 

reach a high level of integrated signal, and 

how efficient spectroscopic characterization 

observations will be.

Early work on advanced space coronagraph concepts emphasized achieving high 

contrast on telescopes with idealized unobscured apertures. Using a Lyot design with 

a band-limited occulter mask together with 

two DMs, the NASA Terrestrial Planet Finder 

project demonstrated contrast of 5  x  10-10 

with 10% bandpass at an IWA of 4 l/D. 

This established that the performance needed to image an Earth analog is feasible. It 

also kicked off a flurry of new coronagraph 

designs and tests. The best demonstrated 

contrasts at an IWA of 3.1 l/D1 on an unobscured aperture currently stand at:

• 1.2 × 10-10 at a bandpass of 2%

• 6 × 10-10 at a bandpass of 10%

• 1.3 × 10-9 at a bandpass of 20%

The addition of the Coronagraph 

Instrument (CGI) to the WFIRST mission 

offered a major opportunity to the exoplanet 

imaging community—the application of 

state-of-the-art technology on a 2.4-meter 

diameter space telescope—and also new 

challenges. The WFIRST telescope has a large 

central obscuration (30% of diameter) and 

six secondary support struts. Together, these 

pose an exceptionally difficult coronagraph 

design problem. Furthermore, WFIRST will 

operate in a high-vibration environment 

that will dynamically tilt and deform the 

1 See the NASA Exoplanet Program Technology 

Appendix: https://exoplanets.jpl.nasa.gov/exep/

technology/technology-overview/

CGI wavefront during observations. In 

response to these challenges, coronagraph 

development has made rapid progress over 

the past 4 years, leading to the conception 

and laboratory demonstration of new design 

solutions for obscured apertures, and the 

application of Low-Order Wavefront Sensing 

and Control (LOWFS). LOWFS uses light 

rejected by the coronagraph mask to actively 

stabilize the wavefront during coronagraph 

observations. LOWFS and related active 

stabilization methods are discussed in 

Section 11.3.

WFIRST CGI recently demonstrated 

a contrast <1  × 10-8 at an IWA of 3 l/D 

with a bandpass of 10% using 2 different 

coronagraph designs and LOWFS control. 

The demonstration was performed on 

a testbed that includes a WFIRST-like 

shadowed pupil and deliberately-induced 

dynamic disturbances that trace to those 

expected on orbit. As documented in Shi et 

al. (2017) and Seo et al. (2017), the hybridLyot coronagraph design achieved 5 × 10-9 

contrast averaged over 3–9 l/D with a 10% 

bandwidth in the dynamic environment. 

Another major testbed milestone was 

reached in 2017 when the CGI team 

integrated a shaped-pupil Lyot coronagraph 

with an integral field spectrograph and 

demonstrated the first 1 × 10-8 contrast over 

a broad (18%) spectroscopically-dispersed 

bandpass (Cady et al. 2015; Groff et al. 

2017).

The progress made by the WFIRST 

project in developing obscured aperture 

coronagraphs has been driven by rigorous lab 

testing. A similar ground testing campaign, 

using a high-quality LUVOIR-traceable 

testbed, will be needed to demonstrate and 

debug LUVOIR coronagraphs. One such 

facility, the JPL Decadal Survey Testbed, 

will be used to demonstrate coronagraph 

performance at levels that trace to LUVOIR 

and HabEx science goals (Crill & Siegler 
LUVOIR The Large UV Optical Infrared Surveyor

11-6 The LUVOIR Interim Report

2017). This testbed will also provide the 

data for model validation activities, to 

ensure that the models used to engineer the 

coronagraphs and testbeds capture the key 

effects governing coronagraph performance. 

In parallel, the Space Telescope Science 

Institute is now commissioning the High 

Contrast Imager for Complex Aperture 

Telescopes (HiCAT) testbed, aimed at system 

level demonstration at modest performance 

levels (non-vacuum), but realizing the nested 

loop wavefront control architecture that 

is necessary for LUVOIR. Caltech is also 

developing the High Contrast Spectroscopy 

Testbed, shown in Figure 11.2, that will 

demonstrate high-contrast, high-spectral 

resolution instrument concepts.

LUVOIR coronagraph instruments will 

need 10 to 100 times the starlight suppression 

performance of the WFIRST CGI, requiring 

continuing development of key coronagraph 

elements beyond the WFIRST technology 

freeze. Some of these further developments 

are already underway, including the 

Segmented Coronagraph Design Analysis 

(SCDA) study, which sponsors independent 

coronagraph design work at Universities and 

NASA Centers, exploring alternate design 

pathways, and comparing performance in the 

context of telescopes like LUVOIR (Shaklan 

2016). SCDA has already yielded several 

coronagraph designs that theoretically 

achieve high contrast (10-10), with bandpass 

up to 15%, IWA of 3–4 λ/D, and with core 

throughput as high as 20%2—all with 

segmented apertures traceable to LUVOIR. 

The performance of these coronagraphs can 

be degraded by numerous effects such as 

leaked light from spatially-resolved stars, 

mask fabrication and alignment errors, DM 

calibration errors, and dynamic wavefront 

error disturbances. The SCDA design teams 

2 The “core throughput” of the coronagraph is 

defined here as the ratio of the energy in the planet 

point-spread function core to the total incident energy on the obscured primary mirror, ignoring losses 

due to mirror reflectivity, filter transmission, spectrograph transmission, and detector quantum efficiency.

Figure 11.2. (a) The Caltech High Contrast Spectroscopy Testbed telescope simulator and 

wavefront control modules. A segmented deformable mirror (SDM) simulates the primary mirror 

of a segmented aperture telescope. A Boston Micromachines kilo-DM (DM1) will be used for 

wavefront control (not yet installed in photo). (b) A compact coronagraph and fiber injection unit 

consisting of an optional apodizer (Apod), focal plane mask (FPM), and Lyot stop (LS) followed 

by a tip-tilt mirror (TTM) used to couple planet light into a single mode optical fiber, which feeds 

into a spectrograph. The hardware shown will be deployed at W. M. Keck Observatory as part 

of the KPIC instrument in 2018. Credit: D. Echeverri and J.-R. Delorme, (Exoplanet Technology 

Lab, Caltech).
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-7

are pursuing solutions that are robust to 

these effects.

For LUVOIR-A, the preferred coronagraph 

designs include the apodized pupil Lyot 

coronagraph (APLC) (N’Diaye et al. 2015; 

Zimmerman et al. 2016). The APLC uses a 

mask technology that is similar to the shaped 

pupil coronagraph on WFIRST CGI. It has the 

virtue of being relatively tolerant to resolved 

stellar diameter (maintaining most of the 

10-10 dark zone at star diameters up to 0.1–

0.2 l/D) but has a relatively large IWA (> 3 

l/D) and a throughput penalty originating in 

the transmission loss of the apodizer mask. 

Figure 11.3 shows an example of an APLC 

mask design and performance.

The second preferred design is the 

vector vortex coronagraph (VVC), which can 

reach the same contrast as the APLC but 

with smaller inner working angle (~2 l/D) 

and over a wide bandpass, limited only by 

the wavefront control system performance 

(Ruane et al. 2016). For centrally obscured 

telescope apertures, however, the VVC 

has significantly more sensitivity to stellar 

diameter than the APLC.

The best overall solution for LUVOIR 

might be to include multiple coronagraph 

Figure 11.3. Example apodized pupil Lyot coronagraph design. In the upper left is the LUVOIR-A 

telescope pupil. The top center and top right show the Lyot mask and apodizing mask, respectively. 

The bottom left shows the annular high-contrast region in the coronagraph focal plane that would 

result after wavefront control. The bottom right is an azimuthal average of the radial cut through 

the dark-hole region and shows the effects of finite stellar diameter on the contrast. The vertical 

dashed line shows the diameter of the focal plane mask occulting spot. Credit: N. Zimmerman 

(NASA GSFC).
LUVOIR The Large UV Optical Infrared Surveyor

11-8 The LUVOIR Interim Report

design types on board. Like the WFIRST 

CGI, the mask would be selected according 

to the demands of each observation. Thus, 

LUVOIR could use its VVC for smaller, more 

distant stars, or longer wavelength nearinfrared observations in general, whereas 

the APLC would be used for nearby stars 

large enough to be spatially resolved by the 

telescope.

The phase-induced amplitude apodization—complex mask coronagraph (PIAACMC) is a third Lyot coronagraph design 

variant that has been developed for high performance on segmented apertures (Guyon et 

al. 2010). The prospects for this new design 

family are promising, particularly in terms of 

inner working angle and throughput. The 

LUVOIR Study Team anticipates new PIAACMC designs tailored to LUVOIR to evaluate 

within the next year alongside the APLC and 

apodized VVC.

Yet another coronagraph design family 

that is a candidate for LUVOIR are nulling 

coronagraphs, such as the visible nulling 

coronagraph (VNC) (Lyon et al. 2008). 

Instead of an apodizing and focal-plane 

mask architecture, the VNC concept relies on 

pupil shearing interferometry to destructively 

interfere on-axis starlight. The VNC offers 

excellent inner working angle (2 l/D) and 

relatively high throughput (no apodizing 

mask), but at the cost of greater optical 

complexity. The VNC remains under active 

development both in terms of design concept 

and laboratory demonstration (Hicks 2016).

These advances have moved the 

coronagraph state of the art forward, 

as summarized in Table 11.2. Testbed 

demonstrations have established TRL 4 

for the WFIRST CGI coronagraph designs, 

against the performance objectives of 

WFIRST. While successful launch and 

operation of the WFIRST CGI will be 

extremely valuable to LUVOIR technology 

development, it is not a prerequisite for, nor 

will it fully qualify all aspects of, LUVOIRTable 11.2. LUVOIR coronagraph technology status and path ahead. Green shaded options are 

in the LUVOIR-A baseline design. Yellow options may provide alternative or potentially enhancing 

solutions given enough development. Red options would only be considered with substantial 

development and gains in system performance.

Current 

TRL

Path to 

TRL 4

Path to 

TRL 5

Path to 

TRL 6

Hybrid Lyot 4

Shaped Pupil 4

Apodized Pupil 3

Vector Vortex 3

Phase-Induced 

Amplitude Apod. 3

Visible Nulling 3

Photonic 

Solutions 2 Requires study to determine potential benefits.

Electrostrictive 

DMs 5 ü ü WFIRST will demonstrate 48×48 electrostrictive DM to TRL 9.

MEMS DMs 4 ü

Environmental 

Testing

Active funding through TDEM and SBIR 

programs.

High-contrast image 

post-processing

Improve total image 

contrast by >10×

Low initial contrast

Speckle coherence 

considerations

Hubble-derived 3 Analysis Test w/ 

CGI data

Current methods provide 10-500×

improvement, but at 1e-5 initial contrast.

Centrally-obscured aperture

Wavefront drift sensitivity

Resolved stellar diameter

Model maturity

Algorithm convergence time

Testbed 

Demos

Enhanced 

Testbeds, 

including 

LOWFS

Active funding through TDEM program.

WFIRST will demonstrate 1e-8 contrast 

with an obscured aperture to TRL 9.

Shaped Pupil is directly traceable to 

LUVOIR Apodized Pupil architecture.

Hybrid Lyot studies for LUVOIR aperture 

are underway.

Under study via SCDA. Apodized pupil 

is traceable to WFIRST shaped pupil 

concept.

ü WFIRST CGI 

Development

Testbed 

with 

subscale 

segmented 

telescope 

and 

relevant 

dynamic 

and 

thermal 

disturbance

Deformable Mirrors 

(DMs)

Accuracy:

< 5 pm

Stroke:

> 1 μm

Format:

> 64 × 64

Actuator count

Actuator yield

Actuator stability

Electronics packaging

Open-loop commandability

Electrical connections

Segmented aperture 

coronagraph 

architecture

Contrast:

< 1e-10

Inner Working Angle:

< 4 λ/D

Throughput:

> 20%

Bandpass:

> 10%

Development status and path ahead

Technology Driving Requirements Technical Challenges Solution Paths Note
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-9

specific coronagraphs. Rather, WFIRST 

has shown the great value of high-quality 

testbeds for development of missionspecific coronagraphs. This is the path 

forward for LUVOIR, using rigorous ground 

testing to complete development of LUVOIR 

coronagraphs. APLC and VVC designs 

tailored to LUVOIR have been successfully 

demonstrated at the proof-of-concept level 

using computer models, establishing TRL 

3—laboratory demonstrations are needed to 

advance to TRL 4, 5, and 6.

Finally, we note rapid progress in photonic 

devices, driven by the communications 

industry. Design and analysis of idealized 

coronagraphs using devices such as 

photonic lanterns and universal linear 

optical processors indicate real potential 

for new methods to help solve some of 

the hardest problems in obscured-aperture 

coronagraphy. Further work is needed in this 

area, to explore performance with realistic 

devices and begin experimentation.

The next steps for the LUVOIR-specific 

coronagraph designs include: more detailed 

analysis and modeling, emphasizing robustness to noise effects; and then demonstration on testbeds to establish TRL 4. Further 

testing, culminating in closed-loop operation 

with a scaled testbed segmented telescope, 

is needed to establish TRL 6.

Path to TRL 4: Continue the SCDA 

study to model coronagraph performance 

with realistic noise inputs, including optical 

errors, dynamic disturbances and stellar 

diameter. Perform testbed demonstrations 

of the coronagraph designs and achieve 

moderate contrasts (10-8 or better) over 15% 

bandpass at relevant inner working angles. 

Correlate the testbed demonstration with 

models that predict 10-10 contrasts.

Path to TRL 5: Perform testbed 

demonstrations of coronagraph designs 

that achieve flight requirements on contrast, 

bandpass, IWA, and throughput, in the 

presence of expected dynamic and thermal 

instabilities.

Path to TRL 6: Develop a prototype 

coronagraph instrument that incorporates 

flight-like masks, deformable mirrors, and 

optics, including a segmented aperture 

front-end telescope system. Achieve flight 

requirements on contrast, bandpass, IWA, 

and throughput in the presence of expected 

dynamic and thermal instabilities.

11.2.2 Deformable mirrors

Deformable mirrors (DMs) are small mirrors 

with reflective face sheets whose optical 

figure can be changed over a small range 

using actuators. When used in coronagraphs, 

one DM is typically placed at a pupil image, 

where it strongly affects the phase of the 

electric field, providing direct correction of the 

largest source of wavefront error, namely the 

primary mirror. In this configuration a highcontrast dark hole can only be obtained in 

one half of the field of view. Adding a second 

DM at a plane away from the pupil image 

allows correction not only of the phase, 

but of the beam amplitude, which varies 

across the beam. The combination of two 

DMs also enables grating-like diffraction to 

null “speckles”—concentrations of light that 

occur in the dark-hole detection region. This 

results in a dark hole that is now symmetric 

with respect to the optical axis and in theory 

has wider bandpass. In general, the optical 

shape of the DMs is determined by

• obtaining an estimate of the complex 

electrical field at the science camera 

(in order to avoid non-common-path 

errors),

• using a precise diffractive model of the 

DM response through the coronagraph 

in order to find the DM settings that will 

best cancel the aberrated complex field, 

and

• iterating this process in order to obtain 

a solution that is robust to noise in the 
LUVOIR The Large UV Optical Infrared Surveyor

11-10 The LUVOIR Interim Report

estimate or model uncertainty. The 

estimation is in general done using 

spatial or temporal modulation of the 

images at the science camera

The DMs in a coronagraph also set the 

outer size limit of the dark hole. The outer 

working angle (OWA) is theoretically defined 

by the spatial Nyquist frequency of the 

coronagraph DMs, as the reciprocal of twice 

the actuator spacing. LUVOIR will require high 

actuator density, from 64 × 64 to 128 × 128 

actuators, to provide dark-hole diameters 

between 32 and 64 λ/D (440–880 mas 

at a wavelength of 1.0 μm for LUVOIR-A). 

Note that the IWA and OWA both scale 

with wavelength. Coupled with the narrow 

observing bands required for high contrast, 

this means that multiple observations at 

different wavelengths are needed to map out 

the full detection region.

Once a dark hole is achieved, the DMs 

must maintain contrast by correcting slow 

drifts in the rest of the optical system, using 

picometer-level actuator commands. The 

measurements that allow this to happen are 

provided by concurrent wavefront sensing, 

and/or metrology, discussed in Section 

11.3. The CGI testbed has shown that a DM 

feed-forward control can also be useful, to 

provide fast response based on mechanical 

measurements (Shi et al. 2017). The CGI 

testbed has also shown that DM actuation 

error and actuator creep, though small, can 

limit coronagraph performance. Ultimately, 

high spatial resolution and/or repeated 

rounds of the contrast acquisition algorithm 

may be required to counter actuator errors 

not measurable by other means.

State of the Art:

(1) Electrostrictive PMN-based (leadmagnesium-niobate) DMs have been 

used to demonstrate 6 × 10-10 contrast at 

10% bandwidth with unobscured pupils, 

and 1 × 10-8 contrast with the obscured 

WFIRST CGI pupil. These are being 

baselined for the WFIRST CGI instrument 

(TRL 5).

(2) Micro-electro-mechanical systems 

(MEMS) DMs have been used in a 

laboratory setting to achieve 5  ×  10-9

contrast in the visible nulling coronagraph 

(TRL 4). Continuous facesheet MEMS 

DMs are also routinely used on groundbased observatory systems to achieve 

moderate contrasts (10-5–10-6) (TRL 4).

Path to TRL 5: Continue to mature 

DM technology by flight qualifying the 

devices and electronics. Improve actuator 

counts to 64 × 64 or larger. Increasing 

the actuator count of electrostrictive DMs 

is accomplished by mosaicking actuator 

blocks behind a common facesheet. MEMS 

DMs actuator counts can be increased 

with photolithography, but scaling electrical 

interconnects is a challenge. Figure 11.4

shows a wiring diagram for an 8,000-actuator 

device currently under development. 

Other goals include miniaturization of drive 

Figure 11.4. Wiring diagram for a ~100 × 100 

actuator MEMS deformable mirror device. 

The final device would have ~8k actuators in 

the clear aperture. Credit: P. Ryan (Boston 

Micromachines Corporation).
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-11

electronics and demonstrating feed-forward 

control precision and repeatability consistent 

with LUVOIR wavefront sensing and control 

requirements. Environmental tests of MEMS 

DMs will demonstrate robustness to launch 

vibrations.

Path to TRL 6: Use either candidate 

flight-traceable DMs in a coronagraph 10-10 

contrast demonstration with a segmented 

aperture telescope front end and closedloop control.

11.3 Active optics: An architecture 

for ultra-stability

The hardest technical challenge facing 

LUVOIR is the need to preserve 10-10 

contrast during coronagraph observations. 

High contrast is established at the beginning 

of each observation using “speckle nulling” 

controls, such as the electric field conjugation 

algorithm, which drives the coronagraph’s 

two DMs to suppress residual starlight in 

the dark hole, compensating phase and 

amplitude irregularities in the telescope and 

coronagraph optics. The DMs must make 

and hold wavefront adjustments as small 

as a few picometers RMS. Once contrast is 

established it must be stably maintained for 

long periods of time, potentially many days, 

while the coronagraph accumulates photons 

from the planets, often at a rate of a few 

photons a minute.

Analysis shows that the required stability 

is a function of the spatial frequency of the 

disturbances, so that a low spatial frequency 

wavefront error drift can be as large as 100 

pm RMS without excessive contrast degradation, while segment-to-segment phasing 

drift must be held below about 40 pm RMS 

(Nemati et al. 2017). All of this is required 

on a spacecraft with vibration disturbances from spinning attitude control reaction 

wheels, with thermal deformations from an 

environment with slow temperature changes 

and using devices such as DMs and materials such as invar, carbon composites and 

adhesives that, at the picometer level, are 

prone to creep (Sokolowski et al. 1993).

Against this challenge LUVOIR has 

several powerful tools. The first is a design 

that creates a stable thermal environment, 

even as the telescope is repointed from star 

to star. This is accomplished with a large 

flat sunshield, and a gimbal between the 

sunshield and the telescope, as described in 

Chapter 8. Second is the use of thermally 

stable materials, coupled with milli-Kelvinlevel thermal controls, especially for the 

primary mirror segments. These minimize 

thermally driven deformations in LUVOIR 

mirrors and structures. Third is the use of 

passive isolation between the spacecraft 

attitude control system to attenuate highfrequency vibrations before they disturb the 

telescope.

However, these tools alone will not 

achieve ultra-stability. For that, LUVOIR 

will also require active measurement and 

control of the optics and wavefront during 

and between observations, to preserve the 

wavefront established during speckle nulling 

control in the face of thermal and other drift 

effects. To keep vibrations from limiting 

contrast, LUVOIR will also need active 

disturbance isolation between the payload 

and the spacecraft as a whole. These 

capabilities will be provided by active optics 

technologies described in this section:

• wavefront sensing and control running in 

parallel with coronagraph observations; 

• active, high bandwidth metrology of the 

optical configuration; and 

• active, high-attenuation vibration 

isolation at the spacecraft/telescope 

interface.
LUVOIR The Large UV Optical Infrared Surveyor

11-12 The LUVOIR Interim Report

11.3.1 Finding the minimum-cost, 

maximum-performance solution

LUVOIR has many tools that can be 

combined in multiple ways to provide the 

ultra-stability needed for coronagraphy. 

Which combination will provide the needed 

performance at the lowest risk, cost, and 

complexity requires resolving trade-offs that 

will define the architecture for LUVOIR’s 

active optics system. Final resolution of 

these design trades requires advancing the 

TRL of the individual technologies, at the 

component and subsystem levels, ultimately 

to achieve TRL 6 for the combined active 

optics observatory architecture at the 

mission PDR. This will be done by analyzing, 

building and testing components and 

subsystems: first testing components; then 

testing integrated subscale systems; finally 

testing at the engineering or prototype level 

in system-level, subsystem-level assemblies, 

or testbeds. For the LUVOIR-A point design 

study, the design team has made choices 

that emphasize simplicity. More complexity 

will be added only as needed to meet 

performance or reduce cost. 

This testing is done to verify component 

and subsystem performance—and in the 

process, to validate and verify computer 

models of each component and subsystem. 

These models can then be combined, 

intermixing structures, thermal, optics, 

dynamics, and controls discipline modeling 

tools in an integrated computing environment: 

so-called STOP modeling (StructuralThermal-Optical Performance). Ultimately, 

with component models grounded in testing, 

system-level STOP models can accurately 

predict system-level performance, quantifying 

the performance aspects of trades between 

different design options. Even now, at the 

very earliest stages, integrated modeling is 

proving essential in identifying architectural 

approaches, and setting performance 

allocations for the various components 

and subsystems, including contrast, image 

quality, and wavefront error budgets (Redding 

2016).

STOP modeling is not a new technology 

or approach, but LUVOIR will present new 

challenges, especially the need to accurately 

predict performance in the picometer 

wavefront regime, where no material is 

entirely stable. This is a step beyond current 

practice. Sources of creep will include 

DM actuator nonlinearities, desorption of 

moisture from composite materials, natural 

creep of materials such as Invar, and epoxy 

strain relief. These creep effects have the 

potential to cause changes that degrade 

contrast, if not measured and controlled 

at the appropriate time intervals—and the 

LUVOIR STOP models will need to capture 

these effects.

11.3.2 Wavefront sensing and 

control concurrent with coronagraph 

observations: LOWFS, OBWFS, and 

artificial guide stars

One way to stabilize the coronagraph contrast 

is by continuously measuring and correcting 

changes in the optical wavefront, using 

concurrent wavefront sensing and control 

(WFSC). This approach has the advantage 

that it can directly measure all of the changes 

that occur in the beam train, whether in 

the telescope or in the coronagraph. It has 

several challenges, however, such as the long 

integration times needed to make individual 

wavefront measurements when observing 

the dimmest target stars. 

The WFIRST CGI testbed is already 

demonstrating the effectiveness of low order 

wavefront sensing and control (LOWFS) 

in measuring and controlling low-order 

wavefront disturbances (Shi et al. 2016). 

LOWFS uses starlight rejected by the 

coronagraph’s occulting spot. As shown in 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-13

Figure 11.5, the core of the starlight image 

is reflected off the backside of the occulting 

spot of the focal plane mask, which includes 

a small dimple at the center to create a 

spherical reference wavefront. This forms a 

Zernike wavefront sensor (ZWFS) (Wallace et 

al. 2011). The interference fringes that result 

provide a high-precision measure of the 

wavefront errors present in the beam. The 

spatial frequency of the measured wavefront 

is limited, since the occulting spot, which is 

only the width of the image core, functions as 

a low-pass spatial filter. LOWFS is not able 

to resolve higher spatial frequency effects, 

such as segment-to-segment phase errors, 

or deformations within segments.

Results on the WFIRST CGI testbed 

have demonstrated LOWFS line-of-sight 

sensitivity to 0.01 mas, consistent with a 

feedback closed-control bandwidth of ~10 

Hz and feedforward control up to 100 Hz, 

and sensitivity to low order spatial modes 

(focus, astigmatism, coma, trefoil) down to 

~10 pm at 0.05 Hz under the testbed source. 

These results are limited by testbed stability 

and photon noise. Low-light (V=2 mag for 

dark-hole creation and V=5 mag for LOWFS) 

experiments are on-going and the recent 

initial test results are very promising.

Another approach for LUVOIR is to 

sample the full beam from the star, so that 

all of the wavefront spatial frequencies that 

can be controlled are measured. This is done 

using a dichroic beam splitter to split the light, 

sending one band to the coronagraph and 

another to an Out-of-Band Wavefront Sensor 

(OBWFS): UV light for wavefront sensing, 

while observing in the visible, for instance. 

The OBWFS sensor uses the LOWFS 

architecture, including a ZWFS, but without 

the spatial filtering effect of the reflection from 

the occulting spot. It measures low, mid, 

and high spatial frequency wavefront errors, 

though the accuracy and speed with which 

each term can be measured depends on the 

magnitude of the guide star. Figure 11.6

shows the spectral type and visual magnitude 

of the expected population of LUVOIR-A 

exoEarth target stars—note that the average 

is V~6 mag, and all targets are brighter than 

V~11 mag. Monte-Carlo simulation analysis 

shows that OBWFS is potentially capable 

of resolving segment-scale disturbances 

to the 1 pm level in 96 seconds with a V=5 

mag guide star (Table 11.3). This assumes 

LUVOIR’s idealized 15-m aperture with no 

throughput losses.

Figure 11.5. Schematic of LOWFS integrated into coronagraph occulter assembly. LOWFS 

replaces the usual absorbing occulter spot with a reflecting spot (indicated in red) that has a small 

dimple (dark blue) yielding a small phase-only disturbance in the beam reflected to the LOWFS 

detector.
LUVOIR The Large UV Optical Infrared Surveyor

11-14 The LUVOIR Interim Report

The LOWFS approach is attractive 

because it has a minimum of non-common 

path optical error since nearly all of the 

wavefront error (and wavefront error drift) 

measured in the LOWFS sensor is common 

to both the coronagraph and the sensor. 

The target low order wavefront will be near 

zero, consistent with the use of the classical 

linearized ZWFS control algorithms (Wallace 

et al. 2011).

For OBWFS, the target wavefront will 

be larger, in the tens of nanometers. Some 

of this is due to the non-common path, as 

the dichroic and other optics will be unique 

to the sensor, and as the wavefront sensor 

and coronagraph are operating in different 

wavelengths. Plus, the speckle-nulling 

control will use the DMs to generate gratinglike features in the mid and high spatial 

frequency wavefront, features that help reject 

the effects of the shadowed aperture, and 

that compensate residual mid- and highspatial frequency errors of the telescope—

these features must be maintained. OBWFS 

will need to accurately measure picometer 

differential wavefront changes in the 

presence of tens of nanometers of fixed 

control wavefront error. This will tend to 

push operation of the ZWFS out of the linear 

regime, requiring use of higher dynamicrange, higher-performance nonlinear sensing 

algorithms. Nonlinear wavefront sensing 

algorithms that appear to meet OBWFS 

performance requirements are now being 

investigated.

The great advantage of OBWFS and 

LOWFS is that they can continuously 

measure the complete LUVOIR telescope 

and coronagraph wavefront: they directly 

observe all the effects that degrade contrast, 

while the coronagraph is observing. The 

greatest challenge they face is operating fast 

enough to keep up with these disturbances. 

A target star magnitude ≤ 5 allows segment 

control loop closure times of 96 seconds or 

less, and DM control loop closure times of 11 

minutes or less (Table 11.3). For fainter stars, 

sensing times increase, eventually making it 

difficult to keep up with expected thermal 

and material creep effects. Fortunately, as 

Figure 11.6 indicates, 90% of the LUVOIR-A 

stars are brighter than V=10. For dim science 

stars, a combination of LOWFS to measure 

line of sight and low order aberrations 

using rejected science light, and OBWFS 

tracking high order aberrations at a much 

longer exposure length may be more photon 

efficient.

One way to get around the dim guide 

star challenge is to provide an artificial guide 

Figure 11.6. Population of target stars for the LUVOIR-A exoEarth campaign. Most of the stars 

that will be used for low-order and out-of-band wavefront sensing are between V=4 mag and V=6 

mag, and all target stars are brighter than V~11 mag.

Spe

0

A F G K M

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Nstars = 259 | Nobservations = 1539

dmax = 24.6 pc | θmax = 2.4 mas Spectral Type

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# of Stars

0.06 0.11 0.19 0.34 0.60 1.05 1.85 3.24 5.70 10.00

θstar (mas)

0

200

400

600

800

# of Observations

0.06 0.11 0.19 0.34 0.60 1.05 1.85 3.24 

sity

−Distance Plots

K

G

F

A

HZ Completeness

0.00 0.33 0.67 1.00

K

G

F

A

Exposure Time (h)

0 27 54 81

K

G

F

A

Detection Coronagraph

0 1 2 3 4

K

G

F

A

Character

0 1 2 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-15

star. A SmallSat or CubeSat equipped with 

a laser or bright beacon and placed near 

the line of sight between the telescope and 

the target star, would be able to provide a 

bright calibration source. The source could 

be made bright enough for OBWFS to 

measure even the > 64 × 64 actuator DM 

wavefront to picometer sensitivity in under a 

minute. Studies are now underway exploring 

the practical implementation issues of this 

method, including possible source brightness, 

formation flight requirements between the 

telescope and source spacecraft, and how 

best to implement the wavefront sensing 

on this source3. Preliminary indications are 

that a guide star satellite could be extremely 

beneficial, but at the moment it is not included 

in the baseline architecture.

Path to TRL 4: LOWFS has been 

demonstrated by the WFIRST CGI project, 

establishing TRL 4 for LUVOIR. OBWFS 

should be demonstrated using lab testbeds, 

with results correlated to models. 

Path to TRL 5: Testbed demonstration 

using a LUVOIR-traceable aperture, with 

realistic dynamic and thermal disturbances.

Path to TRL 6: Demonstration at a system level, with a subscale segmented telescope and a coronagraph instrument traceable to the LUVOIR ECLIPS design. The 

system level demonstration should incorporate other metrology methods, such as edge 

3 See: https://www.nasa.gov/directorates/spacetech/esi/esi2016/Laser_Guide_Star

sensors and laser truss metrology, to show 

that separate control loops can be incorporated into a single control system.

11.3.3 Metrology of the telescope 

optics: Edge sensors and laser 

trusses

Another powerful tool for LUVOIR ultrastability is high bandwidth metrology of the 

telescope optical alignments, coupled with 

precision rigid body actuation to stabilize 

the position and orientation of each optic or 

optical assembly in 6 degrees-of-freedom. 

Metrology does not require a guide star to 

sense and control the largest sources of 

instability, such as dephasing of the primary 

mirror segments or despace of the secondary 

mirror. As such it is complementary to 

OBWFS and LOWFS, providing much 

higher bandwidth, maintaining alignments 

during maneuvers, and ensuring telescope 

performance during general astrophysical 

observations, when the coronagraph is not 

operating. 

Two metrology methods are especially 

well suited for LUVOIR. The first metrology 

method uses segment edge sensors to make 

high precision measurements of the position 

of each pair of adjacent segment edges (see 

Figure 11.7). Closed-loop control feeds the 

edge sensor measurements back to segment 

rigid body actuators to keep the segments 

aligned to each other. Further development 

of current edge sensor technologies is 

Table 11.3. OBWFS performance summary showing the estimated minimum integration times 

required to achieve target accuracy for particular spatial resolution levels for typical and near 

worst-case stellar magnitudes. Per Figure 11.6, most of LUVOIR’s target stars will be between 

V=4 mag and V=6 mag.

Mode Spatial Resolution Minimum Integration Time 

for V=5 mag

Minimum Integration Time 

for V=10 mag

For 10 pm 

RMS error

For 1 pm 

RMS error

For 10 pm 

RMS error

For 1 pm 

RMS error

OBWFS Segment Piston, Tip, and Tilt 6 s 96 s 96 s 158 min

OBWFS DM actuator modes 10 s 11 min 48 min 80 hrs
LUVOIR The Large UV Optical Infrared Surveyor

11-16 The LUVOIR Interim Report

needed to improve performance to the 

picometer-level sensitivity needed for space 

coronagraph applications.

While segment edge sensors are part 

of the LUVOIR-A baseline, the specific 

implementation of the edge sensor is still 

under study. Capacitive edge sensors have 

long been used on ground-based segmented 

telescopes, notably the Keck Telescopes on 

Mauna Kea, where they provide 1 nm noise 

and 3.2 nm/week drift in relative segment 

edge piston measurements. The Keck edge 

sensors use interleaved sensing elements 

to ensure high sensitivity and stability (Minor 

et al. 1990). Edge sensors planned for the 

upcoming Thirty Meter Telescope use face-on 

sensing plates, plus gap measurements—a 

non-interleaved approach better adapted for 

deployed-aperture space telescopes (Shelton 

et al. 2008). Capacitive gap measurement 

devices have demonstrated 15 pm precision 

for applications on the Laser Interferometer 

Space Antenna (LISA) mission, although 

for much smaller gaps than are planned 

between LUVOIR segments.

Challenges for capacitive edge sensors 

include the need for close and stable gaps 

between the segments, including the possible 

need to interleave sensor structures, and 

high voltage for high sensitivity. Other edge 

sensor designs, using inductive (Wasmeier 

et al. 2014) or optical measurements (Burt 

et al. 2012), are also reasonable candidates.

The second metrology method is laser 

truss metrology, using a network of laser 

distance gauges to measure the rigid body 

state of the segments relative to the secondary 

mirror, and measuring the secondary mirror 

Figure 11.7. Ground-based segmented telescopes commonly use non-contacting capacitive 

edge sensors, such as the Thirty Meter Telescope devices shown here. Configured with two 

sensors per adjacent segment edge, they are capable of measuring primary mirror segment-tosegment alignments. The hybrid edge sensor/laser truss metrology architecture sketched on the 

right uses laser metrology on the 12 segments shown tilted, and segment edge sensors on all 

segments. It offers better performance than either system alone, provided both have picometer 

precision in the most sensitive axes.

Figure 11.8. Laser metrology truss. With 6 laser 

distance gauges between each segment and 

corner cubes attached to the secondary mirror, 

each segment can be independently aligned 

to the secondary. Another 6 gauges from the 

backend instrument bench to the secondary 

mirror ties the primary and secondary mirror 

to the instrument optics. These measurements 

are fed back to the segment and secondary 

mirror rigid body position actuators to keep the 

entire observatory in close alignment.

Figure 8: Laser metrology truss. With 6 laser distance gauges between each segment and corner cubes 

attached to the secondary mirror, each segment can be independently aligned to the secondary. Another 6 

gauges from the backend instrument bench to the secondary mirror ties the primary and secondary mirror 

to the instrument optics. These measurements are fed back to the segment and secondary mirror rigid 

body position actuators to keep the entire observatory in close alignment.

Figure 9: Two examples of laser metrology beam launchers. Left: a beam launcher using integrated 

photonic devices for mixing outgoing and incoming beams. Credit: JPL. Right: a compact version of the 

SIM beam launcher devicesCredit: Lockheed Martin

Sensor - Drive

Sensor - Sense
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-17

relative to the instrument optics behind the 

primary mirror (see Figure 11.8). Laser 

metrology can measure alignment of all 

of the major optical elements relative to a 

common reference, such as a master optical 

bench, or reference point on the backplane 

support frame. Then the rigid body actuators 

on the segments and secondary mirror can 

effectively “rigidize” the entire telescope in 

inertial space.

Laser metrology draws its heritage 

ultimately from the Space Interferometry 

Mission (SIM) project, which demonstrated 

picometer precision on large ground-based 

testbeds, with large and heavy beam 

launchers and corner cubes. Subsequent 

development achieved much smaller and 

more compact laser metrology devices for 

attachment to lightweight mirror segments, 

targeting performance at the sub-nanometer 

level. The LISA Pathfinder mission has 

further refined laser metrology technology, 

flying laser gauges and electronics that 

achieve picometer accuracy. Combining 

elements of the SIM and LISA approaches 

could provide the picometer-precision 

alignment measurements that LUVOIR will 

need to preserve coronagraph high contrast 

performance during extended observations, 

and do it in a compact, unobtrusive package 

that will not interfere with image quality. 

Figure 11.9 shows two example beam 

launcher devices from laser metrology 

systems.

Analysis shows that a hybrid metrology 

approach, illustrated in Figure 11.7, using 

segment edge sensors on every segment, 

and using a laser metrology truss on a few 

selected segments, can outperform either 

system on its own, provided that both 

systems have picometer-level sensitivity. 

This approach provides the end-to-end 

system measurement capabilities of a laser 

metrology truss, while the less complex 

segment edge sensor devices may be better 

positioned to measure lateral movements 

of the segments relative to each other. 

The optimal blend of measurements will 

depend on the ultimate precision of practical 

hardware for both systems and is a focus for 

upcoming technology studies.

Path to TRL 4: For edge sensors, 

develop one or more approaches (capacitive, 

inductive, optical) capable of picometer 

sensitivity, and demonstrate sensitivity in the 

lab. For laser metrology, develop improved 

beam launcher systems to reduce thermallyFigure 11.9. Two examples of laser metrology beam launchers. Left: a beam launcher using 

integrated photonic devices for mixing outgoing and incoming beams. Credit: JPL. Right: a 

compact version of the SIM beam launcher devices. Credit: Lockheed Martin.
LUVOIR The Large UV Optical Infrared Surveyor

11-18 The LUVOIR Interim Report

driven errors, and improved phasemeter 

electronics.

Path to TRL 5: Demonstration of 

picometer sensitivity on a lab-scale testbed 

combining metrology and direct optical 

measurements. Depending on method, 

vacuum environment may be relevant.

Path to TRL 6: Demonstration at a system 

level, with a subscale segmented telescope 

and a coronagraph instrument traceable to 

the LUVOIR ECLIPS design. The system 

level demonstration should incorporate 

other wavefront sensing methods, such as 

LOWFS and OBWFS, to show that separate 

control loops can be incorporated into a 

single control system.

11.3.4 Non-contacting disturbance 

isolation

Even with the very best optics and optical 

controls, LUVOIR coronagraph performance 

could be limited by image and wavefront 

jitter due to spacecraft on-board vibrations. 

These come primarily from attitude control 

hardware. Reaction wheel assemblies, 

control moment gyros, and thrusters create 

vibrations that jitter the telescope optics 

and reduce image quality and contrast. The 

LUVOIR baseline design uses a non-contact 

Figure 11.10. Non-contacting vibration isolation “floats” the telescope across a narrow gap from 

the spacecraft. The payload controls the telescope line-of-sight by pushing against the spacecraft 

inertia using a set of six non-contact voicecoil actuators, while the spacecraft controls its inertial 

attitude such that interface stroke and gap are maintained. Requirements for spacecraft attitude 

control are no more stringent than those for conventional spacecraft, and do not derive from 

payload pointing requirements. Payload line-of-sight isolation from spacecraft disturbances is 

broadband, even down to low frequency, and is not affected by interface measurement noise. 

Power and signal are carried across the gap using very soft connections and may include the 

possibility of wireless transmission. Credit: L. Dewell (Lockheed Martin).
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-19

Table 11.4. LUVOIR ultra-stable system technology status and path ahead. Green shaded options 

are in the LUVOIR-A baseline. Yellow options may provide alternative or potentially enhancing 

solutions given enough development.

Current 

TRL

Path to 

TRL 4

Path to 

TRL 5

Path to 

TRL 6

Ultra-stable optomechanical systems 

architecture

Contrast Stability:

< 1e-11 during 

coronagraph 

observations

Minimize wavefront drift 

during maneuvers

Preserve optical quality 

during general 

astrophysics 

observations

Changing thermal 

environment

Vibration environment

Sensor accuracy

Actuator stability

Material creep

Risk, Cost, Complexity

Stable structures 

and materials

Active thermal 

control

Mix of wavefront 

sensing, 

metrology, and 

control

2

System 

modeling 

and 

simulation 

to TRL 3; 

Hardware 

subsystem 

demos to 

TRL 4

Subsystem 

prototype 

demos with 

relevant 

vibration 

and 

thermal 

drifts. 

Validation 

by 

correlated 

models.

Architecture will use wavefront sensing 

and control, metrology, active thermal 

control, and stable material technologies 

to find lowest risk and lowest cost 

solution that meets performance 

requirements.

LOWFS 4 ü

Limited in spatial resolution, but high 

bandwidth.

WFIRST will demonstrate concept at 

TRL 9, though additional development 

for LUVOIR likely needed.

OBWFS 3

Lab test 

with coronagraph 

instrument

Slow, but high spatial resolution 

observes control actions through entire 

system.

SmallSat/CubeSat flight 

technologies

Precision stationkeeping

Precision laser pointing

Laser guide star 

+ OBWFS 2 Smallsat demo

Requires separate SmallSats or 

CubeSats to provide bright guide star 

within OBWFS field of view

Segment edge 

sensors 3

Demo 

measurement of 

relevant 

degrees of 

freedom

Need to develop sensor-head geometry 

capable of measuring desired degrees 

of freedom to picometer levels.

Laser truss 

metrology 3

Improve 

beamlauncher 

size / 

stability, 

phasemeter 

electronics

Combine proven SIM and LISA-derived 

tech for compact, picometer-precision 

gauges

Picometer resolution 

rigid body actuators

Stroke: 

> 10 mm

Resolution: 

< 10 pm

Creep: 

< 1 pm / 10 min

Long performance lifetime

Low size/weight/power

Launch loads

Mechanical 

actuator for large 

stroke + piezo tip 

actuator for 

precision

3

Prototype 

design 

demo of 

stroke and 

resolution

Environmental 

testing; 

Lifetime 

and load 

tests

Mech actuator heritage from JWST 

combined with commercial piezo 

actuators (e.g. Mad City Labs, Physik 

Instrumente)

Dynamic isolation 

systems

Isolation:

40 dB/decade

Slew Rate:

3 ° / min

Power and signal cables 

bridging interface

Large moments of inertia

Actively 

controlled noncontact interfaces4 ü

Demo 

isolation 

with 

realistic 

disturbance 

inputs and 

with power 

and cable 

transmitted 

across the 

interface

Build on demonstrated Disturbance Free 

Payload technology from Lockheed 

Martin, for example.

Microthruster attitude 

control to eliminate 

reaction wheel 

disturbances

Thrust:

1 N (coarse mode)

100 uN (fine mode)

Resolution:

0.01 uN 

Lifetime

Redundancy

Contamination

Electrospray 

thrusters for fine 

mode, electric 

propulsino for 

coarse mode

5 ü ü

Demo 

thrusters 

with 

sufficient 

force and 

resolution 

for LUVOIR

Heritage from LISA Pathfinder

System studies needed to design for 

LUVOIR

Technology Driving Requirements Technical Challenges Solution Paths

Development status and path ahead

Note

Dim guide stars

Non-common path wavefront 

errors

High dynamic range

On-board implementation

Wavefront sensing 

and control concurrent 

with coronagraph 

observations

Contrast Stability: 

< 1e-11 during 

coronagraph 

observations

Bandwidth: 

> 1 Hz (LOWFS)

Spatial Resolution:

> 12 cyc/ap (OBWFS) 

Demo with 

coronagraph 

instrument 

and 

relevant 

vibration 

and 

thermal 

drifts; 

realistic 

source 

brightness.

Systemlevel test 

with 

subscale 

segmented 

telescope 

and coronagraph 

instrument; 

Include 

relevant 

dynamic 

and 

thermal 

inputs with 

closed-loop 

control of 

system. 

Will require 

validated 

modes to 

fully verify 

performance onorbit.

Metrology systems

Wavefront error 

stability:

< 10 pm RMS

(within control spatial 

and temporal bands)

Demo 

metrology 

on 

subscale 

segments 

in presence 

of relevant 

vibration 

and 

thermal 

drifts.

Picometer resolution

Picometer stability

Thermal sensitivity

Size/Weight/Power of sensor 

elements

High voltage (edge sensors)

Cyclic error (laser truss)
LUVOIR The Large UV Optical Infrared Surveyor

11-20 The LUVOIR Interim Report

isolation technology, illustrated in Figure 

11.10, to prevent these vibrations from 

reaching the telescope (Dewell et al. 2017).

Path to TRL 4: The non-contact 

isolation technology is at TRL 4, based on 

Lockheed Martin’s Disturbance Free Payload 

concept and lab demonstrations, but system 

studies are required to optimize a LUVOIR 

implementation. Other approaches could be 

considered, and the system studies should 

include assessment of alternate methods.

Path to TRL 5: Demonstration of LUVOIRtraceable performance in a representative 

environment, including gravity offload and 

perhaps vacuum. Especially important will 

be demonstrating that cables providing 

power and signal can cross the gap without 

degrading the isolator performance.

Path to TRL 6: Demonstration at a 

system level, with a subscale segmented 

telescope. The system level demonstration 

should incorporate line-of-sight pointing 

measurements generated by a prototype fine 

guidance sensor traceable to HDI that are 

used to command a fast-steering mirror in 

addition to the VIPPS to show an integrated 

pointing control system.

11.3.5 Disturbance avoidance: 

Micro-thrusters for attitude control

Another method for eliminating vibrations 

from spinning-mass attitude control hardware 

is to eliminate such devices entirely, replacing 

them with extremely precise, micro-Newton 

to Newton level reaction control thrusters. 

This method has been flight proven by LISA 

Pathfinder, which used colloidal thrusters to 

achieve thrust of 30 micro-Newtons or less, 

with resolution down to 0.1 micro-Newtons. 

Other mature thruster technologies are 

available for pointing control when higher 

thrust levels are required, so that a LUVOIR 

sized telescope could be both rapidly 

repointed and precisely controlled without 

the vibrations inherent to reaction wheels or 

control moment gyros. 

Micro-thrusters are a potential breakthrough alternative for attitude control, one 

that could reduce or eliminate the largest sources of dynamic disturbances for 

LUVOIR. Adoption of this approach needs 

to be considered at the system level to fully 

assess its advantages. These could include: 

use of lower stiffness structures and mirrors, 

for mass savings; precision pointing without 

the need for a fine steering mirror; and possibly improved image quality in all observing 

modes. At the same time, new contamination issues may arise; and the basic LUVOIR 

structural layout will need to be reconsidered. 

11.3.6 Active control of the telescope 

wavefront 

The telescope wavefront control actuation has 

a large trade space. Primary mirror segment 

and secondary mirror rigid body control is 

required to phase the segments and align 

the telescope following launch and maintain 

alignment during the life of the mission. Loworder segment figure actuation may be used 

to compensate for mirror fabrication errors, 

such as radius-of-curvature mismatch 

between segments, or residual gravity 

release effects. Higher-order segment figure 

actuation offers resilience to other possible 

wavefront errors, such as segment edge 

defects, and could even be used as part 

of the coronagraph speckle-nulling control. 

Wavefront control may also be applied at 

the telescope exit pupil using a deformable 

mirror external to the coronagraph. An active 

secondary mirror with high-order figure 

actuation, could also be used, though such 

corrections may not be effective over the 

entire telescope field of view due to pupil 

registration, distortion, and magnification 

limitations (McComas & Friedman 2002). 

Local segment control (i.e., sensing of the 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-21

primary mirror segment rigid body motion 

and control of the segment motion) requires 

precision rigid body actuators, perhaps like 

those used on JWST, but with an additional 

ultra-fine actuation stage.

Control at the telescope is expected to 

be bandlimited in the spatial and temporal 

domains. In the spatial domain, the band is 

limited by the actuation spatial resolution, 

set by the segment geometry for rigid body 

control, and by actuator density if segment 

figure actuators or a deformable mirror is 

used. In the temporal domain the bands 

are constrained by the integrating action 

of observations and the chopping action 

of angular- or field-difference observing 

strategies. The exact temporal bandwidth 

is dependent on the science observation 

and architecture, but the control bandwidth 

is likely below 1 Hz, with passive stability 

needed at frequencies above the control 

bandwidth. The most stringent control 

capability within the 1 Hz bandwidth is 

expected to be segment piston at ~10 pm 

RMS of wavefront, followed by segment tilt at 

25 pm RMS wavefront (Nemati et al. 2017).

This level of actuation for LUVOIR is 

TRL 3. An example is Mad City Labs, Inc.4

closed-loop piezo actuators, which show 

5 picometer step resolution and have been 

independently demonstrated to achieve 

<10 pm RMS on a 0.5 Hz sinusoid—good 

enough for LUVOIR. This actuator design 

was developed for a past flight application, 

but more development is needed for LUVOIR, 

including lightweighting and integration with a 

large-stroke actuator (such as the ones flown 

on JWST). The most important aspect of the 

Mad City Labs actuator is the demonstration 

of the small step resolution which provides 

confidence the LUVOIR requirements can 

be met. Figure 11.11 shows an actuator 

concept that uses PZT devices in concert 

with a flexure design to achieve picometerlevel step sizes while also isolating the PZT 

devices from launch loads.

Path to TRL 4: Develop flight prototype 

rigid body actuators, using one or more 

methods (PZT, mechanical, or a hybrid), and 

test in the lab. 

Path to TRL 5: Demonstration of 

picometer-level actuation of a subscale 

segment with a prototype rigid body actuator 

in six-degrees of freedom. Metrology and 

direct optical measurements of segment 

displacements will verify performance. 

Vacuum testing of the actuator may be 

necessary.

Path to TRL 6: Demonstration at a 

system level, with a subscale segmented 

telescope. The system level demonstration 

should incorporate metrology methods, 

such as edge sensors and laser truss 

metrology, to show closed-loop control of 

4 See http://www.madcitylabs.com/nanometz.

html

Figure 11.11. Conceptual picometer actuator 

using PZT devices. The gray inserts on either 

end are the PZT devices. The gold structure 

is a flexure design that provides a mechanical 

reduction in step size between PZT actuation 

and physical motion at the actuator interfaces. 

The flexure also removes the PZT device 

from the load path, reducing forces on the 

device during launch. Credit: Ball Aerospace 

Technology Center.
LUVOIR The Large UV Optical Infrared Surveyor

11-22 The LUVOIR Interim Report

the segment position(s). Vacuum testing may 

be necessary. 

11.3.7 Primary mirror segments

LUVOIR primary mirror segments and 

their supporting subsystems can draw on 

heritage from multiple sources, with some 

key options that will need to be traded to 

assure best performance and affordable 

cost, complexity, and risk. Here we note 

that the JWST mirror segments are made 

from beryllium, a material chosen for its high 

stability at cryogenic temperatures. Beryllium 

is not the best candidate for LUVOIR 

because at the operational temperature 

of 270 K, beryllium has a very high CTE. 

Beryllium mirrors are also time consuming 

and expensive to fabricate; it took about 8 

years to complete a fully integrated JWST 

segment assembly.

Many space telescopes have used 

mirrors made of Ultra Low Expansion (ULE) 

glass, a material that is thermally very 

stable, and that can be figured and polished 

to UV requirements. ULE is commonly 

“lightweighted,” fabricated with a honeycomb 

core between two thin facesheets, to provide 

needed high stiffness at a low mass. One 

facesheet is polished and coated, to provide 

the optically reflective surface (see Figure 

11.12).

The LUVOIR-A design assumes the use of 

lightweighted ULE for each of its 120 primary 

mirror segments, building on technology 

first demonstrated by the Advanced Mirror 

Segment Demonstrator project, which built 

and tested prototype JWST mirror segments 

(Matthews et al. 2003). Each segment will 

be mounted via bonded invar pads that 

are then mounted to support flexures. The 

struts connect the mirror to six rigid body 

actuators, arranged as three separate 

bipods. All segments are then connected 

to a common backplane structure, a highly 

stable, stiff, composite structure that forms 

the backbone of the primary mirror assembly. 

This approach allows each segment to be 

independently positioned in six degreesof-freedom, so they can be aligned and 

phased even if the backplane structure 

deforms. Another important part of each 

mirror segment will be an active thermal 

control system, with one or more heaters 

and temperature sensors acting to maintain 

the temperature of each segment at 270 K. 

Analysis indicates that thermal stability of 1 

mK will provide ~1 pm mirror figure stability 

during operations (Eisenhower et al. 2015).

One consideration for LUVOIR is whether 

or not its segments should be equipped 

with surface figure actuators. To achieve 

diffraction-limited performance at 500 nm 

requires a surface figure error of ~10 nm 

RMS over the fully-phased 120-segment 

primary mirror surface. Mirrors are limited in 

Figure 11.12. A ULE mirror segment 

undergoing ion figuring for surface figure and 

radius of curvature correction. Capture Range 

Replication technology enables replication of 

mirrors to within capture range of final finishing 

processes for figure and surface finish, 

eliminating most of the grinding and polishing. 

This technology is especially useful when 

multiple mirrors with the same prescription are 

needed, as is the case with segmented optical 

systems. Credit: Harris Corporation.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-23

their figure quality by a number of effects, 

including:

• Unpredicted/uncorrected gravity sag

• Mismatch of the radius of curvature 

between segments

• Print-through of mount points caused 

by thermal effects or adhesive and Invar 

creep.

Surface figure actuators enable onorbit correction of these errors, insuring 

against contingencies, and relaxing difficult 

fabrication requirements. Surface figure 

actuators introduce weight and complexity, 

however. Lightweighted, surface-figure 

actuated ULE mirror segment systems have 

already been built and tested.

Another, highly actuated approach 

to primary mirror segments has been 

developed, using a Silicon Carbide (SiC) 

substrate technology equipped with many 

surface figure actuators. These “Actuated 

Hybrid Mirrors,” shown in Figure 11.13

repeatedly demonstrated 15 nm surface 

figure, after correction of fabrication and 

gravity sag errors of 1800 nm—a correction 

factor of >100×. Actuated Hybrid Mirrors 

use a replication-based fabrication process, 

with a nanolaminate foil face sheet bonded 

to the SiC substrate (Hickey et al. 2010). 

Better performance is expected if the SiC 

substrate is clad with pure silicon instead of 

the nanolaminate, and then superpolished 

to meet as-built figure of <100 nm RMS 

(Wellman et al. 2012). Assuming the same 

correction factor as the nanolaminate 

version, operational surface figures of <10 

nm would be achievable.

The advantage of highly actuated mirrors 

is the high level of figure control, enough to 

meet the required surface figure error performance at a wide range of temperatures, in 

1-g as well as 0-g, even with relaxed polishing requirements. The high actuator density 

would even allow the use of the primary mirror as a third DM for shaping coronagraph 

wavefronts. A disadvantage is the relatively 

high coefficient of thermal expansion of SiC 

at LUVOIR’s 270 K operating temperature. 

Analysis suggests that the high thermal controllability of SiC can be thermally stable at 

the needed level, though with 10× tighter 

temperature controls (~100 μK) than for the 

ULE. The passive stability of SiC improves as 

the operating temperature drops, approaching a coefficient of thermal expansion of 0 

at around 100 K. Thus, SiC mirrors are well 

suited for cold or cryogenic applications. A 

final consideration is the simple fact that active SiC mirrors do not have the long heriFigure 11.13. SiC mirror segment technology. (Left) An AHM mirror being set up for test. (Right) 

A SiC mirror substrate showing its open back rib structure. Credit: JPL.
LUVOIR The Large UV Optical Infrared Surveyor

11-24 The LUVOIR Interim Report

tage of ULE. Nonetheless, they offer important capabilities and provide an alternative for 

LUVOIR.

Path to TRL 6: The path ahead for mirror 

segments will ultimately require building and 

testing full-scale segments at the subsystem 

level, including integrated thermal controls, 

rigid body actuators, and figure actuators 

if needed. Testing multiple segments 

together will be necessary to establish TRL 

6, to verify radius of curvature matching 

and manufacturing repeatability, especially if 

figure actuation is not provided.

11.3.8 UV reflective coatings

General astrophysics and exoplanet science require high-throughput observations 

between 100 nm and 2.5 μm. The coating 

should achieve >50% reflectivity at 105 nm 

while not compromising performance at 

wavelengths > 200 nm compared to existing 

state-of-the-art. The coating process must 

be scalable to meter-class segments and 

repeatable to ensure uniform performance 

across an aperture comprised of >100 segments.

State-of-the-art enhanced Al+LiF coatings exhibit the desired performance with 

>85% reflectivity between 103 and 130 nm 

(Quijada et al. 2014). However, LiF is hygroscopic and will suffer from environmental 

degradation, impacting reflectivity. Atomic 

layer deposition of thin MgF2 or AlF3 overcoats will provide environmental stability 

while not affecting performance (Moore et al. 

2014). Coating characterization is needed to 

perform polarization performance modeling 

and its impact on high-contrast imaging.

As to mirror cleaning, existing approaches 

(e.g., CO2 snow, or electrostatic wands 

with AC excitation), work well for removing 

dust or water, but do not remove molecular 

contamination. Promising methods (e.g., 

electron or ion beams) could clean molecular 

layers as well as dust on substrates.

Table 11.5. LUVOIR mirror technology status and path ahead. Green shaded options are in the 

LUVOIR-A baseline. Yellow options may provide alternative or potentially enhancing solutions 

given enough development. Red options would only be considered with substantial development 

and gains in system performance.

Current 

TRL

Path to 

TRL 4

Path to 

TRL 5

Path to 

TRL6

No surface figure 

actuation 5 ü ü

Radius-ofcurvature only 

actuation

4 ü

Low-order 

surface figure 

actuators

4 ü

Actuated Silicon 

Carbide mirror 

segments

Silicon cladding

Epoxy creep

Actuator stability

Electronic harnessing

Mass

Polished, Si-clad 

SiC mirrors 3 Si cladding 

and polish

Heritage from Actuated Hybrid Mirrors 

with nanolaminate facesheets

3.5-m Herschel SiC mirror demonstrates 

flight heritage of base material

Zerodur glass-ceramic 

mirror segments

Radius of curvature matching

Gravity sag prediction 

Print-through figure errors

Epoxy and invar creep

No surface figure 

actuation 4 ü

Open-back construction requires 

heavier, thicker mirrors for equal 

stiffness.

Good thermal stability.

UV reflective coatings

Average Reflectivity:

> 50% (103-115 nm)

> 80% (115-200 nm)

> 88% (200-850 nm)

> 96% (850-2500 nm)

Protective coating

Cleaning

Aluminum 

reflective coating 

with protective 

layers such as 

LiF, MgF, AlF3

3

Demo 

coating on 

multiple 

runs to 

show 

repeatability

Life test in 

relevant 

radiation 

and I&T 

environments

Coating on 

multiple fullscale 

segments

Enhanced Al+LiF coatings demonstrate 

reflectivity performance

Need to demonstrate thin capping 

layers of MgF2 or AlF3.

Technology Driving Requirements Technical Challenges Solution Paths Note

Development status and path ahead

Segmented 

mirror 

system test 

with 2 or 

more 

segments 

and all 

baselined 

hardware 

(actuators, 

thermal 

control, 

edge 

sensors, 

laser 

metrology, 

etc.)

Figure Error:

< 10 nm RMS

Figure Drift:

< 40 pm / 10 min

Stiffness:

> 200 Hz first free mode

Substrate Areal 

Density:

< 12 kg/m2

Heritage from Advanced Mirror System 

Demonstrator and Multiple Mirror 

System Demonstrator programs.

Environmental and 

optical 

performance 

testing of 

fullyintegrated 

mirror 

segment

Ultra-Low Expansion 

(ULE) glass mirror 

segments

Radius of curvature matching

Gravity sag prediction 

Print-through figure errors

Epoxy and invar creep
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-25

Path to TRL 4: Demonstrate the 

deposition of a protected Al+LiF+MgF2 or 

Al+LiF+AlF3 far-UV-optimized coating in 

separate coating runs to verify repeatability of 

the process. Characterize coating reflectivity, 

uniformity, and polarization performance 

to verify the coatings achieve LUVOIR 

requirements.

Path to TRL 5: Verify coating stability 

over time in space-simulated vacuum and 

ambient integration and test environments, 

as well as in a relevant radiation environment. 

Path to TRL 6: Demonstrate coating 

process on multiple meter-class prototype 

segment assemblies.

11.4 Instrument technologies

The LUVOIR instrumentation has broad general astronomy and astrophysics capabilities 

as well as specialized coronagraph modes, 

as outlined in Chapter 9. These capabilities 

can be provided within current technologies 

for instrument elements such as mirrors, lenses, diffraction gratings, grisms, mechanisms, 

and structures. Except for the coronagraph 

elements described earlier, new instrument 

technology development is required only for 

detectors and for the LUMOS microshutter 

array, which is used for multi-object spectroscopy.

11.4.1 Detectors for LUVOIR 

instruments

The LUVOIR Study Team has baselined 

detector technologies with established 

heritage: CCDs, CMOS arrays, HgCdTe 

photodiode arrays, and microchannel plates. 

But as with any generational advance in 

instrumentation, the goal is to improve these 

technologies in areas of array size, sensitivity, 

noise performance, and radiation tolerance. 

These are the main areas of focus for 

LUVOIR detector technology development. 

At the same time, we note certain exciting, 

lower-TRL technologies that may provide 

high performance payoff in the future 

with adequate development; these will be 

monitored going forward.

11.4.1.1 Microchannel plates

In the far-UV, large-format, high dynamic 

range detectors are needed for multi-object spectroscopic observations at wavelengths as short as 100 nm. Microchannel 

plate technologies have been demonstrated in sub-orbital sounding rocket missions 

SISTINE, FORTIS and CHESS (Fleming et al. 

2011; Hoadley et al. 2016). These suborbital 

efforts have matured independent aspects 

of microchannel plate detector technologies, 

including array size, photocathode sensitivity, and read-out electronics count rates, 

and establish TRL 4 for far-UV LUVOIR applications. Incorporating all aspects of recent demonstrations into a single, large 

format, high-count rate, high quantum efficiency detector array for a sounding rocket 

demonstration could raise the TRL to 6 for 

LUVOIR’s applications. Micro-channel plates 

are baselined for the LUVOIR LUMOS instrument far-UV multi-object spectroscopy and 

imaging channels.

11.4.1.2 CCDs, electron multiplying 

CCDs, and hole multiplying CCDs

While CCD devices have a rich spaceflight 

heritage, many of LUVOIR’s applications 

require key features not available in traditional 

devices. Single photon counting, large 

format detectors will enable key exoplanet 

observations, including multiplexed spectral 

observations of exoplanet systems. The 

electron-multiplying CCD (EMCCD) detectors 

baselined for the WFIRST CGI (Nemati 2014) 

provide the baseline solution for the LUVOIR 

ECLIPS visible channel, but their susceptibility 

to radiation damage might limit their useful 

life on orbit, at least for the most sensitive 

exoplanet observations (Nemati et al. 2016). 

Additional radiation testing and design work 
LUVOIR The Large UV Optical Infrared Surveyor

11-26 The LUVOIR Interim Report

to further improve radiation tolerance is 

necessary to achieve long-life performance 

and will significantly enhance the exoplanet 

science yield potential for LUVOIR. Larger 

format arrays would also enhance the 

efficiency of high-resolution integral field 

spectrograph instrument designs. Holemultiplying CCDs are promising, in that they 

have inherently higher radiation tolerance, 

but they will require further development 

to demonstrate similar photon counting 

capabilities and array sizes as EMCCDs.

EMCCDs and HMCCDs can also be 

d-doped for enhanced sensitivity in the nearUV (200–400 nm). The d-doping process is 

well demonstrated on CCD devices and has 

flight heritage. A d-doped EMCCD device, 

shown in Figure 11.14, will fly on the next 

FIREBALL sounding rocket experiment 

(Nikzad et al. 2017).

11.4.1.3 CMOS and sCMOS 

detectors

CMOS devices are experiencing rapid development, driven by commercial applications. They generally offer larger formats and 

smaller pixels than CCD devices, but at the 

penalty of higher noise. Some CMOS devices, however, have demonstrated sub-electron read noise in some fraction of their pixels—so-called scientific or sCMOS devices. 

Improvement in this area could make them a 

possibly more robust alternative for LUVOIR 

exoplanet science. CMOS and sCMOS devices can also be d-doped for enhanced UV 

performance. Figure 11.15 shows a CMOS 

device prior to being d-doped.

11.4.1.4 HgCdTe photodiode and 

avalanche photodiode arrays

In the NIR, Teledyne HgCdTe HAWAII 4RG 

(“H4RG”) detectors developed for WFIRST 

already exhibit exceptionally good noise 

performance (single-digit read noise, 10-3 

dark current). Further characterization of 

these large-format tile-able arrays should be 

pursued to understand their viability for NIR 

exoplanet science. Efforts to minimize read 

noise in the readout electronics would further 

improve science yield.

HgCdTe Avalanche Photodiode arrays 

offer a photon counting option in the NIR, 

similar to EMCCDs in the visible. Current 

devices, however, exhibit dark currents that 

Figure 11.14. A d-doped, 2 Megapixel EMCCD 

device with visible-rejection filters to optimize 

for high in-band (120–150 nm) quantum 

efficiency, and high out-of-band rejection. 

Credit: S. Nikzad (JPL).

Figure 11.15. An 8-inch wafer containing 

a monolithic CMOS sensor, prior to being 

d-doped using molecular beam epitaxy. Credit: 

S. Nikzad (JPL).
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-27

Table 11.6. LUVOIR detector and microshutter technology status and path ahead. Green shaded 

options are in the LUVOIR-A baseline. Yellow options may provide alternative or potentially 

enhancing solutions given enough development. Red options would only be considered with 

substantial development and gains in system performance.

Current 

TRL

Path to 

TRL 4

Path to 

TRL 5

Path to 

TRL 6

LUMOS Far-UV Multiobject Spectrograph

and

LUMOS Far-UV 

Imager

Wavelength Range:

100 - 200 nm

Array Size:

200 mm × 200 mm,

Tileable to larger arrays

Resol Size:

£ 20 μm

Read Noise:

0 eDark Current:

£ 0.1 counts/cm2

/s

Sensitivity

Array sizes

Dynamic range / bright object 

sensitivity

Micro-channel 

plates 4 ü

Sounding 

rocket test 

of 200 mm 

sized 

arrays; 

Demo of 

tiling 

capability

Full qualification of 

flightpackaged 

devices 

Long development heritage through 

sounding rocket program, HST, etc.

delta-doped CCD 5 ü ü

Full qualification of 

flight 

prototype 

devices; 

Demo tiling 

of arrays 

Sounding rocket heritage devices. 

Would need to show ability to tile 

multiple devices.

Smaller pixels and better radiation 

tolerance would be desireable.

delta-doped 

CMOS 4 ü

Sounding 

rocket 

demo; 

radiation 

testing

Full qualification of 

flight 

prototype 

devices; 

Demo tiling 

of arrays 

ECLIPS UV Channel

Wavelength Range:

200 - 525 nm

Array Size:

3 1k × 1k

Quantum Efficiency:

3 50%

Read Noise:

< 1 eDark Current:

£ 1e-4 e-/pix/s

Radiation hardness

High sensitivity in near-UV 

while maintaining 

performance through 500 nm

delta-doped 

EMCCD 5 ü ü

Full qualification of 

flight 

prototype 

devices; 

Demo tiling 

of arrays 

Sounding rocket heritage devices. 

Better radiation tolerance is desireable.

EMCCD 5 ü ü

Demo 

improved 

radiation 

hardness 

and larger 

array size

Under development by WFIRST in 1k x 

1k formats

Radiation tolerance is low; lifetime of 

current devices is limited < 5 years

Path to 4k x 4k devices is established

HMCCD 3

Lab demo 

of device 

with similar 

capabilities 

to existing 

EMCCD 

devices

Flight 

package 

device with; 

Radiation 

testing

Full qualification of 

flight 

prototype 

device 

Potential to deliver same performance 

as EMCDD, but with inherent radiation 

hardness.

HgCdTe 

Photodiode Array 6 ü ü ü

Devices developed for WFIRST widefield instrument

Lower read noise and dark current 

desireable; potentially achievable 

through readout electronics optimization

HgCdTe 

Avalanche 

Photodiode Array

4 ü

Reduce 

dark 

current to 

acceptable 

levels; 

Radiation 

testing

Full qualification of 

flight 

prototype 

device

Currently exhibit desired read noise, but 

need significant improvements in dark 

current

Larger array sizes also needed

LUMOS Near-UV 

Multi-object 

Spectrograph

Wavelength Range:

200 - 400 nm

Array Size:

3 8k × 8k,

Tileable to larger arrays

Quantum Efficiency:

3 50%

Pixel Size:

£ 7 μm

Read Noise:

£ 5 eDark Current:

£ 1e-3 e-/pix/s

Sensitivity

Array sizes

Radiation hardness

ECLIPS Visible 

Channel

Wavelength Range:

500 - 1000 nm

Array Size:

3 4k × 4k

Read Noise:

< 1 eDark Current:

£ 1e-4 e-/pix/s

Radiation hardness

Sensitivity at 1000 nm for 

exoplanet characterization

Large formats for use in 

integral field spectrographs

Wavelength Range:

1000 - 2000 nm

Array Size:

3 4k × 4k

Read Noise:

< 3 eDark Current:

£ 1e-3 e-/pix/s

Read noise

Dark current

ECLIPS NIR Channel

Instrument Channel Driving Requirements Technical Challenges Solution Paths

Development status and path ahead

Note
LUVOIR The Large UV Optical Infrared Surveyor

11-28 The LUVOIR Interim Report

are ~10,000× higher than is required for 

exoplanet imaging. It is believed this is largely 

due to the readout electronics and should be 

mitigated with better optimization (Rauscher 

et al. 2016). Larger array sizes would also 

need to be developed.

11.4.1.5 Other detector technologies

Newer, less mature technologies that could 

offer significant performance advantages 

in the long term include visible-band high 

temperature photon counting arrays, such as 

Current 

TRL

Path to 

TRL 4

Path to 

TRL 5

Path to 

TRL 6

CCD/EMCCD 5 ü ü

Full qualification of 

flight 

prototype 

device in 

large 

format, tiled 

array

CCD devices have high heritage and 

very good noise performance.

Lack ability to define arbitrary region-ofinterest for high-speed readout.

Better radiation hardness is desireable.

CMOS 4 ü

Demo large 

format 

devices 

with 

sufficiently 

low darkcurrent at 

operating 

temps.; 

Radiation 

testing

Full qualification of 

flight 

prototype 

device in 

large 

format, tiled 

array

Existing commercial devices have good 

performance, but need to be radiation 

tested and/or hardened

HDI NIR Channel

Wavelength Range:

1000 nm - 2500 nm

Array Size:

3 4k × 4k

Tileable to larger arrays

Pixel Size:

£ 10 μm

Read Noise:

£ 5 eDark Current:

£ 1e-3 e-/pix/s

Read noise and dark current

High-speed region-of-interest 

readout for fine guiding

HgCdTe 

Photodiode Array 6 ü ü ü

Devices developed for WFIRST widefield instrument meet all requirements 

for LUVOIR HDI NIR Channel

Microwave 

Kinetic 

Inductance 

Detectors (MKID) 

and

Transition Edge 

Sensors (TES)

4 ü

Demo large 

format 

devices; 

Radiation 

testing; 

Demo cryocooling 

operations 

do not 

interfere 

with 

picometer 

stability

Full qualification of 

flight 

prototype 

device in 

appropriate 

format for 

LUVOIR 

application

Broadband, zero read noise, zero dark 

current devices

Built-in energy resolution provides onchip low-resolution spectroscopy

Operate at sub-Kelvin temperatures, 

requiring a cyrocooler which may be 

incompatible with picometer stability 

requirements

Quanta Image 

Sensor 3

Demo 

required 

quantum 

efficiency 

and basic 

performance 

specs

Demo flightpackaged 

device; 

radiation 

testing

Full qualification of 

flight 

prototype 

device in 

appropriate 

format for 

LUVOIR 

application

Room temperature, high-speed photon 

counting arrays

Could replace CMOS or EMCCD 

devices in near-UV (with delta-doping) 

or visible

Microshutter Arrays

Shutter Format:

420 × 840

Tileable to larger arrays

Electrostrictive actuation

Shutter yield and lifetime

Next-Gen 

Microshutter 

Arrays

4 ü

Environmental 

testing; 

Demo of 

tiling

Flight 

prototype 

qualification

Solution Paths

Development status and path ahead

Note

HDI UVIS Channel

Radiation hardness

Array sizes

Read noise

High-speed region-of-interest 

readout for fine guiding

Other Detector 

Devices

Low-TRL

Cryocooling vibration 

compatibility with picometer 

stability requirements

Instrument Channel Driving Requirements Technical Challenges

Any set of the above 

requirements

Wavelength Range:

200 - 1000 nm

Array Size:

3 8k × 8k

Tileable to larger arrays

Pixel Size:

< 7 μm

Read Noise:

< 5 eDark Current:

£ 1e-3 e-/pix/s
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-29

the Quanta Image Sensor (QIS) (Masoodian et 

al. 2017). With d-doping (applicable to virtually 

all silicon detectors), these could potentially 

provide photon counting detectors with high 

QE in the UV as well. Another potentially 

breakthrough technology that shall be 

monitored is the GaN material family-based 

photon counting UV avalanche diode arrays. 

They are intrinsically solar blind, with potential 

higher operating temperature and radiation 

tolerance. Cryogenic superconducting 

detectors such as microwave kinetic 

inductance detectors (MKIDs) and transition 

edge sensors (TES) are inherently noise free 

and provide native spectral resolution in the 

detector substrate, potentially enabling zero 

read noise exoplanet observations with builtin spectral resolution. However, the need for a 

cryocooler to achieve operating temperature 

may introduce unacceptable vibration into 

the system.

Path to TRL 4, 5, and 6: Specific 

recommendations for each detector 

technology are summarized in Table 11.6. 

We note, however, that recent detector 

development has been driven in large part 

by academic institutions. It is recommended 

that mission developers work closely with 

academic partners to achieve the biggest 

gains in advancement.

11.4.2 Microshutter arrays

The LUVOIR UV Multi-Object Spectrograph 

(LUMOS) instrument requires a 2 × 3 tiledarray of next-generation microshutters (Li et 

al. 2014) to enable the multi-object mode of 

observation. These microshutters leverage 

the heritage from JWST’s Near Infrared 

Spectrograph (NIRSpec) instrument (Kutyrev 

et al. 2008) but add improvements to array 

size and shutter reliability.

The JWST NIRSpec uses a 2 x 2 tile of 

microshutter arrays (MSAs), with each tile 

having a format of 171 × 365 shutters. A 

permanent magnet is swept across the array 

to actuate the shutters. Next-generation 

microshutter arrays that eliminate the 

moving magnet and instead rely on lowvoltage electrostrictive actuation have been 

demonstrated in the lab, with plans to fly on 

a sounding rocket in the near future. The 

demonstrated next-gen MSAs are directly 

applicable to LUVOIR, although they require 

engineering development to improve the 

format to 420 × 840 shutters and allow for 

array tiling.

Path to TRL 5: Environmental testing of 

the next-gen MSAs, including vibe, shock, 

acoustic, and thermal-vacuum testing, 

would constitute a component validation in 

a relevant environment.

Path to TRL 6: A tiled array of MSAs, 

each consisting of 420 x 840 shutters, needs 

to be demonstrated and subjected to the full 

range of flight qualifying environmental tests.

11.5 The technology path ahead

So, what is the right mix of LUVOIR technologies? Will an architecture with LOWFS 

and edge sensors alone be adequate? Or 

will OBWFS and laser truss metrology also 

be required? Will segment figure actuators 

be needed? Questions like these, when answered, will help to define the final performance, cost, and complexity of LUVOIR. 

They are being studied by the LUVOIR Study 

Team, following the methods in Section 

11.3.1 above.

The good news for now is that LUVOIR 

has many tools to work with, putting its scientific objectives within reach. Final determination of the best LUVOIR architecture will 

occur as the component technologies are 

demonstrated to meet the needed performance. It will happen through design trades, 

flowing out performance from components 

to exoEarth yield and other scientific metrics, 

following NASA system engineering practice, 

quantified by accurate, validated system-lev-
LUVOIR The Large UV Optical Infrared Surveyor

11-30 The LUVOIR Interim Report

el modeling and analysis, and grounded in 

rigorous testing.

As the LUVOIR Study Team continues 

to develop both architecture designs, it may 

be that additional technology needs will be 

identified, or that some of the technology gaps 

assessed here may be closed. The LUVOIR 

Study Team will continue to assess the impact 

and readiness of these technologies as the 

LUVOIR architectures evolve. It will conduct 

an inventory of technology development 

investments in each area, defining specific, 

actionable tasks that can be completed to 

achieve the objectives presented here, and 

estimating the cost and schedule required to 

execute those tasks. A complete technology 

development roadmap, with schedule and 

cost, will be included in the final report.

References

Acton, D. S., et al (2012). Space Telescopes 

and Instrumentation 2012: Optical, 

Infrared, and Millimeter Wave, SPIE, 

8442, 84422H

Bolcar, M. R. (2017). UV/Optical/IR Sp 

Telesc Instruments Innov Technol 

Concepts VIII, SPIE, 10398, 103980A

Burt, D. P., Dobson, P. S., Docherty, K. E., 

et al. (2012). Opt Lett, 37, 1247

Cady, E., Mejia Prada, C., An, X., et al. 

(2015). JATIS, 2, 11004

Crill, B., & Siegler, N. (2017). UV/Optical/IR 

Sp Telesc Instruments Innov Technol 

Concepts VIII, SPIE, 10398, 103980H

Dean, B. H., Aronstein, D. L., Smith, J. S., 

et al. (2006). Space Telescopes and 

Instrumentation I: Optical, Infrared, and 

Millimeter, SPIE, 6265, 626511

Dewell, L. D., Tajdaran, K., Bell, R. M., et 

al. (2017) UV/Optical/IR Sp Telesc 

Instruments Innov Technol Concepts 

VIII, SPIE, 10398, 103980B

Eisenhower, M. J., Cohen, L. M., Feinberg, 

L. D., et al. (2015). UV/Optical/IR Space 

Telescopes: Innovative Technologies 

and Concepts VII, SPIE, 9602, 96020A

Feinberg, L. D., et al (2008). UV/Optical/

IR Space Telescopes: Innovative 

Technologies and Concepts III, SPIE, 

6687, 668708

Fleming, B. T., McCandliss, S. R., Kaiser, 

M. E., et al. (2011). UV, X-Ray, GammaRay Sp Instrum Astron XVII, SPIE, 

8145, 81450B

Groff, T. D., Prada, C. M., Cady, E., et al. 

(2017). Tech Instrum Detect Exopl VIII, 

SPIE, 10400, 24

Guyon, O., Martinache, F., Belikov, R., & 

Soummer, R. 2010, ApJSS, 190, 220

Hickey, G., Barbee, T., Ealey, M., & 

Redding, D. (2010). Space Telescopes 

and Instrumentation 2010: Optical, 

Infrared, and Millimeter Wave, SPIE, 

7731, 773120

Hicks, B. A. (2016). JATIS, 2, 11015

Hirshorn, S. R., Voss, L. D., & Bromley, L. 

K. (2007). NASA Systems Engineering 

Handbook, Rev. 2, NASA/SP-2016-

6105 Rev. 2

Hoadley, K., France, K., Kruczek, N., et al. 

(2016). Sp Telesc Instrum 2016 Ultrav 

to Gamma Ray, SPIE, 9905, 99052V

Kutyrev, A. S., Collins, N., Chambers, J., 

et al. (2008). Sp Telesc Instrum 2008 

Opt Infrared, Millimeter, SPIE, 7010, 

70103D

Li, M. J., Brown, A. D., Kutyrev, A. S., et al. 

(2014). MOEMS Miniaturized Syst IX, 

SPIE, 7594, 75940N

Lyon, R. G., Clampin, M., Melnick, G., 

et al. (2008). Space Telescopes and 

Instrumentation 2008: Optical, Infrared, 

and Millimeter, SPIE, 7010, 701045

Masoodian, S., Ma, J., Starkey, D., et al. 

(2017). Adv Phot Count Tech XI, SPIE, 

10212, 102120H
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 11-31

Matthews, G., Barrett, D., Bolton, J., et al. 

(2003). Opt Manuf Test V, SPIE, 5180, 

169

Mazoyer, J., Pueyo, L., N’Diaye, M., et al. 

(2017). AJ, 155, 7

McComas, B. K., & Friedman, E. J. (2002). 

Opt Eng, 41, 567

Minor, R. H., Arthure, A. A., Gabor, G., et 

al. (1990). Adv Technol Opt Telesc IV, 

SPIE, 1236, 1018

Moore, C. S., Hennessy, J., Jewell, A. D., 

et al. (2014). Sp Telesc Instrum 2014 

Ultrav to Gamma Ray, SPIE, 9144, 

91444H

N’Diaye, M., Pueyo, L., & Soummer, R. 

(2015). ApJ, 799, 225

Nemati, B. (2014). Sp Telesc Instrum 2014 

Opt Infrared, Millim Wave, SPIE, 9143, 

91430Q

Nemati, B., Effinger, R., Demers, R., et 

al. (2016). High Energy, Opt Infrared 

Detect Astron VII, SPIE, 9915, 99150M

Nemati, B., Shaklan, S. B., Stahl, M. T., 

& Stahl, H. P. (2017). UV/Optical/IR 

Sp Telesc Instruments Innov Technol 

Concepts VIII, SPIE, 10398, 103980G

Nikzad, S., Jewell, A. D., Hoenk, M. E., et 

al. (2017). JATIS 3, 1

Quijada, M. A., Del Hoyo, J., & Rice, 

S. (2014). Space Telescopes and 

Instrumentation 2014: Ultraviolet to 

Gamma Ray, SPIE, 9144, 91444G

Rauscher, B. J., Canavan, E. R., Moseley, 

S. H., et al. (2016). JATIS, 2, 41212

Redding, D. C., et al. (2003). Aerospace 

Conference Vol. 4-1729, IEEE, 0-7803-

7651-X/03

Redding, D. (2016). Modeling, Systems 

Engineering, and Project Management 

for Astronomy VII, SPIE, 9911, 99110F

Ruane, G., Jewell, J., Mawet, D., Pueyo, 

L., & Shaklan, S. (2016). Advances in 

Optical and Mechanical Technologies 

for Telescopes and Instrumentation II, 

SPIE, 9912, 99122L

Seo, B.-J., Cady, E. J., Gordon, B., et al. 

(2017). Tech Instrum Detect Exopl VIII, 

SPIE, 10400, 104000F

Shaklan, S. (2016). Segmented 

Coronagraph Design and Analysis 

(SCDA): A study by the Exoplanet 

Exploration Program

Shelton, C., Mast, T., Chanan, G., et al. 

(2008). Ground-based Airborne Telesc 

II, SPIE, 7012, 701210

Shi, F., An, X., Balasubramanian, K., et al. 

(2017). Tech Instrum Detect Exopl VIII, 

SPIE, 10400, 13

Shi, F., Balasubramanian, K., Hein, R., et al. 

(2016). JATIS 2, 11021

Sokolowski, W. M., Jacobs, S. F., Lane, M. 

S., et al. (1993). Qual Reliab Opt Syst, 

SPIE, 1993, 115

Wallace, J. K., Rao, S., Jensen-Clem, R. 

M., & Serabyn, G. (2011). Opt Manuf 

Test IX, SPIE, 8126, 81260F

Wasmeier, M., Hackl, J., & Leveque, S. 

(2014). Ground-based Airborne Telesc 

V, SPIE, 9145, 91451R

Wellman, J., Weaver, G., & Redding, D. 

(2012). 229th Meeting of the American 

Astronomical Society, Austin, TX

Zimmerman, N. T., N’Diaye, M., St. Laurent, 

K. E., et al. (2016). Sp Telesc Instrum 

2016 Opt Infrared, Millim Wave, SPIE, 

9904, 99041Y
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 12-1

Chapter 12
LUVOIR The Large UV Optical Infrared Surveyor

12-2 The LUVOIR Interim Report

12 LUVOIR Cycle 1

The signature science cases outlined in this 

document represent transformative leaps 

in their individual fields. In Hubble Space 

Telescope parlance, any one of these cases 

represents a Large or Treasury program. 

Here, we imagine how aspects of each 

signature science case can combine as 

part of a robust multi-scale, guest observerdriven portfolio to impact an enormous 

range of science in LUVOIR’s first year of 

observations.

12.1 Doing the impossible

LUVOIR’s combination of aperture, resolution, and sensitivity allow it to perform science that is otherwise impossible. For “impossible,” consider two different operating 

definitions. First, impossible can be taken as 

its traditional meaning: science that fundamentally requires an observatory of LUVOIR’s 

size and capability. The second definition 

speaks to science that can be done by observatories of lesser capability but will not 

likely be done due to the observations requiring prohibitively large amounts of time. 

For example, Figure 12.1 shows the allocation of observing time on HST as a function 

of program size in orbits. As a general rule, 

small observations are executed frequently, 

Treasury-scale observations very rarely. For 

fully commissioned guest observer facilities, 

this trend is usually invariant with aperture. 

However, by virtue of its immense capability 

through a combination of aperture and instrumentation, one of LUVOIR’s assets is its 

ability to move to the left programs that sit 

on the right of Figure 12.1 for smaller observatories. For example, the iconic Hubble 

Ultra Deep Field (HUDF) which required 

over 11 days of observing with HST could 

be performed in approximately 1 hour with 

LUVOIR-A. Essentially, otherwise prohibitively large programs become routine, freeing up 

time for programs fitting our first definition of 

impossible. 

12.2 A transformational first year of 

science

We do not know how a future time allocation 

committee will prioritize science programs, 

but we expect a healthy balance across 

exoplanet science, general astrophysics, 

and solar system science. We also note that 

in many cases parallel observations can be 

executed, facilitating exciting combinations 

of large program science. For example, a 25 

hour-long ECLIPS spectroscopic observation 

of an exoEarth candidate can simultaneously 

return 5-band HDI imaging at a depth 2–3 

magnitudes deeper than the HUDF. When we 

consider the power of parallel observations, 

and the multiple science cases addressed 

by deep field observations (let alone archival 

data uses), any description of a set of long 

LUVOIR observations for a single science 

case is likely to significantly underestimate 

the total return for that time investment. 

Figure 12.1. Number of HST programs 

in Cycles 17-23 as a function of program 

execution time in orbits. Credit: STScI
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report 12-3

We illustrate LUVOIR’s immense potential 

science return with a list of observing 

programs that require total exposure times 

of ~100 hours or fewer to execute, assuming 

the LUVOIR-A Architecture. These programs 

are drawn from the signature science 

cases described in this report. They do not, 

however, span the whole range of programs 

that the community may choose to execute 

as a guest observer-driven facility, nor are 

they intended to express any prioritization 

of science topics. They simply represent 

an example menu of large-scale program 

options that could be done in LUVOIR’s 

first year, while leaving greater than three 

quarters of the time free for as-yet-to-bedetermined programs at any scale, assuming 

a very conservative observatory efficiency of 

50%. As neither the telescope+instrument 

sensitivities are finalized at this stage nor 

the exact targets known in many cases, 

these example projects are approximate, 

preliminary, and purely illustrative. It is 

important to emphasize that in nearly every 

case below, a parallel observation can be 

executed, usually with either HDI or LUMOS.

• The exoEarth search begins: ECLIPS 

makes ~ 25 visits to a variety of FGK stars 

(plus Proxima Centauri) within 7 parsecs. 

This provides ≥ 7-sigma photometric 

detections in three wavelength bands 

for any planets as bright or brighter than 

modern Earth within the ECLIPS field-ofview (FoV) at the time of each visit.

• From darkness, light: LUMOS observations of 10 fields provides observations 

of 10,000 galaxies between 0 < z < 1, 

building the faint-end UV luminosity function for galaxies over 8 billion years of 

cosmic time and definitively constraining 

the amount of ionizing light they leak.

• Water in motion: Time-dependent, high 

spatial resolution, NIR and/or UV spectroscopic imaging of icy moons in the solar system resolves geyser activity. Such 

observations could be done in coordination with an in-situ spacecraft, providing 

geyser location and timing information.

• Reionization revealed: Deep observations of a single HDI field reveals ~600 

z=7 galaxies with enough statistical power to begin to reveal the effects (if any) 

of reionization suppression of early star 

formation.

• Diverse worlds: ECLIPS spectroscopy 

of ≥ 10 known planetary systems reveals 

the detailed atmospheric characteristics 

of a dozen or more planets including 

Venuses, mini-Neptunes, and Jupiters.

• Feedback at the source: Using its 

microshutter array, LUMOS maps the 

galactic outflows where they arise in 

star forming regions and follows the 

winds out to their interactions with the 

surrounding medium, directly resolving 

the complex physics of galactic feedback 

at unprecedented physical scales.

• Habitable worlds: For stars with promising planets from the exoEarth search, 

atmospheric characterization commences with ECLIPS spectroscopy in an integral field spectrograph band centered 

near 980 nm to search for water vapor 

absorption. A methane absorption feature also appears near this wavelength, 

allowing quick identification of any giant 

planets in the FoV.

• The nurseries explored: LUMOS obtains FUV-NUV observations of molecular 

diversity in the inner few AU of hundreds 

of young stellar and planetary systems.

The scientific impact of these example 

programs in the first year alone would 

be immense—and in many cases truly 

transformative. Moreover, this hypothetical 

sampling only represents a tiny fraction of 

what would be possible with a serviceable 

observatory intended to last decades. As 
LUVOIR The Large UV Optical Infrared Surveyor

12-4 The LUVOIR Interim Report

such, LUVOIR is poised not just to continue 

in the tradition of the great observatories, 

but to re-define nearly every aspect of our 

knowledge of the cosmos.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-1

13 Appendix A: Further LUVOIR science cases

To better capture the full range of LUVOIR’s capabilities, this appendix includes short additional 

LUVOIR science programs contributed by the community and LUVOIR team members. The 

topics range from remote sensing of solar system bodies, to exoplanet observations, to a 

wide range of general astrophysics studies. In some instances, the authors have chosen to 

provide further details—or put their own perspective—on a case that is mentioned in the 

main science chapters. The cases are of varying length and appear in no particular order. 

This is a living document that will grow with time.
LUVOIR The Large UV Optical Infrared Surveyor

A-2 The LUVOIR Interim Report

13.1 Observations of Venus with LUVOIR

Giada Arney (NASA GSFC), Valeria Cottini (NASA GSFC), Shawn Domagal-Goldman (NASA 

GSFC), Lori Glaze (NASA GSFC), Eric Lopez (NASA GSFC), Victoria Meadows (UW), Ravi 

Kopparapu (NASA GSFC), Roser Juanola Parramon (NASA GSFC)

13.1.1 Introduction

At its closest approach, Venus is the nearest 

planet to Earth, yet much still remains 

unknown about Earth’s twisted sister. Many 

important Venus science questions can 

be addressed with sys-Earth telescopic 

observations. A sufficiently small solar 

elongation viewing angle for LUVOIR (≤ 45°) 

enables such observations of Venus. This, 

together with a darkening neutral density 

filter, opens the door to exciting and much 

needed data on the Venusian atmosphere. 

Three interesting case studies that could 

be investigated with LUVOIR are described 

briefly below as examples of the types of 

Venus science LUVOIR can make possible.

1. Observations from the JAXA Akatsuki 

orbiter have revealed an unusual stationary 

bow-shaped wave at the Venus cloud tops 

(65 km) observable at multiple wavelengths 

from the UV at 283 nm (corresponding to 

a SO2 absorption band) to the longwave 

infrared at 8–12 mm (Fukuhara et al. 2017). 

Normal wind speeds at these high altitudes 

whip across the planet at roughly 100 m/s, 

but this UV-bright, bow-shaped feature 

remains stationary relative to the surface far 

below. The center of the bow-shaped feature 

(Figure 13.1) is located above the western 

slope of equatorial highland region Aphrodite 

Terra, and is interpreted to be a stationary 

atmospheric gravity wave associated with 

lower atmosphere wind flows over the terrain. 

However, the propagation of such waves to 

the cloud tops is difficult to reconcile with 

current understanding of convection in the 

Venus atmosphere (Seiff et al. 1985). Thus, 

the dynamics of the Venus atmosphere may 

be more complex than previously thought. 

Monitoring the temporal evolution of features 

such as this could provide new insights into 

the physics of Venus atmosphere circulation. 

2. Long temporal baseline (1970s–

2012) monitoring of Venus across multiple 

Venus missions at l = 215 and 283 nm has 

revealed quasi-periodic variations in high 

altitude (70 km) SO2 abundance (Marcq et 

al. 2012) with reported variations from ~400 

ppbv to less than 100 ppbv. This variability 

may be related to poorly understood 

oscillations in atmospheric circulation, and/

or volcanic injections of SO2 into the upper 

atmosphere. The amount of SO2 currently 

in the atmosphere has been estimated 

to be in excess of equilibrated conditions 

by a factor of 100, implying a source (i.e., 

volcanism) within the past 20 million years 

(Bullock & Grinspoon 2001). Monitoring of 

SO2 variations may therefore shed light on 

multiple processes related to atmospheric 

Figure 13.1. A bow-shaped UV bright wave 

in the Venus upper atmosphere (highlighted 

with solid white line) seen with Akatsuki above 

the highland region Aphrodite Terra could 

be observed with LUVOIR and shed light on 

Venus atmospheric dynamics. Circles indicate 

displaced air parcels. Credit: Fukuhura et al. 

(2017)
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-3

dynamics and/or volcanic processes. Such 

a monitoring campaign demands a UVenabled space-based platform that can 

observe Venus over years or decades. 

3. Besides SO2, several other trace gases 

can be observed in the Venus spectrum, 

providing insights into chemical, dynamical, 

and photochemical processes that occur 

on Venus. For example, significant and 

surprising variability has been observed 

in the abundances and distributions of 

trace gases in the Venusian sub-cloud 

atmosphere (Arney et al. 2014). These 

variations hint at poorly understood chemical 

and physical mechanisms operating in the 

lower atmosphere. They also suggest the 

presence of H2SO4 virga events (rain that 

evaporates before reaching the surface). 

Between 1–2.5 mm, one can observe 

upwelling thermal radiation from the subcloud (0–45 km) atmosphere on the Venus 

nightside. This enables deep-atmosphere 

observations of HDO, H2O, SO2, HCl, CO, 

and OCS, which have been observed to 

vary both spatially and temporally on poorly 

constrained timescales. 

Because exo-Venus planets may be one 

of the most common types of exoplanets 

(Kane et al. 2014), better understanding 

the planet next door will enable us to better 

interpret observations of exo-Venus worlds.

13.1.2 The role of LUVOIR

Currently, there is no planned NASA mission 

to Venus. Periodic monitoring of Venus over 

long time baselines can reveal important 

information about variations in atmospheric 

species that may constrain theories of 

dynamical, chemical, and geophysical 

processes occurring on Venus. LUVOIR-A 

is baselined to point to a minimum solar 

elongation angle of 33°, which is small 

enough to allow observations of Venus at 

even less than its maximum elongation. 

Observations towards crescent phase will 

enable observations of nightside thermal 

radiation upwelling from below the cloud 

deck. Certain UV observations (e.g., of high 

altitude SO2 variations and of the “bow” 

shaped feature) require a space-based 

observatory, as they cannot be performed 

from the ground.

LUVOIR could achieve extremely good 

spatial resolution on Venus. Figure 13.2

shows a view of Venus from the Akatsuki 

orbiter at a wavelength of ~2 mm showing 

variations in opacity of the lower cloud deck 

on the planet’s nightside. LUVOIR could 

achieve a spatial resolution of 4 km/pixel 

at this wavelength (although note that the 

scattering footprint for radiation upwelling 

from the sub-cloud atmosphere is ~100 

km2).

13.1.3 The science program

Venus is significantly brighter than other 

sources LUVOIR will observe (mv = -4.4 on the 

dayside), necessitating neutral density filters 

to view this interesting target. To underscore 

this point, the brightest source the online 

LUMOS ETC includes has an AB magnitude 

of 15, and the brightest source included for 

HDI is magnitude 20. Venus is an extremely 

bright target for LUVOIR. However, LUVOIR 

is also considering observations of Jupiter, 

which has mv = -2.7, so Venus is comparable 

Figure 13.2. A view of Venus from JAXA’s 

Akatsuki. LUVOIR-A would obtain comparable 

spatial resolution. Credit: R. Juanola Parramon 

(NASA GSFC)/Damia Bouic/JAXA Akatsuki
LUVOIR The Large UV Optical Infrared Surveyor

A-4 The LUVOIR Interim Report

to other bright solar system targets under 

consideration.

The angular diameter of Venus is ~30" 

when the planet is close to maximum 

elongation. The LUVOIR-A LUMOS 

spectrometer currently has a wavelength 

range 100–400 nm, and it may be extended 

to 1000 nm. LUMOS has several available 

resolutions (R = 500, 16000, 63200, 100000), 

enabling moderate and very high-resolution 

spectroscopy. For comparison, previous 

observations probing isotopes in the Venus 

atmosphere have used R = 105 (Krasnopolsky 

et al. 2010), while measurement of gas 

species in the nightside thermal windows 

can be accomplished with R = 2000 (Arney 

et al. 2014), so the range of resolutions used 

by LUVOIR are useful for a variety of studies. 

The LUMOS field-of-view (FOV) is 2' x 2', so 

Venus fits comfortably within it. 

LUVOIR-A HDI has a FOV of 2' x 3', which 

also allows for full views of Venus in a single 

frame. It has two channels: UVIS covering 

200–950 nm and NIR covering 800–2500 

nm. Nyquist sampled pixels at 400 nm (2.73 

mas/pixel) and at 1200 nm (8.20 mas/pixel) 

mean that 10000 pixels will span Venus at 

400 nm (~ 1.2 km/pixel) and 3600 pixels 

at 1200 nm (3.3 km/pixel). By comparison, 

the Venus Monitoring Camera aboard 

ESA’s Venus Express orbiter could obtain 

0.2–45 km/pixel depending on how far the 

spacecraft was from the planet (Markiewicz 

et al. 2007). LUVOIR allows for orbiter-quality 

monitoring of Venus.

LUVOIR-A ECLIPS will have a significantly 

smaller FOV than LUMOS and HDI, but it 

could still in principle observe Venus using 

open slots in the filter wheel. The ECLIPS 

imager has a FOV of 1.4" x 1.4" in the UV, 

2.7" x 2.7" in the visible, and 5.6" x 5.6" in 

the NIR. Useful observations with ECLIPS 

could be obtained via a mosaic technique 

to cover the entire disk of Venus, or smaller 

sub-regions could be targeted.

References

Arney, G., Meadows, V., Crisp, D., et al. 

2014, J Geophys Res Planets, 119, 

1860

Bullock, M., & Grinspoon, D. 2001, Icarus, 

150, 19

Fukuhara, T., Futaguchi, M., Hashimoto, G. 

L., et al. 2017, Nat Geosci, 10, 85

Kane, S. R., Kopparapu, R. K., & DomagalGoldman, S. D. 2014, Astrophys J, 

794, L5

Program at a Glance

Science goal: Obtain high spatial and high spectral resolution data of Venus at UV-VISNIR wavelengths to monitor for atmospheric variability possibly related to atmospheric 

dynamics, chemistry, and/or volcanic activity. 

Program details: Venus will have an angular diameter ~30" at maximum solar elongation, 

and its day-side magnitudes are: U = -2.79, V = -3.68, B = -4.38, R = -4.95, I = -5.08, Rc 

= -4.73, Ic = -5.04. 

Instrument(s) + configuration(s): ECLIPS and LUMOS could be used to obtain medium 

to very high-resolution spectra. LUMOS can fit Venus in its FOV; ECLIPS would require 

mosiacing. HDI can obtain high spatial resolution images from the UV to NIR.

Key observation requirements: Observations of Venus will require neutral density filters 

and the ability to observe at a sufficiently small solar elongation angle (< 45°). This is 

compatible with the design of the notional LUVOIR-A.
The Large UV Optical Infrared Surveyor LUVOIR

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Marcq, E., Bertaux, J., Montmessin, F., & 

Belyaev, D. 2012, Nat Geosci, 6, 1

Markiewicz, W. J., Titov, D. V, Fiethe, B., et 

al. 2007, ESA Spec Publ, 1

Seiff, A., Schofield, J. T., Kliore, A, J., et al. 

1985, Adv Sp Res, 5, 3
LUVOIR The Large UV Optical Infrared Surveyor

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13.2 Exo-cartography for terrestrial planets

Claire Marie Guimond (McGill University), Nicolas B. Cowan (McGill University)

13.2.1 Introduction

LUVOIR and its predecessors will have yielded 

us alien pale blue dots; the next step is to 

study their habitability. There are undoubtedly 

cases where understanding a planet requires 

understanding the diversity of its regions, 

especially since oceans and continents imply 

long-term habitability. Planets without a mix 

of water and land may not have a significant 

silicate weathering feedback, which regulates 

the CO2 greenhouse on Earth and keeps her 

climate temperate (Cowan 2015). 

With exo-cartography, we can infer the 

number, reflectance spectra, and longitudinal 

locations of major surface types (Fujii et al. 

2017; Cowan and Strait 2013; Kawahara 

and Fujii 2011). This works because directly 

imaged planets show diurnal brightness 

variations as different surface and cloud 

features rotate into view (see Cowan and 

Fujii 2017). The lightcurve collected from 

the planet is the disk-integrated reflectance 

per exposure (Figure 13.3). In theory, we 

can invert lightcurves to piece out latitudelongitude albedo maps (Kawahara and Fujii 

2010; Figure 13.4). 

13.2.2 The role of LUVOIR

The very large aperture of LUVOIR will enable 

reflected light surface mapping and spin 

determination for terrestrial planets (Pallé et 

al. 2008; Oakley and Cash 2009; Cowan et 

al. 2009, 2011; Kawahara and Fujii 2010, 

2011; Fujii and Kawahara 2012). Previous 

mapping papers have adopted the optimistic 

1% photometric uncertainty (S/N of 100) for 

1-hr integrations. For a 15-m telescope, this 

will only be possible for a super-Earth at 1 

pc. However, Cowan et al. (2009) claimed 

they could do essentially the same science 

with 3% photometry in 1-hr integrations (24 

data per rotation, each of S/N = 33). 

As Figure 13.5 shows, for an Earth twin 

at 10 pc, we can only expect an S/N of ~10 

with one rotation, but for more slowly rotating 

planets and/or larger radii, this value can 

double or triple. Further, decreasing the time 

resolution (i.e., longitudinal sample rate) by a 

factor of 16 increases the per-integration S/N 

by a factor of 4—this would set the number 

of pixels in the final map. Stacking multiple 

epochs of observations can be problematic, 

as clouds strongly influence reflected light 

Figure 13.3. Simulated light curve and resulting fit from an Earth analog with 0 degree (face-on) 

inclination and 90 degree obliquity. The simulated data and measurement errors are shown in 

the five insets, representing five epochs, each lasting one rotation, collected over the course of a 

year with a one hour cadence. The shaded region in each inset shows the central 90% posterior 

credible interval for the constructed light curve. From Farr et al. (2018, in prep.).
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-7

fluxes, and these atmospheric features are 

prone to change between epochs (Oakley 

and Cash 2009).

Thus only with a 15-m class space 

telescope such as LUVOIR can we start 

to map super-Earth exoplanets. For the 

smallest targets, only spin orientation and 

low-resolution longitudinal maps will be 

retrievable.

13.2.3 The science program

This program will target the Earth-twin 

planets we expect to have been detected 

(Stark et al. 2014) in the habitable zone of 

G stars (MV ~ 4.8). We limit observations to 

one rotation period to better avoid confusion 

with diurnal cloud variation. Hence, the 

rotation period sets total integration time. We 

assume a sampling rate of four exposures 

per rotation—e.g., 6 hours of exposure for 

a 24-hour rotation—as the bare minimum to 

detect rotational variation. 

We adopt the wavelength ranges 400–

500 nm (UVIS channel) and 800–900 nm 

(UVIS/NIR channels). The inverted reflectivity 

difference across these bands has been 

Figure 13.4. Demonstration of albedo map 

retrieval using a simulated lightcurve of the 

Earth (top), with 10th percentile (middle) 

and 90th percentile (bottom) of the marginal 

posterior distributions for the albedo of each 

pixel. The 10th percentile map clearly shows 

a reflecting region (the Sahara Desert) while 

the 90th percentile clearly shows dark regions 

corresponding to the Pacific, Atlantic, and 

Indian Oceans. These maps therefore establish 

the presence of continents and oceans on the 

the planet. Adapted from Farr et al. (2018, in 

prep.).

Figure 13.5. Signal-to-noise ratio at the poisson limit for two bandpasses as a function of rotation 

period and planet radius, for a planet at 10 pc with semi-major axis of 1 AU, geometric albedo of 

0.3, nominal telescope diameter of 15 m, and coronagraph throughput of 15%.
LUVOIR The Large UV Optical Infrared Surveyor

A-8 The LUVOIR Interim Report

shown to suppress the cloud signal and 

roughly recover the continental distribution 

(Kawahara and Fujii 2011).

The observations use LUVOIR’s Extreme 

Coronagraph for Living Planetary Systems 

to suppress light from the host star, in 

conjunction with the High Definition Imager. 

The maximum angular separation between 

an Earth-twin on a 1-AU circular orbit 10 pc 

away is 100 mas, which would be within the 

outer working angle of the coronagraph. For 

the highest signal, planets will be imaged 

at the projected separation corresponding 

to the coronagraph’s inner working angle. 

We will pursue targets with planet-star flux 

contrast ≥ 10-10, due to expected instrument 

limitations.

References

Cowan, N. B. 2015, ArXiv e-prints, arXiv:1511.04444

Cowan, N. B., & Fujii, Y. 2017, ArXiv 

e-prints, arXiv:1704.07832

Cowan, N. B., & Strait, T. E. 2013, ApJ, 

765, L17

Cowan, N. B., Agol, E., Meadows, V. S., et 

al. 2009, ApJ, 700, 915

Cowan, N. B., Robinson, T., Livengood, T. 

A., et al. 2011, ApJ, 731, 76

Farr, B., Farr, W. M., Cowan, N. B., 

Haggard, H. M., Robinson, T. D. 2018, 

in prep.

Fujii, Y., Lustig-Yaeger, J., & Cowan, N. B. 

2017, AJ, 154, 189

Fujii, Y., & Kawahara, H. 2012, ApJ, 755, 

101

Kawahara, H., & Fujii, Y. 2010, ApJ, 720, 

1333

—. 2011, ApJ, 739, L62

Oakley, P. H. H., & Cash, W. 2009, ApJ, 

700, 1428

Pallé, E., Ford, E. B., Seager, S., MontañésRodríguez, P., & Vazquez, M. 2008, 

ApJ, 676, 1319

Stark, C. C., Roberge, A., Mandell, A., & 

Robinson, T. D. 2014, ApJ, 795, 2

Program at a Glance

Science goal: Construct rough longitudinal maps of Earth twins using rotational mapping 

techniques.

Program details: Observe brightness variations over one rotation period, using point 

source multi-band photometry, for the nearest handful of < 2 RE planets in the habitable 

zones of G stars.

Instrument(s) + configuration(s): Differential HDI photometry with ECLIPS.

Key observation requirements: Two imaging bands of 400–500 nm and 800–900 nm 

with signal-to-noise  10, and planet-star contrast  10-10. 
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13.3 Extragalactic massive stars

Miriam Garcia (CAB, CSIC-INTA), Chris Evans (UKATC, STFC)

13.3.1 Introduction

Massive stars are cosmic engines and make 

valuable probes of the Universe. They are 

mighty sources of ionizing radiation and 

mechanical energy, giving rise to striking 

bubbles, giant HII regions, and galactic-scale 

outflows during their lives. In death they are 

the progenitors of supernovae and longduration γ-ray bursts, with the latter so bright 

they can be detected in galaxies up to z = 9 

(Robertson & Ellis, 2012). These dramatic 

explosions release chemical elements that 

are essential to life (e.g., oxygen) into the 

interstellar medium, and their remnants 

(neutron stars, pulsars, black holes) are the 

sites of extreme physics and sources of 

gravitational waves (Abbott et al. 2016).

A critical area of research in the field 

is characterizing the role of metallicity 

(abundance of elements heavier than H and 

He) on the lifecycle of massive stars. The 

Universe has become increasingly enriched 

in metals since the Big Bang so, to correctly 

interpret observations of distant galaxies, we 

need models of stellar physics and evolution 

that match their metal content. The ultimate 

goal is to investigate the paradigm of the first 

generation of (effectively metal-free) massive 

stars in the Universe (so-called Population 

III), a suggested source of reionisation at 

z > 6 (Haiman & Loeb, 1997). 

A key observable of massive stars is 

their radiatively-driven stellar winds, via 

which solar masses of material can be lost 

throughout their lives, directly influencing 

their evolutionary sequence, feedback to the 

local medium, and pre-explosion core mass. 

The wind momentum depends strongly on 

metallicity (Kudritzki & Puls, 2000), implying 

a strong metallicity dependence to evolution 

and feedback. Only UV spectroscopy 

from space can provide the data required 

to assess the properties of such winds 

(e.g., Fullerton et al. 2006; Sundqvist et 

al. 2014). IUE, FUSE, and HST-STIS/COS 

have made unprecedented and long-lasting 

contributions to these studies. 

Figure 13.6. Left: Resolving massive stars in the ultra-metal poor galaxy I Zw 18 at 18 Mpc is 

a long-standing goal for studies of stellar evolution. Right: UV fluxes (1500Å) for massive stars 

(triangles = B-type supergiants; diamonds = O-type stars) at increasing distances, compared to 

five orbits of HST spectroscopy (at R~2000). At least a 12m aperture is required for UV spectra 

of individual massive stars in I Zw 18.
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Studies in nearby galaxies such as the 

SMC (60kpc), IC1613 (750kpc), WLM, 

Sextans A and NGC3109 (~1.2Mpc) have 

enabled studies of the properties of massive 

stars with metallicities of 10–20% solar (e.g., 

Venn et al. 2003; Bresolin et al. 2007; Evans 

et al. 2007; Camacho et al. 2016) providing 

valuable empirical templates (e.g., Leitherer 

et al. 2001) and important tests of theory (e.g., 

Mokiem et al. 2007). However, significant 

uncertainties remain over their wind behavior 

at low metallicity, even when UV data are 

available (Tramper et al. 2011; Herrero et al. 

2012; Garcia et al. 2014; Bouret et al. 2015) 

because of the sensitivity limits of current 

facilities. Moreover, we need to go at least 

an order-of-magnitude lower in metallicity to 

match conditions in distant absorption-line 

systems (e.g., Prochaska et al. 2003) and 

to move toward robust models of Pop. III 

stars, but these are simply out of reach of 

current facilities (HST, 8–10m ground-based 

telescopes).

13.3.2 The role of LUVOIR

The ambitious goal in this topic is studies of 

individual massive stars in I Zw 18, a starforming galaxy at 18 Mpc (see Figure 13.6) 

with a metallicity of 2–3% solar (Vilchez 

& Iglesias-Paramo). The task will require 

outstanding spatial resolution and sensitivity. 

Optical/NIR spectroscopy, needed to 

determine some key physical properties 

(temperatures, luminosities) of individual 

stars, will be feasible with future groundbased, multi-object spectrographs aided by 

adaptive optics (e.g., TMT, ELT). However, 

UV data are crucial to build the complete 

description of the stars.

Such observations require the sensitivity 

and spatial resolution of at least a 12-m 

aperture facility in space (see Figure 

13.6). High-quality UV spectroscopy over 

1200–1800 Å of individual luminous stars 

will constrain their mass-loss rates, shocks, 

structure, and velocity fields of their winds. 

Only with access to UV spectra of these 

objects can we fully characterize their 

natures and test theoretical predictions 

for the properties of such stars in the early 

Universe.

13.3.3 The science program

LUMOS onboard LUVOIR will enable the first 

thorough characterization of the winds of 

metal-poor (from 10% down to 2–3% solar 

metallicity) massive stars with mid-resolution 

(R = λ/Δλ ≥ 5000) with UV spectroscopy. The 

combination of the large aperture of LUVOIR 

and the multiplex of LUMOS will allow us to 

mine metal-poor galaxies out to the outer 

edges of the Local Group, other neighboring 

groups, and ultimately I Zw18 (see Figure 

13.6). The data will provide definitive 

answers to pressing questions regarding the 

properties and evolution of high-mass stars 

including: 

• The first observational parameterization 

of radiation-driven winds at very 

Program at a Glance

Science goal: The physical properties of massive stars at very low metallicities, akin to 

those in the early Universe. 

Program details: UV spectroscopy of individual massive stars in the galaxy I Zw18 at a 

distance of 18 Mpc.

Instrument(s) + configuration(s): Multi-object spectroscopy with LUMOS (G155L).

Key observation requirements: UV spectroscopy spanning 1000 to 2000 Å (essential), 

to 2500 Å (desirable).; R ≥ 5000; S/N > 20 per resolution element at 1500 Å.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-11

low metallicities (<10% solar), with 

consequences for models of Pop. III stars 

and stellar evolution in the early Universe, 

and population-synthesis models used to 

interpret observations of high-z galaxies. 

• Direct tests of ‘chemically-homogeneous 

evolution’ at such low metallicities, and 

a characterization of their winds. This 

evolutionary channel has been invoked to 

explain the binary system in the first LIGO 

detections (e.g., de Mink & Mandel, 2016) 

but lacks observational confirmation.

References

Abbott et al. 2016, Phys. Rev. Lett., 116, 

061102

Bouret et al. 2015, MNRAS, 449, 1545 

Bresolin et al. 2007, ApJ, 671, 2028

Camacho et al. 2016, A&A, 585, A82

de Mink & Mandel, 2016, MNRAS, 460, 

3545

Evans et al. 2007, ApJ, 659, 1198

Fullerton et al. 2006, ApJ, 637, 1025

Garcia et al. 2014, ApJ, 788, 64

Haiman & Loeb, 1997, ApJ, 483, 21

Herrero et al. 2012, A&A, 543, A85

Kudritzki & Puls, 2000, ARA&A, 38, 613

Leitherer et al. 2001, ApJ, 550, 724 

Mokiem et al. 2007, A&A, 473, 603 

Prochaska et al. 2003, ApJL, 595, L9

Robertson & Ellis 2012, ApJ, 744, 95

Sundqvist et al. 2014, A&A, 568, A59

Tramper et al. 2011, ApJL, 741, L8

Venn et al. 2003, AJ, 126, 1326

Vílchez & Iglesias-Paramo 1998, ApJ, 508, 

248
LUVOIR The Large UV Optical Infrared Surveyor

A-12 The LUVOIR Interim Report

13.4 Ultraviolet haloes around edge-on galaxies

Benne W. Holwerda (University of Louisville) in collaboration with the SKIRT team (University 

of Gent)

13.4.1 Introduction

In the latest iterations of radiative transfer 

models of edge-on galaxies across from 

ultraviolet to the sub-millimeter regime (e.g., 

Popescu+ 2011, Holwerda+ 2012, Recently, 

De Geyter+ 2015, Mosenkov+ 2016, 2018), 

it has become clear that there is a dustobscured ultraviolet component as well as a 

diffuse halo (see e.g., Seon+ 2014 and Seon 

& Draine, 2016, Figure 13.7). 

Models of the ultraviolet radiative transfer 

are complicated by both the localized nature 

of the origin in galaxies and the scatter from 

the dusty ISM (see Figure 13.8). Similarly, 

the diffuse ultraviolet emission from inter-arm 

regions can either be from localized O-stars 

or scattered (Crocker+ 2015). 

A way to test the nature of the diffuse 

UV emission in any of these galaxies 

that is completely independent from the 

panchromatic radiative transfer analysis is 

UV polarimetry, suggested by Baes & Viane 

(2016), Hodges-Kluck & Bregman (2014). 

If the UV halo emission is dominated by 

scattered radiation, we would expect a strong 

linear polarisation signature, similar as seen 

in reflection nebulae. Unfortunately, there 

is currently no UV polarisation instrument 

available. The total emission at these 

wavelengths is probably dominated by direct 

light from the stellar halo, but it is possible 

that the signature of a very extended dust 

distribution might still be visible in polarised 

light. Detailed polarised radiative transfer 

simulations should be used to test this. 

13.4.2 The role of LUVOIR

LUVOIR can image the diffuse ultraviolet 

haloes, obtain spectra to identify the 

predominant stellar population behind them, 

and measure polarization of the halo to 

determine how much of the light is scattered 

or not. 

And because LUVOIR has the sensitivity 

and spatial resolution far surpassing existing 

UV observatories, it can map diffuse, 

extended ultraviolet haloes around galaxies at 

Figure 13.7. The NGC 891 ultraviolet halo from Seonet al. (2014).

12

13.4 Ultraviolet haloes around edge-on galaxies

Benne W. Holwerda (University of Louisville) in collaboration with the SKIRT team (University of 

Gent)

13.4.1 Introduction

In the latest iterations of radiative transfer models of edge-on galaxies across from ultraviolet to 

the sub-millimeter regime (e.g., Popescu+ 2011, Holwerda+ 2012, Recently, De Geyter+ 2015, 

Mosenkov+ 2016, 2018), it has become clear that there is a dust-obscured ultraviolet component 

as well as a diffuse halo (see e.g. Seon+ 2014 and Seon & Draine, 2016, Figure 1).

Models of the ultraviolet radiative transfer are complicated by both the localized nature of the 

origin in galaxies and the scatter from the dusty ISM (see Figure 2). Similarly, the diffuse ultraviolet 

emission from inter-arm regions can either be from localized O-stars or scattered (Crocker+ 2015). 

A way to test the nature of the diffuse UV emission in any of these galaxies that is completely 

independent from the panchromatic radiative transfer analysis is UV polarimetry, suggested by 

Baes & Viane (2016), Hodges-Kluck & Bregman (2014). If the UV halo emission is dominated by 

scattered radiation, we would expect a strong linear polarisation signature, similar as seen in 

reflection nebulae. Unfortunately, there is currently no UV polarisation instrument available. The 

total emission at these wavelengths is probably dominated by direct light from the stellar halo, but 

it is possible that the signature of a very extended dust distribution might still be visible in 

polarised light. Detailed polarised radiative transfer simulations should be used to test this. 

13.4.2 The role of LUVOIR

LUVOIR can observe the diffuse ultraviolet haloes in imaging, obtain spectra to identify the 

predominant stellar population behind them and polarization observations of the halo to 

determine how much of the light is scattered or not. 

And because LUVOIR has the sensitivity and spatial resolution far surpassing existing UV 

observatories, it can map diffuse, extended ultraviolet haloes around galaxies at much earlier 

epochs in galaxy-formation, answering the question if dusty cirrus is common in the past. 2 SEON ET AL.

-15 -10 -5 0 5 10 15

x (kpc)

-10

-5

0

5

10

z (kpc)

FUV 2h

22m20s

2h

22m40s

2h

23m00s

42°

15′

42°

20′

42°

25′

-15 -10 -5 0 5 10 15

x (kpc)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Radial profile (×103 CU)

FUV

-10 -5 0 5 10

z (kpc)

100

101

102

103

104

Vertical profile (CU)

FUV

-15 -10 -5 0 5 10 15

x (kpc)

-10

-5

0

5

10

z (kpc)

NUV 2h

22m20s

2h

22m40s

2h

23m00s

42°

15′

42°

20′

42°

25′

-15 -10 -5 0 5 10 15

x (kpc)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Radial profile (×103 CU)

NUV

-10 -5 0 5 10

z (kpc)

100

101

102

103

104

Vertical profile (CU)

NUV

Figure 1. GALEX maps and intensity proÞles of NGC 891. The images were rotated such that the major axis of the disk is horizontal and scaled as asinh.

Contour levels correspond to I = 250, 450, 700, 1000, 1500, 2500, 4000, and 7000 photons cm−2 s−1 sr−1 Å−1(continuum unit; CU) above the background

intensity. The contours were made after smoothing the images by a Gaussian function with full width at half maximum (FWHM) of 10 pixels (corresponding

to 0.7 kpc). However, the displayed images were not smoothed. Concentric contours and white regions are artifacts due to the masking of foreground stars. A

distance of 9.5 Mpc to NGC 891 was assumed. Black solid lines represent the observed proÞles. The red curves are the best-Þt two-exponential functions for the

vertical proÞles. The vertical proÞles were obtained by averaging over the whole disk and the radial proÞles over |z| < 5 kpc.

images (Xilouris et al. 1998). The asymmetry is consistent

with the idea of a trailing spiral structure with the dust

behind OB stars and the fact that the SW side is the receding

side (van der Kruit & Searle 1981; Kamphuis et al. 2007).

the point spread function (PSF) of GALEX is approximately

Gaussian with an extended wing at large radii. However, the

extended wing is too small to account for the extended UV

proÞle. The full width at half maximum (FWHM) of the PSF

Figure 1: The NGC 891 ultraviolet halo from Seonet al. (2014).
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-13

much earlier epochs in galaxy-formation, 

answering the question of whether dusty 

cirrus is common in the past.

13.4.3 The science program

The science program consists of deep 

imaging to detect ultraviolet haloes around 

galaxies. Ultraviolet spectroscopy of the 

haloes will determine the dominant stellar 

Figure 13.8. A sketch of the model from Seon & Draine (2016). It shows ultraviolet rays directly 

or scattered into the line-of-sight towards the observer.

populations and polarization imaging will be 

of extreme importance as well. Exposure 

times depend on the surface brightnesses 

needed for each galaxy.

References

Crocker et al. 2015, AJ, 808, 76

Baes & Viane, 2016, A&A, 587, 86 

Program at a Glance

Science goal: To find and characterize the origin of the diffuse halo and inter-arm ultraviolet 

light in disk galaxies 

Program details: To detect and characterize the ultraviolet halo, one targets edge-on 

spiral disk galaxies (e.g., NGC 891) and the inter-arm regions in well-resolved face-on 

galaxies. 

Instrument(s) + configuration(s):

LUMOS imaging – for detection and characterization of the extent. 

LUMOS spectroscopy – to determine the predominant stellar type responsible for the 

diffuse ultraviolet emission. 

POLLUX polarimetry – to determine how much of the diffuse light is generated in situ

and how much scattered off cirrus clouds. 

Key observation requirements: Key requirements are both FUV and NUV imaging to 

extremely low surface brightnesses ~32 mag/arcsec2, R=3000 spectroscopy for stellar 

populations. A polarimeter to measure polarized light fraction. 

13

13.4.3 The science program

The science program consists of deep imaging to detect ultraviolet haloes around galaxies.

Ultraviolet spectroscopy of the haloes will determine the dominant stellar populations and 

polarization imaging will be of extreme importance as well. Exposure times depend on the surface 

brightnesses needed for each galaxy.

2 SEON & DRAINE

L-Rs

Rs

L

L

L

scattered light

scattered light

direct light

high

density

low density

stars

observer

gure 26. Scattering-dominant geometry. Direct starlight from a spherical region with a radius of Rs is attenuated by a dense cylinder with the same ra

Starlight is scattered not only by the high-density cylindrical region but also by a low-density region outside of the high-density region and measured by 

server. The optical depths of the low- and high-density media are defined by ⌧low,V = V ⇢lowL and ⌧high,V = V ⇢high(L -Rs), respectively, where ⇢low and ⇢

densities of the media and V is extinction coefficient at V-band. An additional variable ⌧ 0

high,V is also defined by V ⇢lowL to represent the optical dept

high-density medium at the limit of Rs = 0. There is no dust in the source region.

comes larger (shallower) and E(B-V) becomes smaller as

s and/or Rs/Rd increase. The properties were already noted

Sections 3.1 and 3.2. Therefore, large variations in the vs

B-V) diagram shown in Figures 8 and 10 of Salmon et al.

015) may be attributable to diversity of Ms and Rs/Rd rather

an to variation in the intrinsic dust properties.

Figure 25 shows the vs AV relation, which is also conve

ent in interpreting observational results. In the figure, we

so compare with the relation derived from Equation (10)

r RV = AV /E(B -V)=4.05 ± 0.80 as in the Calzetti curve.

he vs AV relation leads to conclusions similar to those de

ribed for Figure 24, except that the LMC-WD dust type

with Ms ⇠ 10 and Rs/Rd ⇠ 0.8) now best matches the relation

ferred from Equation (10). However, we note that intrinsic

st properties are not well constrained only with the obser

tional data of vs E(B-V) or vs AV . Both the MW-WD

d SMC-WD dust types can explain the observational data of

lmon et al. (2015) equally well, but the attenuation curves

rived with the SMC-WD dust type are not consistent with

e overall shape of the Calzetti curve as shown in Figure 10.

he MW-WD model with a UV bump strength reduced by a

ctor of ⇠ 0.3 - 0.4 is preferred to explain the observational

enuation curves of star forming galaxies, as described in

ctions 3.2 and 3.6.

4. DISCUSSION

We showed that the primary determinant of the attenuation

rve is not the underlying extinction curve but the absorption

rve. Inoue et al. (2006) studied the UV color variation by

rying the wavelength dependence of the scattering albedo

hilkithMW titifiditf

2011). Fischera et al. (2003) state that it is somewhat s

prising that their attenuation model works well even with

the inclusion of scattered light. In fact, this is not surpris

because absorption is the most important factor in produc

attenuation curves.

As noted in Buat et al. (2012), radiative transfer models, 

cluding ours and those of Witt & Gordon (2000), indicate t

SED fitting codes must allow for variations of the attenuat

curve to fit the UV continuum of galaxies. Our radiative tra

fer results for a wide range of configurations considered in 

present paper indicate that the modified Calzetti curve (Eq

tion 6) is versatile enough to reproduce theoretical attenuat

curves as well as to model galactic SEDs.

In the following, we further discuss three topics that 

relevant to our results.

4.1. Correlations with Galaxy Properties

It appears that the UV bump feature is present in typi

star-forming galaxies with generally weaker strengths than 

the MW extinction curve. The UV strength seems to be 

sociated with galaxy activity. Noll et al. (2009b) found t

galaxies with evidence of a UV bump feature host older s

lar populations than galaxies lacking an evident bump. It w

also shown that more active galaxies with higher specific s

formation rates (SFRs) tend to have weaker UV bumps (W

et al. 2011; Buat et al. 2012; Kriek & Conroy 2013).

This trend can be explained by one of the following s

narios or by their combinations: (1) variations in the gr

size distribution, (2) age-dependent extinction, and (3) rad

tive transfer effects. First of all, variations in the grain s

ditibtithtdThblbt

Figure 2: A sketch of the model from Seon & Draine (2016). It shows ultraviolet rays directly or 

scattered into the line-of-sight towards the observer.

Program at a Glance

Science goal: To find and characterize the origin of the diffuse halo and inter-arm ultraviolet 

light in disk galaxies 

Program details: To detect and characterize the ultraviolet halo, one targets edge-on spiral 

disk galaxies (e.g. NGC 891) and the inter-arm regions in well-resolved face-on galaxies. 

Instrument(s) + configuration(s):

LUMOS imaging – for detection and characterizing the extent. 

LUMOS spectroscopy – to determine the predominant stellar type responsible for the diffuse 

ultraviolet emission. 

POLLUX polarimetry – to determine how much of the diffuse light is generated in situ and how 

much scattered off cirrus clouds. 

Key observation requirements: Key requirements are both FUV and NUV imaging to extremely 

low surface brightnesses ~32 mag/arcsec2

, R=3000 spectroscopy for stellar populations. A 

polarimeter to measure polarized light fraction. 
LUVOIR The Large UV Optical Infrared Surveyor

A-14 The LUVOIR Interim Report

De Geyter et al. 2015, MNRAS, 451, 1728

Hodges-Kluck & Bregman, 2014, AJ, 789, 

13

Holwerda et al. 2012, A&A, 541, 5

Mosenkov et al. 2016, A&A, 592, 71

Mosenkov et al. 2018, in prep.

Popescu et al. 2011, A&A, 527, 109

Seon et al. 2014, ApJ, 785, 18

Seon & Draine, 2016, ApJ, 833, 201
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-15

13.5 Prospects for mapping terrestrial exoplanets with LUVOIR

Jacob Lustig-Yaeger (University of Washington), Victoria S. Meadows (University of 

Washington), Guadalupe Tovar (University of Washington), Edward Schwieterman (University 

of California, Riverside), Yuka Fujii (Tokyo Institute of Technology)

13.5.1 Introduction

Future direct imaging missions offer a means 

to characterize the surface habitability 

of terrestrial exoplanets, probing deeper 

atmospheric regions not accessible to 

transmission spectroscopy. Nulling the star 

with a coronagraph enables directly-imaged 

photometry and spectroscopy to capture 

disk-integrated light emitted and reflected 

from an exoplanet along an ensemble of 

relatively short atmospheric paths, compared 

to transmission spectroscopy which probes 

the upper regions of atmospheres along 

longer slanted paths (Fortney 2005). Photons 

on more direct paths through the atmosphere 

encounter lower optical depths, and 

therefore possess a greater sensitivity to the 

lower atmosphere and surface. Geometric 

arguments alone make direct imaging well 

suited for future habitability and biosignature 

assessments of terrestrial exoplanets.

To complement coronagraph spectroscopy of the disk-averaged surface and atmosphere, time-series observations offer 

a unique window into the two-dimensional 

surface heterogeneity of terrestrial exoplanets. Rotating planets that are spatially heterogeneous induce photometric variability 

in their observable light curves (Ford et al. 

2001; Palle et al. 2008; Oakley et al. 2009). 

Numerous studies have explored time-series 

observations for exoplanet surface identification and mapping (for a recent review, see 

Cowan and Fujii 2017). Multi-wavelength, 

time-series observations of Earth have been 

used to construct longitudinal maps of land, 

ocean, and clouds (Cowan et al. 2009; Fujii 

et al. 2010, 2011). Further generalizing these 

observations and modeling methods have 

shown promise for uncovering the reflectance spectrum and longitudinal distribution 

of individual surfaces on exoplanets (Cowan 

et al. 2013), including the possibility of detecting extrasolar oceans (Fujii et al. 2017). 

Longitudinal mapping using multi-wavelength, time-series observations of terrestrial 

exoplanets offers a path towards identifying 

oceans and assessing habitability. 

13.5.2 The role of LUVOIR

LUVOIR is critical for performing surface 

habitability studies of terrestrial exoplanets 

because it is a large aperture, space-based, 

coronagraph-equipped telescope. A spacebased observatory is necessary for (a) 

achieving the coronagraph contrast ratio 

required to observe Earth-like exoplanets 

(~10-10), (b) escaping the diurnal cycle of 

Earth for continuous observations that 

may span multiple days, and (c) accessing 

wavelengths that are optically thick to 

space from the ground due to atmospheric 

opacity. In particular, the near-UV (~300 nm) 

is sensitive to the cloud variability of Earthlike planets. The large aperture considered 

for LUVOIR (~12–15m) will also allow both 

shorter cadence time-series (~1 hour) – 

providing higher surface resolution to the 

inferred maps—and access to solar-analog 

systems out to a greater distance, which 

increases the yield for such investigations. 

13.5.3 The science program

We propose a science program that enhances 

the science return from spectroscopic 

observations that motivate the design of the 

LUVOIR coronagraph. Approximately 100 

hours will be required to detect the presence 

of O2 in the atmosphere of an Earth-like 
LUVOIR The Large UV Optical Infrared Surveyor

A-16 The LUVOIR Interim Report

exoplanet at 10 pc with the 15-m LUVOIR 

concept. Our program will leverage these 

long integrations to “mine” for rotationally 

induced exoplanet time variability using 

individual ~1 hour exposures from a longer 

composite spectrum. If these observations 

include data within the 0.7–0.8 μm LUVOIR 

bandpass (which contains the 0.76 μm O2 A 

band), we can bin the spectrum into a single 

photometric point and construct a lightcurve 

of a terrestrial exoplanet over the total 

spectrum exposure time. Our simulations 

suggest that this dataset can be used to 

infer the rotation rate of Earth to within ~5% 

(Lustig-Yaeger et al. 2017). Binning the 

time-dependent spectra into two or more 

photometric points is an effective means of 

constructing simultaneous multi-wavelength 

lightcurves, which could potentially infer the 

crude longitudinal distribution and color of 

oceans on an Earth analog (Lustig-Yaeger 

et al. 2017). Other wavelengths can be used 

either instead of or in concert with the 0.7–0.8 

μm LUVOIR bandpass, such as the 0.4–0.45 

μm bandpass, where Rayleigh scattering 

masks surface features and enhances 

sensitivity to heterogeneous cloud coverage. 

However, the 0.7–0.8 μm bandpass is still 

preferred for the initial assessment because 

of its sensitivity to both atmospheric oxygen 

and the planetary surface. 

This science program would allow a 

long spectral exposure, to yield two unique 

studies along both the spectral and temporal 

dimensions. Co-adding in time will give a long 

integration spectrum at the native instrument 

spectral resolution that can be used to study 

the atmospheric composition of the targeted 

exoplanet. Co-adding in wavelength will 

give a long baseline lightcurve with little to 

no spectral resolution that can be used to 

study the targeted exoplanet’s rotation rate 

and surface map. Since the time-resolution 

of a lightcurve depends on the individual 

exposure times that comprise the timeseries data, low noise or noiseless detectors 

that can support shorter exposure times will 

greatly improve the feasibility and scientific 

return of this program. 

References

Cowan, N. B., Agol, E., Meadows, V. S., et 

al. 2009, ApJ, 700, 915

Cowan, N. B., & Strait, T. E. 2013, ApJL, 

765, L17

Cowan, N. B., & Fujii, Y. 2017, ArXiv 

e-prints, arXiv:1704.07832

Ford, E. B., Seager, S., & Turner, E. L. 2001, 

Nature, 412, 885 

Fortney, J. J. 2005, MNRAS, 364, 649

Fujii, Y., Kawahara, H., Suto, Y., et al. 2010, 

ApJ, 715, 866

Program at a Glance

Science goal: Study the rotation rate, surface colors, geography, and habitability of 

directly imaged terrestrial exoplanets.

Program details: Construct time-series observations of habitable terrestrial exoplanets 

using the single spectrum exposures that will be required to build-up much longer 

integration times for spectroscopy. 

Instrument(s) + configuration(s): ECLIPS coronagraph spectroscopy

Key observation requirements: Single coronagraph bandpass (0.7–0.8 μm), R = 100–

200, S/N ≥ 0.5 per spectral element per exposure (S/N ≥ 2.5 per bandpass per exposure), 

coronagraph design contrast 10-10
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-17

Fujii, Y., Kawahara, H., Suto, Y., et al. 2011, 

ApJ, 738, 184

Fujii, Y., Lustig-Yaeger, J., & Cowan, N. B. 

2017, AJ, 154, 189

Lustig-Yaeger, J., Tovar, G., Schwieterman, 

E. W., Fujii, Y., & Meadows, V. S. 2017, 

LPI Contributions, 2042, 4110

Oakley, P. H. H., & Cash, W. 2009, ApJ, 

700, 1428

Pallé, E., Ford, E. B., Seager, S., MontañésRodríguez, P., & Vazquez, M. 2008, 

ApJ, 676, 1319
LUVOIR The Large UV Optical Infrared Surveyor

A-18 The LUVOIR Interim Report

13.6 Spatially resolved maps of star-forming gas with LUVOIR

John M. O’Meara (Saint Michael’s College)

13.6.1 Introduction

Strong HI absorption, specifically the 

Damped Lyman alpha systems, have been 

used for decades to study the bulk of the 

neutral gas in the universe (Wolfe et al. 2005), 

and thus the reservoirs for star formation. 

Large samples of DLA now exist (e.g., 

Noterdaeme et al. 2012) and the metallicity 

for the DLA has been obtained for hundreds 

of systems (Rafelski et al. 2012). Despite 

their long history of study, the size of the DLA 

at high-z has gone largely unconstrained, as 

they are observed toward very small emitters 

on the sky (e.g., quasars or GRBs). If instead 

spatially extended objects such as galaxies 

are used as a background source, this would 

allow for a direct study of the spatial extent 

of any foreground intervening DLA gas, 

along with a map of its variation in metallicity. 

This methodology can be applied to higher 

HI column density Lyman limit systems as 

well, provided their Lyman alpha line alone is 

sufficient to determine N(HI).

Figure 13.9 demonstrates this idea 

showing variable absorption towards a 

strong gravitationally lensed galaxy at z~2.8 

(O’Meara et al. 2018, in prep) with KCWI 

on Keck. Gravitationally lensed galaxies are 

unfortunately rare, limiting our ability to apply this technique en masse and to compare the results statistically to simulations. 

Furthermore, ground based observations of 

the DLA are limited to z>1.6, excluding the 

majority of the history of the universe, and 

with it much of the transition of galaxies from 

star-forming to passive, an epoch where a 

detailed understanding of the ”where and 

when” of star-forming gas is critical.

Figure 13.9. Keck KCWI spectra of a z~2.8 lensed galaxy. Spectra from multiple positions along 

the galaxy show variations in HI column density in an intervening z~2.5 DLA. Metal lines are also 

observed for this system, allowing for constraints to be made on variations in gas metallicity on 

kpc scales, similar to scales in cosmological simulations.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-19

13.6.2 The role of LUVOIR

LUVOIR provides two essential opportunities: 

First, the UV capabilities of LUMOS allow for 

the technique to be applied for HI Lyman 

alpha anywhere from 0 < z < 2.2, essentially 

the last 11 billion years of cosmic history. 

At lower redshifts, the extent on the sky of 

the background galaxy increases, facilitating 

finer and finer spatial sampling. Second, 

LUVOIR’s immense aperture allows for the 

technique to be applied to the more general 

population of galaxies. As a result, instead 

of relying on very rare gravitational lens 

scenarios, galaxies at FUV magnitudes of 

20 (for LUMOS high resolution modes) or 

23 (for low resolution modes still capable 

of detecting HI and metal absorption) are 

reachable in ~1 hr integration time.

13.6.3 The science program

LUMOS shutters will be placed across z< 

2 galaxies (identified either through ground 

based spectra or photometry), ideally in the 

M modes for higher resolution (FUV mag 20.5 

or brighter). Selection of G120M, G150M, 

G180M, G300M will depend on the redshift of 

the intervening absorption, and will be made 

to cover HI Ly-a, along with key metal line 

diagnostics such as OI, SiII, and CII. Sources 

can be as faint as FUV mag 23, if the G145LL 

mode is employed. Observations will reach 

S/N = 10 or greater, which is possible for 

all M gratings for sources FUV mag 20.5 or 

brighter.

Each LUMOS microshutter spectrum 

will be analyzed for intervening HI and metal 

line absorption, and a spatial map of HI and 

metallicity variations will be made for each 

absorber.

Follow-up observations with either 

LUMOS or HDI may be warranted to observe 

the galaxies associated with the HI gas. 

References

Wolfe, A. M. et al. 2005, ARA&A, 43, 861W 

Noterdaeme, P. et al. 2012, A&A, 547L

Rafelski, M. et al. 2012, ApJ, 755, 89

O’Meara, J. M. et al. 2018, in prep.

Program at a Glance

Science goal: Spatially resolved maps of HI and metals in star-forming gas at 0 < z < 2

Program details: LUMOS observations of z < 2 galaxies with intervening HI and metal 

absorption. Multiple LUMOS MOS shutters across each galaxy provide the spatial sampling.

Instrument(s) + configuration(s): LUMOS G120, G150, G180, G300M

Key observation requirements: S/N > 10 per spectrum. Wavelength coverage of HI 

Lyman-alpha and key metallicity tracers (OI, CII, SiII) all in the rest frame FUV.
LUVOIR The Large UV Optical Infrared Surveyor

A-20 The LUVOIR Interim Report

13.7 The quiescent UV spectra of cool dwarf stars

Allison Youngblood (NPP / NASA GSFC)

13.7.1 Introduction

The UV spectra of cool dwarf stars (F, G, 

K, & M) are dominated by emission lines 

originating from the magnetically heated 

upper layers of the stellar atmosphere 

(Figure 13.10); <10% of their photospheric/

blackbody continuum is emitted in the UV (λ

< 3000 K). Chromospheric, transition region, 

and coronal emission lines from species like 

H I, Mg II, C IV, Si IV, and N V are ultimately 

powered by the star’s magnetic dynamo, 

which also controls the rotational evolution 

of these stars via magnetized stellar winds 

(Skumanich 1972). UV emission line fluxes 

and widths can diagnose temperature, 

pressure, density, and kinematics throughout 

the stellar atmosphere, even indicating H2 

fluorescence from the star’s own UV photons 

(e.g., Kruczek et al. 2017, Jaeggli et al. 

2018). When obtained for a wide range of 

masses and ages, UV spectra can elucidate 

the evolutionary processes of cool stars 

(e.g., Guinan et al. 2003).

Of special note is H I Lyman alpha 

(1215.67 Å), the brightest emission line in the 

FUV and NUV spectrum of a cool dwarf. A 

Figure 13.10. The UV spectrum from 1000—3000 Å for a sample of cool dwarfs observed with 

Hubble and Solar Mesophere Explorer. Lyman alpha, N V, Si IV, C IV, and Mg II are labeled with 

tick marks.
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The LUVOIR Interim Report A-21

complete UV characterization of these stars 

cannot be obtained without it, but Lyman 

alpha observations are challenging. The 

Earth’s extended atmosphere glows brightly 

in Lyman alpha, and interstellar H I attenuates 

the entire line core for even the closest stars. 

The core of the Lyman alpha line must be 

reconstructed from the observed line wings 

(e.g., Youngblood et al. 2016), which can 

prove difficult for faint targets.

13.7.2 The role of LUVOIR

LUVOIR’s LUMOS instrument will have a 

sensitivity two orders of magnitude better 

than HST’s STIS, enabling measurements 

of the chromospheric heating rate for even 

the most seemingly inactive cool stars (e.g., 

older M dwarfs; Guinan et al. 2016). LUMOS 

will provide access to more emission lines 

with a wider range of formation temperatures 

(probing from the lower chromosphere to the 

corona) not only because of its increased 

sensitivity but also its increased spectral 

range compared to STIS and COS (e.g., 

sensitive access to 1000–1200 Å). Its 

microshutter array can also create a long 

slit, which is essential for spatially separating 

the stellar Lyman alpha emission from the 

bright geocoronal Lyman alpha emission, 

and the sensitivity should be great enough to 

measure continuum emission.

13.7.3 The science program

To obtain a 1000 Å–3000 Å spectrum of a 

cool dwarf, a combination of at least two 

gratings must be employed. Using the 

medium resolution gratings (R~30,000) 

throughout will resolve line multiplets (e.g., 

C II, O IV) and the line widths themselves. 

Two lower resolution gratings (R~8,000 and 

500) are also suitable alternatives for fainter 

targets.

After 5 hours of integration (representative 

of the integration time for the K and M dwarfs 

of HST’s MUSCLES Treasury Survey; France 

et al. 2016), S/N = 10 for the faintest emission 

lines of an early M dwarf’s FUV spectrum will 

be achieved for stars down to FUVGALEX = 23 

mag (9-m LUVOIR) or FUVGALEX = 25 mag 

(15-m LUVOIR). For reference, the M2 V star 

GJ 832 at d = 5 pc is FUVGALEX = 21 mag. 

LUMOS’s microchannel plate detectors 

enable time-tagging photons as they arrive, 

so any flares that occur during integration will 

be able to be distinguished from quiescent 

emission, warranting their own analysis.

References

France, K., Loyd, R. O. P., Youngblood, A., 

et al. 2016, ApJ, 820, 89

Guinan, E. F., Ribas, I., & Harper, G. M. 

2003, ApJ, 594, 561

Jaeggli, S. A., Judge, P. G., & Daw, A. N. 

2018, arXiv:1802.03779

Kruczek, N., France, K., Evonosky, W., et al. 

2017, ApJ, 845, 3

Skumanich, A. 1972, ApJ, 171, 565

Youngblood, A., France, K., Loyd, R. O. P., 

et al. 2016, ApJ, 824, 101

Program at a Glance

Science goal: Characterizing the quiescent UV spectra of cool dwarf stars (F, G, K, and 

M dwarfs) to better understand the physical conditions in their atmospheres and how they 

evolve in time.

Program details: F, G, K, and M dwarfs covering a wide parameter space of mass and 

age should be targeted. 

Instrument(s) + configuration(s): LUMOS spectroscopy.

Key observation requirements: 1000-3000 Å, R=30,000, S/N > 10.
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A-22 The LUVOIR Interim Report

13.8 Exoplanet diversity in the LUVOIR era

Ravi Kopparapu (UMD/NASA GSFC), Eric Hebrard (University of Exeter), Rus Belikov (NASA 

Ames), Natalie M. Batalha (NASA Ames), Gijs D. Mulders (LPL), Chris Stark (STScI), Dillon 

Teal (NASA GSFC), Shawn Domagal-Goldman (NASA GSFC), Avi Mandell (NASA GSFC), 

Aki Roberge (NASA GSFC)

13.8.1 Introduction

In the search for exoEarth candidates with 

LUVOIR, we will undoubtedly detect a 

multitude of brighter planets. According to 

Stark et al. (2014), for an 8-m size telescope, 

the number of exoEarth candidates detected 

is ~ 20 (see Figure 4 in Stark et al. 2014), 

although this is strongly dependent on the 

value of hEarth. At the same time, the number 

of stars that must be observed to detect 

these exo-Earth candidates numbers in the 

hundreds. If we assume that on average, 

every star has a planet of some size (Cassan 

et al. 2012; Suzuki et al. 2016), then there are 

hundreds of exoplanets of all sizes that can be 

observed. Not considering the ~20 exoEarth 

candidates, the bulk of the exoplanets will fall 

into “non-Earth” classifications, without any 

distinguishing features between them. This 

provides a motivation to devise a scheme for 

classifying exoplanets based on planetary 

size and corresponding comparative 

atmospheric characteristics of exoplanets.

With some exceptions of Venus-type 

exoplanets (Kane et al. 2012), there has not 

been an overarching way to classify planets 

beyond the habitable zone (HZ). Classifying 

different size planets based on the transition/

condensation of different species at different 

stellar fluxes (i.e., orbital distances) provides a 

physical motivation for estimating exoplanet 

mission yields, separate from exoEarth 

candidate yields (Figure 13.11).

13.8.2 The role of LUVOIR

The histogram plot (Figure 13.12) visualizes 

the total scientific impact of the habitable 

planet candidate survey, along with several 

Figure 13.11. The boundaries of the boxes represent where different chemical species are 

condensing in the atmosphere of a planet at a given stellar flux, according to equilibrium chemistry 

calculations Credit: Kopparapu et al. (2018)
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The LUVOIR Interim Report A-23

other classes of exoplanets, based on 

different telescope diameters. The y-axis 

gives the expected total numbers of 

exoplanets observed (yields), which are also 

given by the numbers above the bars. It is at 

the LUVOIR-type telescope sizes (~8-m and 

15-m size) that one can truly see the diversity 

in exoplanet yields, and further characterize 

different classes of planets. We note that in 

general, larger apertures are less sensitive to 

changes in mission parameters than smaller 

apertures.

13.8.3 The science program

With a 4-m class mission, observations 

that are designed to maximize the yield of 

potential Earths will also yield the detection 

and characterization of all of the planet 

types discussed here, with the exception of 

hot Jupiters. Hot Jupiters are not observed 

by a 4-m class mission because the tight 

inner working angle, and because of the low 

abundances of hot Jupiters.

A 15-m telescope will bring the ability 

to not only observe planets, but to test the 

occurrence of different features within each 

of the planet types. It would observe dozens 

of each planet type, providing larger sample 

sizes which enables the study of each planet 

type as a population.

References

Kane, S. R., Kopparapu, R. K., DomagalGoldman, S. 2014, ApJ, 794, L5

Kopparapu, R. K., Hebrard, E., Belikov, R., 

et al. 2018, ApJ, in press 

Stark, C. C., Roberge, A., Mandell, A., 

Robinson, T. D. et al. 2014, ApJ, 795, 

122

Stark, C. C. et al. 2015, ApJ, 808, 149

Program at a Glance

Science goal: Measure atmospheric compositions of a wide range of exoplanets with 

different sizes, orbits, and host stars.

Program details: IFS optical and near-IR spectra of a) all planets in the field during long 

integrations on stars with habitable zone planet candidates and b) a selected set of known 

planets around other stars.

Instrument(s) + Configuration: ECLIPS high-contrast, point-source spectroscopy

Key observation requirements: Spectral bandpass from 200 nm to 2000 nm, R ~ 100, 

Continuum SNR > 10

Figure 13.12. Expected number of exoplanets observed for hot (red), warm (blue) and cold (iceblue) incident stellar fluxes shown in Figure 13.11. The left panel is for a telescope size of 4-m 

and the right is for 15-m.
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13.9 Detecting exomoons with LUVOIR

Eric Agol (University of Washington, NASA Astrobiology Virtual Planetary Laboratory), Tiffany 

Jansen (Columbia University), Brianna Lacy (Princeton University), Tyler D. Robinson (Northern 

Arizona University), Victoria Meadows (University of Washington, NASA Astrobiology Virtual 

Planetary Laboratory)

13.9.1 Introduction

The detection of exomoons has proven 

elusive, and is becoming an increasingly prominent goal of exoplanet studies. 

LUVOIR can open the possibility of the detection of exomoons in the habitable zones 

of main sequence stars via spectroastrometry: the astrometric shift versus wavelength that occurs between wavelengths 

with flux dominated by the exoplanet vs. 

wavelengths dominated by the exomoon. 

The several requirements to reach this goal 

are: 1) a large telescope aperture; 2) astrometric calibration and stability; 3) ability to 

measure the centroid of the planet’s light simultaneously over a broad range of waveFigure 13.13. The flux of the Earth-like moon and the warm Jupiter at quadrature phase angle as 

a function of wavelength in microns (top) and the contribution of flux due to the moon shown as 

a fraction of the total flux (bottom). The maximum fraction of the moon’s flux for the Jovian-Earth 

system occurs at λ = 1.83 μm, contributing 99.1% to the total flux.
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The LUVOIR Interim Report A-25

lengths; 4) ability to revisit the planet many 

times; and 4) extension to near-IR wavelengths. In addition to enabling the detection 

of exomoons, this technique may allow for 

the characterization of the planet via the orbit 

of the exomoon.

The Moon has likely played a critical role in 

the evolution of planet Earth, from influencing 

the geological and chemical composition 

(Smith 1977; Canup 2012), to modifying the 

mass and angular momentum of the Earth/

Moon system (Canup & Asphaug 2001), 

to possibly stabilizing the obliquity of the 

Earth (Laskar et al. 1993). Lunar tides may 

have played an outsized role in influencing 

the evolution of life on Earth (Balbus 2014). 

Speculation that habitable exomoons could 

orbit giant planets makes an intriguing 

alternate niche for life (Williams et al. 1997; 

Kaltenegger 2010). Caveats exist for each of 

these scenarios (Lissauer et al. 2012; Heller 

et al. 2014; Lammer et al. 2014), so a search 

for exomoons is necessary to ascertain the 

importance of moons on planetary physics 

and biology. 

There are a range of proposed techniques 

for detection of exomoons, from transiting 

planet observations (Cabrera & Schneider 

2007; Kipping 2009), to phase function 

measurements in direct imaging Moskovitz 

et al. (2009); Robinson (2011). Each of these 

techniques drawbacks: transiting exomoons 

may be swamped by stellar variability, while 

phase variations may be mimicked by 

variations in planetary atmospheres. 

We recently proposed a new approach for 

the detection of exomoons that could result in 

an instantaneous detection of an exomoon, 

Figure 13.14. Diagram indicating spectroastrometric signal.
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A-26 The LUVOIR Interim Report

and would allow for measurements of the 

exoplanet-moon system: spectroastrometry 

(Agol et al. 2015). This technique relies on 

the observation that a moon can outshine 

a planet at wavelengths where the planet 

is non-reflective, i.e., in strong molecular 

absorption bands (Williams & Knacke 

2004; Moskovitz et al. 2009). The Earth is 

outshined by the Moon at ≈2.7 μm, and the 

Earth would outshine a companion Jovian 

planet in several methane bands between 

0.9–2.7 μm (Figure 13.13). The technique 

is summarized in Figure 13.14, which 

schematically shows how the centroid of 

the PSF may vary between wavelengths 

dominated by the planet versus wavelengths 

dominated by the moon. 

The detection technique proceeds by 

simultaneous measurement of the centroid 

as a function of wavelength. The wavelengths 

at which the planet is dim would have a 

stronger contribution from the moon, and 

thus cause a shift of the centroid towards the 

position of the moon. Thus, even if the moon 

and planet are not resolved, the moon may 

still be detected via the spectroastrometric 

signal. 

The astrometric signal, which is 

proportional to the angular separation of the 

planet and moon, scales as r/d, where r is 

the separation of the moon and planet on the 

sky, and d is the distance to the system. The 

astrometric noise scales as (λ/D)(d/D), as the 

PSF width scales as λ/D, while the precision 

of the measurement of the centroid scales as 

d/D, where D is the diameter of the telescope. 

Thus, the signal-to-noise scales as r(D/d)2/λ, 

and so the ability to search a large volume 

V for exomoons within the nearby Galaxy 

scales as V ∝ d3 ∝ D3. To increase the odds 

of a successful search, then, requires the 

largest diameter telescope possible. 

This relation assumes that the Poissonnoise limit applies, and thus also requires: 

1) high-contrast to avoid additional 

contribution to the noise from scattered light; 

2) precise control of the PSF, the astrometric 

pointing, and the relative astrometric 

precision between wavelengths. Ideally this 

approach will be made more efficient with 

simultaneous astrometric measurements 

at a range of wavelengths, which might 

be accomplished with an IFU or an MKID 

device. One advantage of this technique is 

that only the relative position needs to be 

measured as a function of wavelength, not 

the absolute position. Another advantage 

is that the signal will vary with time as the 

moon orbits the planet, which makes it 

reproducible and difficult to confound with 

other systematic errors or sources of noise. 

This makes this technique more powerful 

than other proposed techniques for detecting 

exomoons, which may require observations 

at specific times or geometries (such as transit 

of the moon in front of the star, or transit of 

the moon in front of the planet), or may not 

have repeatability such as microlensing (Han 

& Han 2002). Spectroastrometry may also 

allow for the measurement of the mass of the 

planet+moon with Kepler’s law, as well as a 

disentangling of the moon/planet spectrum 

(Agol et al. 2015).

13.9.2 The role of LUVOIR

Spectroastrometry of habitable-zone (HZ) 

Earths will not be feasible from groundbased telescopes as these may only probe 

the HZ of late M dwarf stars, for which the 

planets are close enough that they cannot 

harbor stable exomoons. A space telescope 

is required to achieve high contrast (10−11–

10−10) to enable the detection of an exomoon, 

to avoid interference of the atmosphere with 

spectral features that show the sharpest 

variation in the spectroastrometric signal, to 

allow for repeated visits and long exposures, 

and to achieve stability of the PSF.

A large telescope will be necessary due 

to the strong scaling of the astrometric 
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The LUVOIR Interim Report A-27

signal-to-noise with telescope aperture 

(∝ D2), which will allow the search for this 

signal for a larger number of stars. The 

larger aperture allows for a sharper PSF and 

a larger number of photons, both of which 

improve the astrometric precision and allow 

for resolving out surface brightness due to 

(exo)zodi and speckles. 

Relative astrometric precision between 

bands of ≈0.1 mas should be sufficient for 

the expected ≈1 mas shift for the two cases 

examined here. For the Earth-Moon at Alpha 

Centauri example, near-infrared coverage to 

>1.5 mm will be required, while coverage to 

2.7 mm would be preferred to obtain a strong 

Moon/Earth flux ratio. For the Earth-Jupiter 

case, coverage to 1 μm should allow for detection, while 2 μm will give a stronger Earth/

Jupiter flux ratio. A broad range of wavelengths will need to be imaged simultaneously, which an IFU or MKID detector may 

allow for. The inner working angle and field of 

view may accommodate searches closer or 

further from stars, while for the two examples 

we explored we just examined the HZ, which 

is well covered by current LUVOIR specifications. A detailed technical and science case 

for spectroastrometry will require more realistic simulations, which are in progress

13.9.3 The science program

It is impossible to forecast the properties of 

an ensemble of planet-moon systems, and 

so in lieu of this, here are two case studies 

that could drive technical requirements of 

LUVOIR: 1) an Earth-Moon twin orbiting Alpha 

Centauri A (1.34 pc); 2) an Earth-Jovian pair 

orbiting a G2V star at 10 pc with a separation 

of 30% of the Hill sphere. Table 13.1 gives 

the properties of these two systems. One 

could imagine, of course, a broader range of 

possibilities with the Solar System as building 

blocks: e.g., a Mars-sized moon orbiting 

a Neptune-sized ice giant. However, our 

two case studies in some sense bracket a 

range of possibilities, although a true Joviansatellite analog system would probably be 

challenging to detect with this technique. 

Spectroastrometry may be used to search 

for rings of planets since Saturn is dark in 

Program at a Glance

Science goal: Detection of moons orbiting exoplanets.

Program details: Precisely measure the positions of directly imaged exoplanets at 

different wavelengths and times to look for photocenter shifts (spectroastrometry).

Instrument(s) + configuration(s): ECLIPS IFS spectral imaging

Key observation requirements: Contrast > 10-9 for Jupiter orbited by Earth-like moon; 

Contrast > 10-11 for Earth orbited by Moon; NIR wavelengths (> 1.0 μm for Earth-Jupiter 

case, > 1.5 μm for Earth-Moon case); Relative astrometric precision between wavelength 

bands of ≈ 0.1 mas

Table 13.1. The moon and planetary parameters used in our model, where Dtele is the telescope 

diameter, d is the distance from the observer, tobs is the duration of exposure, ε is the telescope 

efficiency factor.
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methane bands, while its ring system is still 

reflective. The pattern of illumination of the 

rings will impart a centroid offset relative to 

the centroid of Saturn.

The Moon-Earth analog centroid offset 

is ≈0.4 mas at 1.4 μm, 0.9 mas at 1.9 μm, 

and 2 mas at 2.7 μm. Thus, near-infrared 

capability is required to apply this technique 

to an Earth-Moon analog system. Relative 

astrometric stability between wavebands 

at better than 80 μas would be required 

to make a detection exceeding 5−σ. The 

planet-star contrast at these wavelengths 

is ≈10−11, and so is going to be affected 

significantly by speckles if a contrast of 

10−10 is achieved. Thus, pushing towards 

a contrast of 10−11 may be necessary for 

detection of a Moon-like exomoon. 

The Earth-Jupiter analog (with Jupiter 

moved into 1 AU) has a centroid offset of 1.3 

mas at 0.86 μm. A 12-meter telescope with 

R=80 could detect this offset at S/N=13 with 

a 24-hour exposure, assuming the Poissonnoise limit. The planet/star contrast at this 

wavelength is ≈10−9, and so will not be as 

affected by speckles for a telescope design 

that approaches 10−10 contrast. 

Our investigation has the drawback that 

we neglected exozodiacal light and speckles, as well as other sources of instrumental noise, so further work to create realistic 

simulations is needed to estimate the impact 

of these on the spectroastrometric signal 

(Jansen et al. in prep.). 

References 

Agol, E., Jansen, T., Lacy, B., Robinson, T. 

D., & Meadows, V. 2015, ApJ, 812, 5 

Balbus, S. A. 2014, Proceedings of the 

Royal Society A: Mathematical, Physical 

and Engineering Sciences, 470, 

20140263 

Cabrera, J., & Schneider, J. 2007, 

Astronomy and Astrophysics, 464, 

1133 

Canup, R. M. 2012, Science, 338, 1052

Canup, R. M., & Asphaug, E. 2001, Nature, 

412, 708

Han, C., & Han, W. 2002, ApJ, 580, 490

Heller, R., Williams, D., Kipping, D., et al. 

2014, Astrobiology, 14, 798

Kaltenegger, L. 2010, ApJ, 712, L125

Kipping, D. M. 2009, Monthly Notices of the 

Royal Astronomical Society, 392, 181

Lammer, H., Schiefer, S.-C., Juvan, I., et al. 

2014, Origins of Life and Evolution of 

Biospheres, 44, 239

Laskar, J., Joutel, F., & Robutel, P. 1993, 

Nature, 361, 615 

Lissauer, J. J., Barnes, J. W., & Chambers, 

J. E. 2012, Icarus, 217, 77

Moskovitz, N. A., Gaidos, E., & Williams, D. 

M. 2009, Astrobiology, 9, 269

Robinson, T. D. 2011, ApJ, 741, 51

Smith, J. V. 1977, in Lunar and Planetary 

Science Conference Proceedings, 

Vol. 8, Lunar and Planetary Science

Conference Proceedings, ed. R. B. 

Merril, 333–369

Williams, D., & Knacke, R. 2004, 

Astrobiology, 4, 400 

Williams, D. M., Kasting, J. F., & Wade, R. 

A. 1997, Nature, 385, 234 
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13.10 LUVOIR for stars, stellar evolution, and the local universe

Bruce Elmegreen (IBM T.J. Watson Research Center)

13.10.1 Introduction

LUVOIR is a ~12-meter class UV-Vis-NIR 

space telescope for the 2030s and beyond. 

Because of the large aperture and short 

observing wavelength, it will have very 

high angular resolution, corresponding to 

an Airy disk of 5 x 10-8 radians for optical 

light (10 milliarcsec). It will also have broad 

wavelength coverage from 100 nm to 2.5 

microns, allowing a wide range of science 

investigations. The field of view is proposed 

to be very wide, making it ideal for surveys: 

the High Definition Imager would have a 

field of view of 2' x 3' with 2.73 mas/pix at 

UVIS and 8.2 mas/pix at NIR. This means 

the image sizes will be 2.9 Gpix and 0.32 

Gpix, respectively, and the file sizes 1–10 

GByte per image. It will also have a multiobject spectrograph from 100 nm–850 nm, 

as well as other instruments. With these 

specifications, taken from Dalcanton et al. 

(2015) and Elmegreen et al. (2017), LUVOIR 

will revolutionize the study of stars, stellar 

evolution, and the local universe.

13.10.2 The role of LUVOIR

The accelerating progress of our view of the 

sky over the last two millennia—the time 

since systematic astronomical studies and 

star catalogs began—is shown schematically 

in Figure 13.15, which plots the number of 

distinguishable pixels in the sky versus time. 

The first major jump from 1 arcmin resolution 

with the eye to 1 arcsec resolution with a 

telescope started in early 1600 with a long 

gap before the Hubble Space Telescope 

improved the resolution further by factor of 

10. The next jump to LUVOIR comes rather 

quickly on this scale, showing another factor 

Figure 13.15. The total number of pixels in the sky, calculated as the ratio of the sky solid angle 

to the angular resolution of the instrument, is plotted versus the year in history. Three major 

technology jumps have occurred, the invention of telescopes, the placement of telescopes 

in space, and the ability to have large telescopes in space. The first two each revolutionized 

astronomy, and the third is likely to as well.
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A-30 The LUVOIR Interim Report

of 10 gain with a 2.6-year doubling time in 

the most recent era. This pace is comparable 

to the advance in general technology, 

reflecting a combination of what’s possible 

and what gains we can expect. From this 

long-term viewpoint, LUVOIR will improve 

our resolution of the sky by the same factor 

over HST as HST improved it over Galileo’s 

first telescope. Some of the science gains 

from this improvement will be discussed in 

the next section.

13.10.3 The science program

Local star formation

Star formation in the solar neighborhood 

will be resolved at 1–10 AU scales, showing 

dusty accretion to protostars, planet-forming regions of young disks, central jets, and 

close binary stars in mutual formation. The 

Orion protoplanetary disks discovered with 

HST—the so-called proplyds (O’dell et al. 

1993)—will be observable at 5 AU resolution 

where their interaction with the hot radiation 

and winds from nearby massive stars will 

be visible. Interactions like these could have 

introduced radioisotopes from supernovae 

into the early Solar Nebula, giving our meteorites their Al-Mg anomalies (MacPherson 

& Boss 2011). The dusty structures of the 

Taurus filaments, which are regions of lowmass star formation, will be observed at 1.4 

AU resolution, which is 10-4 times their size. 

This will easily resolve the transition from supersonic to subsonic turbulence and reveal 

for the first time the thermal, turbulent and 

gravitational processes that form low mass 

stars and their disks.

Imaging stellar surfaces

LUVOIR will be able to resolve and image 

some dozen nearby stars (Ochsenbein & 

Halbwachs 1982; van Belle et al. 1996) to 

look for star spots, limb darkening, rotation, 

flares and other possible features in optical 

and UV light. For example, Betelgeuse has 

been resolved with optical interferometry 

(Haubois 2010), Antares was resolved with 

the VLT1 interferometer (Ohnaka et al. 2013) 

and Mira was resolved with HST (Karovska 

1997). The LUVOIR resolution of 10 mas 

corresponds to a solar radius at a distance of 

0.45 pc. Larger stars like supergiants will be 

resolvable to proportionally larger distances.

Local group galaxies

The Local Group of galaxies, including M31, 

M33, the LMC, SMC, and many smaller 

galaxies, will be resolved at 0.05 pc, allowing 

us to study hundreds of supernova remnants 

and planetary nebula in great detail. From the 

structures and line emission of these nebulae, 

we will learn about the final stages in stellar 

lives, nucleosynthetic element dispersal in 

the surrounding gas, and dust formation in 

dense remnant winds.

The high resolution of LUVOIR will allow 

us to distinguish between individual stars 

as well. According to the calculation of 

integration time versus point source absolute 

magnitude on page 49 of Dalcanton et al. 

(2015), in 1 hour we can see a solar type star 

at a distance of 2.8 Mpc using the F555W 

filter at an apparent AB magnitude of 32. 

Within 2.8 Mpc there are 9 large galaxies, 

i.e., with absolute magnitude less than -16, 

and about 60 small galaxies. At the stellar 

surface density in the solar neighborhood, 

which is about 60 solar masses per pc2, 

there are 150 stars per pc2 larger than 0.01 

solar masses and 10 stars pc-2 larger than 

1 solar mass (using the Kroupa 2001 initial 

stellar mass function). The mean projected 

separations between these stars are 6 mas 

and 24 mas at 2.8 Mpc. The second number 

is larger than the angular resolution of 

LUVOIR, so we will be able to separate solar 

mass stars in galaxy disks within 2.8 Mpc, 

i.e., for dozens of galaxies of various types 

and for the highly diverse conditions in these 

galaxies. By counting these stars and the 

more massive stars, which are separated by 

even larger distances, we can determine the 
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field star mass function and approximate star 

formation history in all regions with fairly low 

extinction. Variations in this field star mass 

function for a given history would suggest 

comparable variations in the stellar initial 

mass function. This will be the first time that 

the stellar IMF can be directly compared to 

environmental conditions and star formation 

rates. Such comparisons should give us 

greater understanding of the origin of the 

IMF.

Nuclear regions of nearby galaxies

LUVOIR will resolve nearby nuclear star 

clusters, disks and black hole activity with 1 

pc resolution or better out to 20 Mpc, which 

includes thousands of galaxies. NGC 300, 

for example, has a nuclear star cluster with 

a 3-pc radius (van der Marel et al. 2007) that 

corresponds to 300 mas—easily resolved by 

LUVOIR. In M31, the nuclear region has a 

108 solar mass black hole with intriguing red 

and blue structures nearby, as observed by 

HST (Lauer et al. 2012). The active galactic 

nucleus in NGC 4261 has an optical disk 

1.7 arcseconds across (Ferrarese et al. 

1996), which is 170 resolution elements. 

The implications are enormous for studies 

of black hole accretion, nuclear disk storms, 

small-scale flaring activity, nuclear spirals 

and torques.

Proper motions with sub-pixel accuracy

Proper motions with HST could reach 0.02 

pixel accuracy (Anderson & King 2000, 

2003) with simultaneous fitting of the average 

stellar point spread function and the stellar 

positions. 0.02 pixel for LUVOIR equals 10-9 

radians or 0.21 mas. This means that with a 

5-year baseline, the proper motion that can 

be measured, in km/s, equals 0.2 times the 

distance in kpc. With this we can observe 

a wide variety of interesting and important 

motions for the first time: the random 

motions of dust features in local star forming 

regions at the sonic speed, 0.1 km/s, the 

expansion of supernova remnants, planetary 

nebulae and “pillars of creation” at 1 km/s, 

the internal motions of Milky Way globular 

clusters to within 2 km/s, the rotation of 

the LMC and SMC to within 10 km/s, and 

the rotation of M31 within 140 km/s. For 

example, proper motions have already 

been measured in the LMC (Kallivayalil et 

al. 2013) although with less precision. This 

is a completely new capability to study the 

dynamics of nearby dust and emission line 

structures, and of stars and stellar systems, 

especially in regions where the Gaia satellite 

will not be able to distinguish individual stars.

Extensions to high redshifts

As noted in the LUVOIR technical summaries 

(Dalcanton et al. 2015), the angular resolution 

will be about 100 pc or better for all redshifts. 

This is equivalent to ground-based resolution 

(1 arcsec) for all galaxies within 20 Mpc, which 

is essentially the Hubble Atlas of Galaxies 

(Sandage 1984). Deep images at high redshift 

will cover the same co-moving volume as the 

Sloan Digital Sky Survey with the same or 

better resolution (Dalcanton et al. 2015). As 

a result, we will be able to see for the first 

time at high redshift the substructure of star 

formation clumps in young galaxies, we will 

resolve young bars and bulges, see multiple 

nuclei from recent mergers, map spiral arms, 

and discover dwarf galaxy companions. The 

radial profiles will be resolved too, allowing 

some understanding of the origin of the 

exponential radial structure in disks. Decades 

of research on local galaxies observed from 

the ground will suddenly have a counterpart 

in the epoch of galaxy formation.

Multi-object spectroscopy

LUVOIR will observe spectral line emission 

from low-lying states of atoms, which are 

primarily in the UV and unobservable from the 

ground. Such observations were a primary 

driver of going into space in the first place as 

mentioned by Spitzer (1946) in his “Report 
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A-32 The LUVOIR Interim Report

to Project Rand: Astronomical Advantages 

of an Extra-Terrestrial Observatory.” Most 

atoms are in these ground states and both 

absorption and emission lines are in the 

UV or FUV. Dalcanton et al. (2015) have a 

diagram of the observable wavelengths 

for many important lines as a function of 

lookback time in the Universe. Most lowlevel ion transitions cannot be observed from 

the ground after a lookback time of some 10 

Gyr, which means these lines are invisible 

for most of the Universe unless they are 

observed from space.

With FUV emission lines, we can 

see interstellar and active galactic nuclei 

emission from 106 K gas at redshifts greater 

than 0.3 and compare it to the observable 

x-ray gas at the same temperature. We can 

observe circumgalactic (CGM) emission in 

Lyman alpha, OVI, and CIV faster than on 

the ground by factors of 10 to 100 because 

of the lower levels of background light in 

space (Dalcanton et al. 2015). An example 

of a Lyman alpha blob in space is shown in 

Cai et al. (2017). 

LUVOIR will also observe absorption lines 

from ground states in the FUV. We expect 

100 times as many background galaxies in 

a deep field at redshifts less than 1 to 2 for 

observations of foreground CGM absorption, 

and 100 times as many QSOs (Dalcanton et 

al. 2015). This will allow us to map the CGM 

around galaxies, trace element formation, 

outflows and inflows and see galaxies 

building up and quenching by outflows for 

the first time. 

Extensive observations of the CGM 

in both emission and absorption will 

revolutionize our understanding of galaxy 

formation and evolution, just as most of the 

other new capabilities mentioned here will 

be revolutionary in their own fields. Not only 

will we see more and smaller galaxies at high 

redshift, but we will also start to fill in the 

space between the galaxies. 

Figure 13.16 shows the impact of this 

change on another plot with a historical 

perspective. This is the sky-covering fraction 

of known astronomical objects as a function 

of year. For all time before the invention of 

Figure 13.16. The fraction of the sky covered with known astronomical objects is plotted versus 

the year in history. As telescopes reach fainter object and deeper fields, more and more objects 

become known. LUVOIR will add to this progression by mapping the circumgalactic gas.
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-33

telescopes, humans could only see some 

5000 stars at 1 arcmin resolution. The 

fraction of the sky covered by these light 

sources was very small, letting us think that 

most of the Universe was empty space. 

When the telescope was invented, the sky 

covering fraction got even smaller because, 

although many more stars could be seen, 

each was 60 times smaller at 1 arcsecond 

resolution, so more blank space could be 

seen as well. However, with larger telescopes 

to collect fainter light (in William Herschel’s 

era), and with photographic images that 

allowed collection of this faint light over long 

periods of time (thousands of times longer 

than the eye could integrate), the sky started 

to be seen as covered with faint galaxies 

and nebulae, filling in the blank space and 

increasing the covering fraction by a factor 

of 100. HST increased it by another factor of 

100 by finding faint galaxies at high redshift 

between all the nearby galaxies and stars, 

covering deep fields at nearly the 10% level. 

LUVOIR will make the next jump with its 

ability to see gas between the galaxies, and 

its possible first-time resolution of unknown 

objects that have appeared up to now to 

be only a smooth background. LUVOIR will 

literally fill the sky with discoveries!

My thanks to Aki Roberge, Daniela 

Calzetti, and Bradley Peterson for their 

invitation to speak on this topic at the AAS 

Winter Meeting 2018. Many of the numbers 

and ideas presented here are from the 

comprehensive summaries by Dalcanton et 

al. (2015) and Elmegreen et al. (2017).

References

Anderson, J. & King, I.R. 2000, PASP, 112, 

1360

Anderson, J. & King, I.R. 2003, PASP, 115, 

113

Cai, Z. et al. 2017, ApJ, 837, 71

Dalcanton, J. & Seager, S. 2015, AURA 

committee publication

Elmegreen, D.M., van Dishoeck, E., 

Spergel, D, & Davies, R. 2017, Kavli 

Foundation Report for the IAU, arXiv:1709.06992

Ferrarese, L., Ford, H.C., Jaffe, W. 1996, 

ApJ, 470, 444

Haubois, X. 2010, APOD “The spotty surface of Betelgeuse”

Kallivayalil, N., van der Marel, R.P., Besla, 

G., Anderson, J. & Alcock, C. 2013, 

ApJ, 764, 161

Karovska, M. 1997, News release ID: 

STScI-1997-26

Kroupa, P. 2001, MNRAS, 322, 231

Lauer, T.R., Bender, R., Kormendy, J., 

Rosenfield, P. & Green, R.F. 2012, ApJ, 

745, 121

MacPherson, G.J. & Boss, A. 2011, PNAS, 

Vol. 108, No. 48, p 19152

Ochsenbein, F. & Halbwachs, J. L. 1982, 

A&AS, 47, 523

Program at a Glance

Science goal: Study local star formation, the stellar initial mass function, and nuclear 

regions in local galaxies, image stellar surfaces to explore activity, probe dynamics of nearby 

dust structures, determine properties of young galaxies, and map the circumgalactic 

medium.

Program details: High-resolution imaging and multi-object spectroscopy of local stars, 

dust and galaxies out to high redshift, young galaxies. 

Instrument(s) + configuration(s): HDI imaging, multi-object spectroscopy

Key observation requirements: Resolution < 10 mas 
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A-34 The LUVOIR Interim Report

O’dell, C. R., Wen, Z., Hu, X. 1993, ApJ, 

410, 696

Ohnaka, K., Hofmann, K.-H., Schertl, D., 

Weigelt, G., Baffa, C., Chelli, A., Petrov, 

R., & Robbe-Dubois, S. 2013, A&A, 

555, A24

Sandage, A. 1984, The Hubble Atlas of 

Galaxies, Carnegie Inst of Washington

Spitzer, L., Jr. 1946, Report to Project 

Rand: Astronomical Advantages of an 

Extra-Terrestrial Observatory

van Belle, G. T., Dyck, H. M., Benson, J. A. 

& Lacasse, M. G. 1996, AJ, 112, 2147

van der Marel, R.P., Rossa, J., Walcher, 

C.J., Boeker, T., Ho, L.C., Rix, H.-W., 

& Shields, J.C. 2007, IAU Symposium 

241, Stellar Populations as Building 

Blocks of Galaxies, ed. A. Vazdekis 

& R. F. Peletier, Cambridge University 

Press: Cambridge, p. 475
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13.11 UV characterization of exoplanet host stars: keys to atmospheric 

photochemistry and evolution

Kevin France (University of Colorado at Boulder)

13.11.1 Introduction

The planetary effective surface temperature 

alone is insufficient to accurately interpret 

biosignature gases when they are observed 

with LUVOIR, particularly for planets orbiting 

low-mass stars (K and M dwarfs). The UV 

stellar spectrum drives and regulates the 

upper atmospheric heating and chemistry 

on Earth-like planets, and it is critical to 

the definition and interpretation of oxygen 

species and other biosignature gases (e.g., 

Seager et al. 2013). As discussed in Chapter 

3, the specifics of the stellar spectrum may 

produce false-positives in our search for 

biologic activity (Hu et al. 2012; Tian et 

al. 2014; Domagal-Goldman et al. 2014; 

Harman et al. 2015). 

13.11.2 The role of LUVOIR

The chemistry of important potential 

biomarker molecules in the atmosphere of 

an Earth-like planet depends sensitively on 

the strength and shape of the host star’s UV 

spectrum. H2O, CH4, and CO2 are sensitive 

to far-UV radiation (FUV; 100–175 nm), while 

the atmospheric oxygen chemistry is driven 

by a combination of FUV and near-UV (NUV; 

175–320 nm) radiation (Figure 13.17). 

Additionally, the temporal variability in UV 

Figure 13.17. Impact of FUV-derived particle impact on long-term ozone depletion (Youngblood 

et al. 2017, Tilley et al. 2018.) Red and blue curves are particle fluences constrained by the FUV 

data.
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emission lines can be an indicator of strong 

charged particle release (e.g., Youngblood et 

al. 2017). Energetic particle deposition into 

the atmosphere of an Earth-like planet during 

a large M dwarf flare can lead to significant 

atmospheric O3 depletions (> 90% for large 

flares; Segura et al. 2010). This alters the 

atmospheric chemistry and increases the 

penetration depth of UV photons that could 

potentially sterilize (or catalyze) surface life. 

Given that particle fluxes are not (typically) 

directly measured for stars other than the 

Sun, UV observations offer the best estimates 

of these important particle environments.

The high sensitivity and temporal resolution of the LUVOIR-LUMOS spectrograph 

will enable a thorough characterization of the 

UV host star spectrum for every potentially 

inhabited star observed by LUVOIR. At present, there are no other UV-capable facilities 

projected to be available to the astronomical 

community in the 2030s, making LUVOIR’s 

contribution essential for this important host 

star characterization. Furthermore, the high 

sensitivity enabled by the large aperture and 

high efficiency of LUMOS (France et al. 2017) 

mean that regular monitoring of these systems will be possible during exoplanet characterization campaigns with minimum impact to the primary exoplanet observations. 

13.11.3 The science program

This study requires the acquisition of a UV 

spectrum for each LUVOIR target with a 

habitable zone planet showing atmospheric 

spectral signatures. The UV spectrum is 

Figure 13.18. FUV flare from the local M dwarf GJ 876 with an effective HZ strength equivalent 

of an X38-class solar flare (France et al. 2016; Youngblood et al. 2017). 
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The LUVOIR Interim Report A-37

then essential to accurately model and 

interpret those features. This is particularly 

true for M and K dwarf host stars, where 

stellar atmosphere models still require a 

comprehensive panchromatic data set to 

optimize the temperature-pressure profile of 

the star’s chromosphere, transition region, 

and corona (e.g., Fontenla et al. 2016). In 

practice, a comprehensive study of lowmass stars is required to understand the 

full parameter-space of age, mass, and 

metallicity of the system. This requires a 

sample of ~400 stars, including the LUVOIR 

Earth-like planet candidates, and a range 

of stellar ages that will likely require going 

beyond LUVOIR’s direct imaging survey 

volume.

As most of these objects are well distributed on the sky, this will be primarily an 

object-by-object survey of F-M stars. We 

will require spectral coverage across the major UV and optical diagnostics for chromospheric (~104

 K), transition region (~105 K), 

and coronal (~106 K) gas. This includes lines 

of O VI (103.2 nm; Figure 13.17), Fe XIX 

(111.9 nm), Lyman-alpha (121.6 nm), Fe XII 

(124.2 nm), NV (123.8 nm), O I (130.4 nm), C 

II (133.5 nm), Fe XXI (135.4 nm), Si IV & O IV] 

(140 nm), C IV (154.8 nm), He II (164.0nm), 

C I (165.7 nm), Fe II (240 and 260 nm), Mg II 

(280 nm), Ca II (394 nm), and H-alpha (656.3 

nm).

Time-resolution of 1 second is required 

because the characteristic time scale for 

strong stellar/solar flares is ~1–5 minutes, 

and 1 second allows us to resolve flare 

lightcurves – important for inferring properties 

of CMEs and to understand the duration of 

energy deposition into the orbiting planet’s 

atmosphere. This requires a photon-counting 

detector.

References

Harman, C., et al. 2015, ApJ, 812, 137

France, K., et al. 2017, SPIE, 10397, 13

France, K., et al. 2016, ApJ, 820, 89

Youngblood et al. 2016, ApJ, 824, 101

Domagal-Goldman, S., et al. 2014, ApJ, 

792, 90

Hu, R. 2012, ApJ, 761, 166

Segura, A., et al. 2010, AsBio, 10, 751

Tian, F., et al. 2014, E&PSL, 385, 22

Seager, S., et al. 2013, ApJ, 775, 104

Tilley, M., et al. 2017, ArXiv

Fontenla, J., et al. 2016, ApJ, 830, 154

Program at a Glance

Science goal: Characterization of the exoplanet host star’s high-energy irradiance 

enabling detailed photochemical and atmospheric evolution modeling.

Program details: FUV and NUV time-resolved spectroscopy of the host star of every 

exoplanetary system targeted by LUVOIR.

Instrument(s) + configuration(s): LUMOS medium-resolution (R = 30,000), singleobject spectroscopy

Key observation requirements: S/N of 20 per spectral resolution element
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13.12 White dwarfs as probes of fundamental astrophysics

Martin Barstow (University of Leicester) & Boris Gänsicke (University of Warwick)

13.12.1 Introduction

White dwarfs (WDs) are the remnants of 

all stars with initial masses less than 8 M, 

and they provide important laboratories for 

the study of stellar evolutionary processes 

and the behavior of matter at extremes of 

temperature and density. As some of the 

oldest objects in the Galaxy they are useful 

cosmological clocks, placing strong limits 

on the ages of globular clusters and disk 

populations. They are implicated in the 

production of Type Ia supernovae, on which 

the cosmological distance scales and the 

existence of dark energy are predicated, 

even though the precise mechanism(s) 

remain unresolved.

In the strong gravitational fields 

associated with white dwarfs, it is predicted 

that their atmospheres should be pure H 

or He (depending on the prior evolution), 

devoid of heavier elements, which sink out 

of the surface layers. However, many studies 

(e.g., Barstow et al. 1993, 2003) have 

demonstrated that white dwarf atmospheres 

containing metals are ubiquitous (Figure 

13.19). While the presence of this material 

was initially attributed to the effect of 

radiative levitation, this mechanism was 

unable to explain the detailed abundances 

or the presence of metals in cool white 

dwarfs, where the radiative effects are 

negligible. It is now evident that many white 

dwarfs are accreting material from extrasolar 

planetary debris (e.g., Jura et al. 2009, 2012; 

Gänsicke et al. 2012; Barstow et al. 2014). 

Consequently, the study of white dwarf 

atmospheres provides a unique opportunity 

to determine the composition of these 

bodies.

It has been shown in theories of quantum 

gravity that fundamental constants, such 

as the fine structure constant (α) and the 

electron/proton mass ratio (μ) can vary in 

the presence of a strong gravitational field. 

Figure 13.19. Si to C ratio for a sample of 17 white dwarfs observed in the far-UV by FUSE. Most 

stars show evidence for accretion of rocky material.

Bulk Earth

Cl Chrondite

Solar
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report A-39

Such variations are expected to manifest 

as small shifts in the wavelengths of atomic 

and molecular transitions. With UV spectra 

containing the absorption lines of many such 

transitions, white dwarfs have been used to 

study the potential effects of these variations 

(Berengut et al. 2013; Bagdonaite et al. 

2014) but the work is limited to a few of the 

very brightest white dwarfs.

13.12.2 The role of LUVOIR

White dwarfs have been studied in the UV for 

around 40 years, initially by IUE and then HST. 

While this work has yielded many important 

and exciting scientific results, it has generally 

been limited to a small and heavily biased 

sample. White dwarfs are Earth-sized, and 

hence intrinsically faint, and even with the 

improved throughput of COS, high-resolution 

ultraviolet spectroscopy can only be obtained 

for either very nearby (<20 pc) or young (<100 

Myr) and therefore hot white dwarfs, severely 

limiting our understanding of the underlying 

physics, and the wider diagnostic power of 

these stellar remnants. LUVOIR will provide 

a unique opportunity to address these 

shortcomings. The larger aperture will enable 

observations of fainter objects, increasing the 

accessible sample, but also yield reduced 

data collection times, making the collection 

of suitable stellar samples more efficient. In 

addition, the dramatic improvement in the 

diffraction limited resolution enabled by the 

larger aperture, coupled with coronagraphic 

capability for the most extreme luminosity 

ratios, will open up the possibility of resolving 

binary systems with smaller separations and/

or at greater distances.

13.12.3 The science program

Knowledge of the WD ages is important in 

measuring the ages of stellar populations. 

However, such results depend on a 

thorough understanding of the evolution 

Figure 13.20. Schematic illustration of the effect of a dependence of the fine structure constant 

on gravity (image courtesy of Julian Berengut).
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A-40 The LUVOIR Interim Report

of WDs themselves and on predictions 

of the cooling rates. In turn, the masses, 

radii, and photospheric compositions 

affect these rates. Studies of WDs in binary 

systems potentially represent a direct test 

of the evolutionary models and the massradius relation, since the WD mass can be 

independently determined from the orbital 

and physical elements of the system. In 

practice, however, few such systems have 

been available to be studied in sufficient detail 

to make these comparisons. An important 

science goal is to build up statistically 

significant samples of binary systems where 

orbits can be determined and follow-up 

spectroscopy carried out.

Priorities in exoplanet research are rapidly 

moving from finding planets to characterizing 

their physical properties. Of key importance 

is their chemical composition, which feeds 

back into our understanding of planet 

formation. Mass and radius measurements 

of transiting planets yield bulk densities, from 

which interior structures and compositions 

can be deduced (Valencia et al. 2010). 

However, those results are model-dependent 

and subject to degeneracies (Rogers & 

Seager 2010; Dorn et al. 2015). Transmission 

spectroscopy can provide insight into the 

atmospheric compositions (Sing et al. 2013; 

Deming et al. 2013), though cloud decks 

detected on a number of super earths 

systematically limit the use of this method 

(Kreidberg et al. 2014). For the foreseeable 

future, far-ultraviolet spectroscopy of white 

dwarfs accreting planetary debris remains the 

only way to directly and accurately measure 

the bulk abundances of exoplanetary 

bodies. Significant progress will be made 

through the acquisition of a large sample of 

high-resolution UV spectra to provide these 

measurements.

Observing potential variations in the fine 

structure constant in white dwarf spectra is 

very challenging, requiring extremely high 

S/N and deep understanding of systematic 

wavelength calibration effects. Statistically, 

there is also benefit in observing a significant 

sample of objects to compare results 

between them. Any observed effect should 

be reproduced in stars of similar gravity. 

Furthermore, extending the sample to the 

extreme range of white dwarf gravities allows 

exploration of the dependence of α & μ on 

gravity, or at least places important limits, 

which can constrain the possible range of 

theories. 

A large UVOIR telescope in the 10 to 

16-m aperture range will enable high S/N 

observations of several thousand white 

Figure 13.21. The bulk abundances of disrupted exo-planetary bodies measured from farultraviolet spectroscopy of white dwarfs (left) are consistent with rocky parent bodies (right: large 

dots = white dwarfs, small dots = solar system meteorites).
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dwarfs, increasing potential sample sizes 

for the above programs by 1 to 2 orders of 

magnitude. The length of typical exposures 

(approx. 0.1 hour) will likely be small 

compared to observational overheads. 

Therefore, attention will need to be paid to 

minimizing target acquisition and readout 

times and optimizing the pattern of slews 

between targets to achieve a high observing 

efficiency so that projects requiring large 

numbers of short exposure are not too costly. 

References

Bagdonaite, J., et al. 2014, Phys. Rev. Lett., 

113, id. 123001

Barstow, M.A., et al. 1993, MNRAS, 264, 

16

Barstow, M.A., et al. 2003, MNRAS, 341, 

870

Barstow, M.A., et al. 2014, MNRAS, 440, 

1607

Berengut, J.C., et al. 2013, Phys. Rev. Lett., 

111, id. 010801

Deming, D., et al. 2013, ApJ, 774, 95

Dorn, C., et al. 2015, A&A, 577, 83

Gänsicke, B., et al. 2012, MNRAS, 424, 

333

Jura, M., et al. 2009, 694, 805

Jura, M., et al. 2012, ApJ, 750, 69

Kreidberg, L., et al. 2014, Nature, 505, 69

Rogers, L.A., & Seager, S. 2010, ApJ, 716, 

1208, 

Sing, D. K., et al. 2013, MNRAS, 436, 2956

Valencia, D., et al. 2010, A&A, 516, 20

Program at a Glance

Science goal: Understanding the physical properties of white dwarfs, their evolution and 

their composition—leading to new insights into fundamental physics and the composition 

of rocky extra-solar planets.

Program details: High resolution UV spectra, some with polarimetry; High resolution 

imaging; coronagraphic imaging for a subset of systems

Instrument(s) + configuration(s): LUMOS/HDI/ONIRS/POLLUX

Key observation requirements: 25:1 in spectra for most surveys, but 100:1 + for fine 

structure constant studies; diffraction limited imaging at v ~ 20
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13.13 Small bodies of the inner solar system

Andrew S. Rivkin (Johns Hopkins University Applied Physics Laboratory), Geronimo L. 

Villanueva (NASA Goddard Space Flight Center)

13.13.1 Introduction

The term “asteroid” covers a wide range 

of compositions from metal to rock to 

primordial mixtures of ice, organics, and 

silicates, and sizes ranging from planetary 

embryos to objects that can fit comfortably 

inside a grad student office. The range of 

research addressing the asteroids is similarly 

broad, and LUVOIR particularly promises to 

allow major advances due to its high spatial 

resolution, high-contrast imaging, and 

sensitivity. 

1) Dawn’s visits to Ceres and Vesta 

demonstrate that regional and local 

variations occur on large asteroids. There 

are over 200 asteroids with diameters >100 

km, and roughly 30 with diameters >200 km. 

We want to understand the homogeneity/

heterogeneity of large asteroid surfaces. 

2) Ongoing activity on Ceres is a matter 

of ongoing debate. Main belt comets are 

known, but we have no data sensitive 

enough to detect ice/water/sublimation. 

3) Binary systems are commonly found 

on NEOs with radar, but they are difficult to 

characterize. The Ida/Dactyl system was 

discovered by Galileo flyby, but unobservable 

from current Earth-based systems (or 

JWST). Study of these systems is important 

to understand collisional evolution.

4) Positional measurements of very 

high precision are of potential use for certain 

objects like hazardous NEOs, or small 

asteroids that are going to make close 

Figure 13.22. The asteroids of the Solar System, categorized by size and number. Credit: M. 

Colombo (DensityDesign Research Lab).
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passes to large asteroids (so mass can be 

determined).

13.13.2 The role of LUVOIR

LUVOIR will permit orders of magnitude 

greater characterization of asteroids than 

what is currently done, with the detection 

capability up to 50 meters bodies (millions 

of asteroids, see Figure 13.22). Specifically, 

LUVOIR can go ~4 times deeper in ~10% 

of the time that HST requires, and it will be 

much more sensitive than any of the ELTs 

for this kind of observation (mainly due to 

reduced background brightness).

13.13.3 The science program

Imaging

• FOV: Primarily useful for recovery of lost 

objects or discovery surveys, neither of 

which is likely to be a driver for LUVOIR 

observations. Observations of impact 

ejecta or active asteroid comae/tails likely 

to need similar or smaller field-of-view as 

cometary observations.

• Filters/Wavelength coverage: Filters 

used during the Eight Color Asteroid 

Survey (ECAS) of the 1980s covered 

wavelengths from 300–1100 nm and 

are still in use on some spacecraft today. 

Nevertheless, their exact placement 

could be revisited. Longer wavelength 

regions include important absorptions 

due to silicates (~2000 nm) and clays 

(~2200–2300 nm). In addition, the ability 

to use a neutral density filter could be 

useful—several targets of interest have 

been unobservable by earlier space 

observatories because they are too 

bright. Objects of potential interest range 

can be as bright as V~7.5. The faint limit 

for discovered and cataloged objects is 

currently at V~21–22, but may be pushed 

fainter with new surveys. 

• Spatial Resolution: A spatial resolution of 

0.01” would allow ~10 km resolution in 

the middle of the asteroid belt and ~15 km 

resolution at its far edge. This resolution 

would resolve the largest 200 asteroids 

to have ~100 pixels or more, and allow 

the satellites of Mars and the very largest 

Trojan asteroids to be similarly resolved. 

For binary system studies, Ida/Dactyl 

Program at a Glance

Science goal: Characterization of the smallest bodies in the asteroid belt (> 50 m), ensuring 

the early detection of potentially hazardous objects (planetary defense) and opening a new 

window into the evolution of our planet and the inner solar system. 

Program details: Observations of asteroids may include targeted imaging of large objects 

to study surface features, targeted spectroscopy of objects to obtain measurements not 

obtainable from Earth due to wavelength or SNR requirements, targeted astrometric 

measurements of near-Earth objects, or parallel observations of objects in the field during 

measurements of astrophysical targets.

Instrument(s) + configuration(s): HDI and LUMOS appear likely to be the instruments 

of main utility for asteroid studies, although the multi-object capability of LUMOS is not 

likely to be used for asteroids.

Key observation requirements: Absorption bands on asteroids typically are 10% or less 

in band depth in the wavelengths in question. Objects of possible interest range from V < 

10 to V > 20, potentially reaching arbitrary faintness (for instance, a small NEO observed 

near aphelion). Tracking rates of objects of interest will range from near-sidereal to > 100 

arcsec per hour. 
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can be used as an example: They are 

separated by roughly 70 milliarcseconds, 

and have a magnitude difference of 6.7 

magnitudes. 

• Depth: Albedo variations on objects 

are typically a few percent. Detection 

of sublimation from active asteroids will 

have requirements similar to comets at a 

similar solar distance.

• Comments: Irregular satellites may also 

be lumped in with asteroids. The giant 

planets have dozens of irregular satellites 

that are thought to be captured asteroids. 

For many of them no physical properties 

are available. 

Spectroscopy

• Wavelength coverage: Major asteroid 

types typically have absorptions near 1 

and 2 μm or else a shallow absorption 

centered near 700 nm when considering 

wavelengths shortward of 2.5 μm. Very 

little asteroid data exists shortward of 

400 nm or so, but HST and spacecraft 

observations of Ceres and Lutetia show 

evidence of features at wavelengths as 

short as 110–120 nm. 

• Resolution: Spectral resolutions are 

typically low for asteroid observations, 

as absorptions are typically very broad. A 

low spectral resolution mode (R ~ 200 to 

250?) will be of use. 

• Multiplexing/IFU: possible, but difficult to 

co-locate several dynamical objects in a 

single FOV.

• Depth: Band depths in the 1–2 μm region 

are typically a few percent.
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13.14 Resolved photometry of young super star clusters

Søren S. Larsen (Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The 

Netherlands)

13.14.1 Introduction

Star clusters were traditionally viewed 

as excellent examples of “simple stellar 

populations”—consisting of stars with a 

single age and a single chemical composition. 

While this still appears to hold true for lowmass open clusters, it is now clear that 

massive globular clusters that inhabit the 

Galactic halo are far more complex systems. 

Color-magnitude diagrams (CMDs) from the 

Hubble Space Telescope show multiple 

populations that reveal themselves through 

parallel main sequences, split red giant 

branches, and other features not reproduced 

by standard models for stellar evolution 

(Gratton et al. 2012; Bastian & Lardo 2017).

Two key capabilities of HST have been 

crucial in uncovering the variety of this 

phenomenon in old GCs: 1) from space, it 

is possible to achieve exquisite photometric 

accuracy that is very hard to reach from the 

ground, especially in crowded environments; 

2) space-based observations provide 

access to important spectral features in the 

UV (CH, CN, and NH molecular bands) that 

are sensitive to the light-element abundance 

variations that trace the multiple populations. 

The Magellanic Clouds provide the 

closest examples of young star clusters 

with masses approaching those of ancient 

GCs. HST imaging has revealed a surprising 

complexity of the CMDs in these clusters, 

too, with extended main sequence turnoffs, parallel young main sequences, and 

other puzzling features (Milone et al. 2016). 

It is unclear, however, to what extent these 

phenomena are related to the multiple 

populations observed in old GCs. 

The stellar populations in ancient GCs 

tend to become increasingly complex with 

increasing mass, and the same may well 

be the case for their younger counterparts. 

However, small number statistics are a 

crucial limitation in young clusters, especially 

for post-main sequence stars, as even a 105 

M cluster only contains 20–30 post-MS 

stars (Larsen et al. 2011). Young clusters 

with masses well above 105 M, also known 

as Super Star Clusters (SSCs), are rare and 

tend to be located beyond the Local Group. 

Figure 13.23. Simulated 1800 s exposures (2"x2") of a 50 Myr old star cluster in the galaxy NGC 

1313 (Larsen et al. 2011), at a distance of 4 Mpc. This cluster has a mass of about 200,000 M.
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HST has played a crucial role in finding these 

objects, but the next step—characterising 

their stellar contents—will require a much 

larger space-based telescope. Nevertheless, 

it is already evident that the CMDs of some 

young SSCs are not well reproduced by 

standard models for stellar evolution (Larsen 

et al. 2011). A number of effects, including 

stellar rotation, binary evolution, age spreads, 

and chemical abundance anomalies may all 

play a role. Current work is already pushing 

HST to the limit of its capabilities, and it is 

clear that there is still a lot to learn. 

13.14.2 The role of LUVOIR

Imaging with a 10-m class LUVOIR will make 

it possible to obtain exquisite photometry 

for individual stars in SSCs well beyond the 

Local Group. In evolved stars, the wavelength 

range 250–450 nm provides access to strong 

molecular features that are sensitive to light 

element abundances (CN, CH, NH). For hot 

stars, observations at longer wavelengths 

(e.g., ground-based AO-assisted imaging) 

lack the Teff sensitivity necessary to properly 

characterize (sub-)populations within the 

SSCs.

The difference between the current stateof-the art and LUVOIR is illustrated in Figure 

13.23, which shows simulated images of an 

actual cluster in the galaxy NGC1313, at a 

distance of 4 Mpc (Larsen et al. 2011). HST 

can only resolve the supergiants and the 

brightest main sequence stars in this 50 Myr 

old cluster and provide a crude CMD, while 

LUVOIR can provide accurate photometry 

reaching far down the main sequence (Figure 

13.24). Blue/UV photometry with LUVOIR 

Figure 13.24. Color-magnitude diagrams for the simulated images in Figure 13.23.

Figure 13.25. Color-magnitude diagrams for the same cluster, but for an age of 1 Gyr and an 

exposure time of 5 hours per filter.
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would establish whether variations in lightelement abundances (C, N) are present in 

this and other young clusters, and would also 

put tight constraints on any age spreads. In 

Figure 13.25, the same cluster as in Figure 

13.23 has been simulated for an age of 1 Gyr. 

Even the brightest stars are now completely 

beyond reach of HST, while LUVOIR will be 

able to reach well below the main sequence 

turn-off in a few hours of exposure time per 

bandpass. The difference between a 6.5-m 

and 15-m aperture is also evident. If past 

experience is any guide, LUVOIR CMDs of 

such clusters will likely reveal surprising new 

features not yet imagined.

13.14.3 The science program

We envision a LUVOIR survey of SSCs in 

a sample of relatively nearby star-forming 

galaxies, out to distances of ~5 Mpc. The 

targets would cover a variety of galaxy types, 

metallicities, environments, etc., ranging 

from dwarf starburst galaxies to large spirals. 

The survey could also include nuclear star 

clusters. The global characteristics of the 

SSC populations in most nearby galaxies 

are already well known, and will be studied 

in more detail by missions such as Euclid 

and WFIRST prior to LUVOIR. Somewhat 

further away, the rich cluster population in 

the Antennae merger would offer interesting, 

but challenging targets.

References

Bastian, N., & Lardo, C. 2017, ARA&A, arXiv:1712.01286

Gratton, R., Carretta, E., & Bragaglia, A. 

2012, A&AR, 20, 1

Larsen, S. S., de Mink, S. E., Eldridge, J. J. 

et al. 2011, A&A, 532, A147

Milone, A., Marino, A., D’Antona, F. et al. 

2016, MNRAS, 458, 4368

Program at a Glance

Science goal: Characterize the stellar contents of massive star clusters in nearby (~5 

Mpc) galaxies via high quality color-magnitude diagrams. 

Program details: Imaging in blue/UV filters (F275W/F336W/F343N/F439W equivalent), 

exposure times from ~30 min to several hours per filter.

Instrument(s) + configuration(s): HDI

Key observation requirements: SNR should be ~100 per filter (errors ~0.015 mag) to 

clearly detect C, N abundance variations in cool stars.
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13.15 The detection of terrestrial planets in the habitable zones of A-stars

Ramses Ramirez (Earth-Life Science Institute, Tokyo Institute of Technology), Aki Roberge 

(NASA GSFC)

13.15.1 Introduction

Most habitable zone (HZ) studies focus on 

terrestrial planets orbiting F,G,K,M mainsequence stars. This is largely because 

an arbitrarily long timescale of 1 Gyr, 

corresponding to the minimum thought 

for life to arise, is also as long as the entire 

main-sequence lifetime of an early F-star. 

However, life on Earth had already appeared 

by ~3.8 Gyr ago, if not sooner. Indeed, life 

may have evolved even earlier had our planet 

not been beset by ongoing impacts during 

the late heavy bombardment. Evidence from 

zircons also suggests habitable conditions 

on Earth by 4.3 Gyr ago, only ~300 Myr after 

formation of the planet. Thus, in order to 

explore the range of conditions under which 

life can arise, we should include A-stars in 

the search for habitable planets.

A-star systems for which exoplanets 

have already been directly imaged include: 

Fomalhaut (7.7 parcsecs), Beta Pictoris (19.4 

pc), HR 8799 (39 pc), HD 95086 (90 pc), 

and HD 15082 (118 pc). Direct imaging is 

currently the best way to discover exoplanets 

around A-stars, as indirect methods like 

radial velocity and transits have proven 

ineffective. The known A-stars planets are 

massive (e.g., Jovian or larger) and located 

far beyond their host stars’ respective HZs. 

Previous A-stars surveys have not found 

smaller terrestrial planets, since the required 

ultra-high contrast is not yet available. 

Should terrestrial planets exist within the HZs 

of these stars, however, we may be able to 

detect them using the ECLIPS coronograph 

to block out extraneous starlight.

Figure 13.26. The classical CO2-H2O habitable zone (blue) with volcanic hydrogen extension 

(red). Adapted from Ramirez and Kaltenegger (2017).
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13.15.2 The role of LUVOIR

The extreme high contrast needed to 

directly observe Earth-like planets around 

Sun-like stars (~ 1 × 10-10) is only possible 

with a space-based telescope and ultrahigh performance starlight suppression 

instrument (like LUVOIR). The situation is 

harder for terrestrial planets around earlytype stars, as the planet-to-star flux ratio is 

even smaller (see below).

13.15.3 The science program

The effective stellar flux of a planet near 

the conservative inner and outer edges of 

the classical HZ of Fomalhaut (A4V; Teff = 

8600K; L = ~16.63 Lsun) is ~1.25 and 0.5 

times that received by the Earth, respectively 

(Figure 13.26), which corresponds to orbital 

distances of ~ 3.65 and 5.8 AU. The inner 

working angle (IWA) of ECLIPS is 3.5 λ/D = 

48 milliarcsec at 1 micron. So HZ inner edge 

is exterior to the IWA for A4V stars out to 

76 parsecs, at wavelengths shorter than 1 

micron. This shows how the large separation 

of the HZ for A-type stars is advantageous 

for high-contrast imaging and spectroscopy.

However, the planet-to-star flux ratio is 

smaller than it is for the Earth around the 

Sun. A planet at the equivalent insolation 

distance has about the same absolute 

bolometric magnitude, no matter the star. 

Equation 15 in Turnbull et al. (2012) gives the 

approximate planet-to-star flux ratio for an 

Earth-twin planet at the inner edge of the HZ. 

Adapting that equation for a 1.4 REarth superEarth exoplanet gives

F

F

L super-Earth

star

 star 1 4 2 10 2 10 ( ) . * × −

Setting the planet-to-star flux ratio to the 

ECLIPS contrast limit of ~ 3 × 10-11, we can 

estimate the most luminous star for which 

an Earth-like super-Earth at the inner edge 

of the HZ can be detected. The limiting 

stellar luminosity is < 13.1 LSun. Therefore, 

such exoplanets can be detected around 

stars with spectral types of ~A7V and later, 

albeit with very long exposure times. We will 

calculate the required exposure times at a 

later date.

References

Ramirez, R., & Kaltenegger, L. 2017, ApJ, 

837, L4

Turnbull, M. C., Glassman, T., Roberge, A., 

et al. 2012, PASP, 124, 418

Program at a Glance

Science goal: Discover terrestrial exoplanets in the habitable zones of A-type stars and 

probe their atmospheres to investigate their habitability.

Program details: Direct imaging and low-resolution Vis/NIR spectroscopy of super-Earth 

exoplanets in the habitable zones of early-type stars with spectral types ~A7V and later, 

out to ~76 parsecs.

Instrument(s) + configuration(s): ECLIPS imaging and IFS spectroscopy

Key observation requirements: Contrast floor < 3 × 10-11, IWA < 3.5 λ/D
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13.16 Quest for the first quasars

Yoshiki Matsuoka (Ehime University)

13.16.1 Introduction

One of the greatest achievements of modern 

optical/infrared astronomy is the discovery 

of ubiquitous supermassive black holes 

(SMBHs) throughout the universe (e.g,, 

Kormendy & Ho 2013). Almost all massive 

galaxies in the local universe harbor a central 

SMBH, as evidenced by stellar and gaseous 

kinematic measurements. In the more 

distant universe, we know the presence of 

numerous quasars and active galactic nuclei 

(AGNs), whose energetic radiation is believed 

to originate from mass accretion onto a 

SMBH. As such, SMBHs are recognized as 

a major constituent of the baryonic universe. 

Furthermore, they may well control the fate 

of the host galaxies, as implied from the 

correlation between SMBH mass and galaxy 

bulge mass, and powerful gas outflows 

observed in quasar host galaxies, among 

other observational facts.

On the other hand, the origin of SMBHs 

is not yet known. The discoveries of SMBHs 

exceeding a billion solar masses (Msun) at 

z > 6, where the universe is less than a billion 

years old, have provoked controversy on 

Figure 13.27. A distant quasar discovered at z ~ 6, where the universe is less than a billion years 

old (Matsuoka et al. 2016, ApJ, 828:26). LUVOIR will probe the even more distant universe, where 

“first quasars” are being formed.
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their seeding mechanism; SMBHs may be 

born as (i) remnants of first stars with 102–103

Msun, which must be followed by extremely 

efficient mass accretion over a long time, (ii) 

products of runaway collapse of primordial 

star clusters, resulting in 103–104 Msun seeds, 

or (iii) massive seeds with 104–105 Msun, 

resulting from direct collapse of primordial 

gas (e.g., Volonteri 2012). Observations 

of “first quasars,” representing the earliest 

stage of SMBH assembly from one or more 

types of the above seeds, are undoubtedly 

a key to disentangling the origin of SMBHs 

(see Figure 13.27).

13.16.2 The role of LUVOIR

Quasars are expected to be extremely faint 

at the early stage of SMBH mass assembly, 

which took place in the very distant universe 

(z > 10). For example, a 105 Msun SMBH at 

z = 10–20, radiating at the Eddington limit, will 

have a rest-frame UV magnitude of 30–32 

AB mag, which will be observed in near-IR 

wavelengths at λ < 3 μm. LUVOIR will be 

the first telescope with sufficient photometric 

sensitivity to detect such faint and distant 

signals.

13.16.3 The science program

The High Definition Imager (HDI) will be used 

for a square-degree scale imaging survey 

through a few near-IR bands. The survey field 

should overlap other multi-wavelength data 

sets, obtained by other space and groundbased instruments, in order to maximize 

various scientific potential when combined 

with the LUVOIR data. We will select z > 

10 sources with the dropout technique; any 

source at z > 10 will be completely dark at λ < 

1.3 μm due to IGM absorption, which gives 

rise to a strong spectral break identifiable with 

near-IR multi-band photometry. Candidate 

first quasars will be searched for with 

broadband colors and inferred luminosity. 

Unfortunately, the emergent spectrum from a 

first quasar has been poorly studied so far. It 

may have an extremely red optical/IR color, if 

surrounding hydrogen gas heavily obscures 

the SMBH (Pacucci et al. 2015). On the other 

hand, if there are escape paths of photons 

from the vicinity of the SMBH, then they 

would make first quasars appear much bluer. 

Alternatively, most luminous dropout sources 

at the relevant cosmic epoch may simply 

be first quasars if a coeval (proto-) galaxy 

cannot produce a similar amount of radiation 

energy; this is indeed the case at the current 

observational frontier of z = 6–7, where 

virtually all sources with high luminosity (restframe UV absolute magnitude M1450  <  -24 

mag; Matsuoka et al. 2016) are quasars. The 

candidate first quasars thus identified will be 

followed up with ground-based extremely 

large telescopes, which will confirm the 

nature of the candidates through highresolution near-IR spectroscopy. The grism 

mode of HDI may also be used to narrow 

down the photometric candidates. Ultimately, 

the spectroscopic information and statistical 

properties, such as the number density as 

a function of SMBH mass and redshift, will 

Program at a Glance

Science goal: We aim to discover “first quasars,” which represent the early evolutionary 

stage of supermassive black holes, with HDI extremely deep imaging.

Program details: a square-degree scale imaging survey in multi near-IR bands

Instrument(s) + configuration(s): HDI, imaging (+ grism spectroscopy)

Key observation requirements: 10σ depth of ~32 AB mag
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be compared with theoretical predictions 

to disentangle the seeding mechanisms of 

SMBHs.

References

Kormendy & Ho 2013, ARA&A, 51, 511

Matsuoka et al. 2016, ApJ, 828, 26

Pacucci et al. 2015, MNRAS, 454, 3771

Volonteri 2012, Science, 337, 544 
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13.17 Protostellar outflows/jets

Christian Schneider (Hamburger Sternwarte) & Gregory Herczeg (Kavli Institute for Astronomy 

and Astrophysics)

13.17.1 Introduction

Stars grow by accreting matter from their 

surrounding protoplanetary disk, which 

requires the efficient redistribution of angular 

momentum—a process still highly uncertain. 

Simulations of disks with non-ideal magnetohydrodynamics suggest a magnetically 

driven disk wind may extract angular 

momentum from the disk, thereby leading 

to accretion (see Figure 13.28 below and 

reviews by Hartmann et al. 2016 and Bai & 

Stone 2013). The accretion onto the star is 

readily observed (e.g., Balmer jump, H-alpha 

emission), but observational constraints 

on the physics of mass transport, with 

implications for planet growth and migration 

as well as the dispersal of the natal envelope, 

need new instrumentation.

13.17.2 The role of LUVOIR

By the time of LUVOIR, sophisticated 

simulations of protoplanetary disks will exist, 

which will require observational tests of 

angular momentum transport from the mass 

loss rates and wind velocities in the inner AU 

around the central star. Current observations 

of disk winds are limited to unresolved line 

emission or line-of-sight absorption, with 

large uncertainties (e.g., Edwards et al. 2006; 

Rigliaco et al. 2013). Similar observational 

challenges limit the interpretation of possible 

jet rotation signatures (Bacciotti et al. 2002, 

Coffey et al. 2012) and jet collimation shocks 

(Schneider et al. 2013). Currently, the 

major instrumental limitations are a lack of 

spatial resolution compared with insufficient 

sensitivity, e.g., the FUV emission lines 

require several orbits with HST for a single 

long-slit spectrum and several slit positions 

are required to sufficiently sample the spatial 

structure of the jet. LUVOIR will overcome 

these issues and is needed even in the era 

of ALMA, JWST or 30-m telescopes as 

none of these are capable of imaging disk 

Figure 13.28. Sketch of a protoplanetary disk and angular momentum transport mechanisms. 

Credit: Simon et al. 2015, MNRAS 454 .
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winds or accretion flows close to the star, 

nor do they cover the relevant temperature 

regime. The primary disk wind diagnostics 

are at UV to optical wavelengths, where high 

spectral and spatial resolution observations 

(preferably with an IFU) are not feasible, even 

with AO systems. Further, disk images in 

FUV CO and H2 emission will reveal accretion 

flows within the disk and onto the star or 

protoplanets in exquisite detail inaccessible 

to ground-based instruments.

13.17.3 The science program

Measure the launching and mass flux in 

disk winds. The launching of MHD winds 

from the disk may drive the accretion flow. 

These winds are thought to be launched 

from a large range of disk radii resulting in 

streamlines with different velocities and 

temperatures. Therefore, a large repertoire 

of diagnostics is needed to sample the total 

mass flux. Measuring the mass flux and 

velocity for objects of different evolutionary 

stages and masses with will allow us to 

measure the angular momentum extracted 

by the disk wind. This mass loss combined 

with accurate accretion rates will provide 

rigorous tests of models for launching disk 

winds and to understand the energy balance 

of jets and accretion. 

Measure jet collimation and rotation 

properties. Jets are collimated by a 

magnetic field anchored in the protoplanetary 

disk. Depending on the anchor/launch point 

in the disk, different streamlines possess 

different collimation properties. Sampling 

the collimation properties and jet rotation 

for different outflow components is needed 

to study jet lunching, the relevance for 

envelope dispersal, and will provide valuable 

information on the disk’s magnetic field. 

Measure the mass flows in the inner 

disk. ALMA and scattered light observations 

revealed that disks have large radial dust 

traps and azimuthal asymmetries likely 

induced by disk physics, chemistry, or planetdisk interactions. While the disk should 

be Keplerian, recent ALMA observations 

suggest radial mass flows. At the highest 

spatial resolution, LUVOIR will be able to 

reveal any non-Keplerian flows in the inner 

AU and onto protoplanets in the disk.

Description of Observations

To measure mass flux rates, we need to 

sample relevant temperature regimes. 

Examples of important emission lines are 

H2, [O I], [S II], [N II], [O III], and C IV with 

line fluxes in the range of 10-14 erg/s/cm-2 for 

the nearby jets and contrast ratios of 1:100 

at 0.1 arcsec from the star. Typical outflow 

velocities range from a few 10 to several 100 

km/s.

With LUMOS, one can efficiently measure 

the collimation and rotation signatures of the 

jet close to the launching region on scales 

of ~10 AU (~100 mas, nearby star forming 

region) with high spectral resolution to trace 

Program at a Glance

Science goal: Understand mass-flows around forming stars

Program details: Spatially and spectrally resolved observations of young stellar objects, 

forming planets, and protostellar jets

Instrument(s) + configuration(s): LUMOS high spectral-resolution spatially resolved 

spectroscopy, other instruments?

Key observation requirements: R > 10,000, spatial resolution better than 0.1 arcsec, 

contrast of 100:1 at 0.1 arcsec.
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different flow components using the microshutter array. Roughly speaking, the flow 

velocity close to the jet base relates to the 

lunching radius. Sampling the collimation 

properties in several velocity channels in the 

range from <10 km/s up to several 100 km/s 

will be critical for this. With expected rotation 

signatures in the km/s range at scales of 0.1 

arcsec (nearby SF region), LUMOS is ideally 

suited to perform high S/N, high spectral 

resolution (R>10,000) observations. Also, the 

region of interest, a few 100 AU translating 

to an apparent size of a few arcsec for the 

nearby star formation regions, is ideally 

matched to LUMOS.

References

Bacciotti et al. 2002, ApJ, 576, 222

Bai & Stone 2013, ApJ, 169, 76

Coffey et al. 2012, ApJ, 749, 139

Edwards et al. 2006, ApJ, 646, 319

Hartmann, Herczeg, & Calvet 2016, Annual 

Reviews, 54 

Rigliaco et al. 2013, ApJ, 772, 60

Schneider et al. 2013, A&A, 550, 1
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13.18 Exploring the high energy processes in microquasars as an 

exemplar case for high time resolution astrophysics

Warren Skidmore (Thirty Meter Telescope) and members of the TMT Time Domain ISDT

13.18.1 Introduction

High time resolution (between a few seconds 

to a few milliseconds) spectroscopic 

observing capabilities spanning the UV 

into the optical have applications across 

many areas of astrophysics, from the solar 

system to the high redshift universe. Here 

we describe one exemplar science case that 

requires observing capabilities that can be 

applied to many other cases.

Black hole X-ray binaries have a stellar 

mass black hole (a few to ~20 M) around 

which an accretion disk exists that is fed 

from a mass losing secondary star (Figure 

13.29). In many systems, high-speed jets of 

material are expelled in a situation very similar 

to that in AGN, and this earns these systems 

the name of microquasars. Microquasars 

provide an opportunity to study the poorly 

understood jet acceleration mechanism 

and the mysterious process that harnesses 

the energy and angular momentum of 

accretion disk material to drive the jets. In 

AGN the region in which these processes 

are taking place is hidden by dust and gas 

but in microquasars they are directly visible. 

However, the timescales involved demand 

special capabilities from astronomical 

instruments.

In microquasars there are multiple processes occurring simultaneously: shocks at 

the base of the jets, flares from the accretion 

disk, reprocessing of X-rays to lower energies, synchrotron and cyclotron processes 

involving optically thick and optically thin 

emitting materials. Each process has different temporal and spectral characteristics. To 

disentangle the observed signals from each 

of the processes it is necessary to examine 

both the temporal and spectral characteristics together using time resolved spectroscopy that includes the UV and has the largest coverage to longer wavelengths.

13.18.2 The role of LUVOIR

A typical brightness of these targets is R~12, 

B~13.6, but much of the variability may be 

constrained to emission lines. A large space 

Figure 13.29. Artist’s impression of the micro-quasar Cygnus X-1, showing material flowing from 

the mass-losing star and forming an accretion disk and the jets being driven out from the vicinity 

of the black hole. Right - Rapid optical variations in V404 Cyg observed with ULTRACAM on the 

4.2-m WHT. Credit: Gandhi et al. (2016)
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based telescope with a moderate resolution 

UV/optical spectrograph is necessary in order 

to have the necessary wavelength coverage 

and acceptable S/N at the required sampling 

frequencies. These include:

• Far UV to blue (100 to 400 nm): Sampling 

frequency of ~30Hz with spectral 

resolution of R~500. 

• Near UV to near IR (200 to 1000 nm): 

Sampling frequency of ~100 Hz with 

spectral resolution of R~500. 

13.18.3 The science program

1. We would separately observe 4 

microquasar targets (target brightness 

R~12, B~13.6, plus 2 to 4 local field stars 

to act as local photometric standards) 

using the LUMOS spectrograph with the 

G145 LL grating in a low multiplexing, 

wide wavelength coverage mode (100 to 

400 nm). Each target would be observed 

for a duration of 2 hours with integration 

times of 0.033 sec giving S/N~20.

2. As above (4 separate targets and local 

standards), except using the R~500 

GRISM mode on HDI and integration 

times of 0.01 s giving S/N~10 to 20.

Further advances could be made using:

• Time resolved spectropolarimetric 

observations, as some of the variable 

emission is likely to be highly polarized. 

• Coordinated simultaneous observations 

between X-ray to look at reprocessed 

signals and map the system (cross 

correlating to accuracies of ~1 millisecond)

However, these observations are not 

described here except to note the importance 

of high accuracy time stamps (better than 

~10 μs).

References

Gandhi, P., et al. 2016, MNRAS, 459, 554

Program at a Glance

Science goal: Identify the observable signals of the stages of the process of converting 

accretion energy into power to drive the jets in microquasars and characterize each of 

those stages.

Program details: 100 to 400 nm, R = 500 LUMOS observations of a science target and 

2 to 4 local field stars with T

exp=0.033 s (100 to 400 nm) and 0.01 s (200 to 1000 nm)

Instrument(s) + configuration(s): 1. LUMOS multi-object (up to 5 targets) with fast 

readout modes and G145 LL grating; 2. HDI grism multi-object (up to 5 targets) with fast 

readout modes

Key observation requirements: Wide wavelength coverage, low spectral resolution, 

short exposure and fast readout modes.
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13.19 Comets and minor planets: the importance of the small things

Noemi Pinilla-Alonso (Florida Space Institute, University of Central Florida, Orlando, USA), 

Alvaro Alvarez-Candal (Observatorio Nacional, Rio de Janeiro, Brazil)

13.19.1 Introduction

Minor planets and comets are rocky and/

or icy objects, usually ranging in size from a 

few meters to a few hundreds of kilometers. 

They comprise near-Earth and main belt 

asteroids, Trojans (of Jupiter and other giant 

planets), trans-Neptunian objects, Centaurs, 

comets, and a recently discovered category 

called the transitional objects (de Leon et al. 

2018).

The study of minor planets over the 

last decades has led to dramatic changes 

in our understanding of the process of 

planet formation and evolution, and the 

relationship between our Solar System 

and other planetary systems. Small bodies 

also serve as large populations of “test 

particles” recording the dynamical history 

of the giant planets, revealing the nature of 

the Solar System impactor population over 

time, and illustrating the size distributions 

of planetesimals, which were the building 

blocks of planets. The number of discoveries 

regarding exoplanets and debris disks is 

continuously increasing, and therefore it 

is crucial to first understand our own solar 

system’s provenance and evolution in order 

to better interpret what is going on in newly 

discovered planetary systems

Telescope observations from the ground 

or in Earth orbit telescopes have increased 

in the last decade and form the basis for 

understanding these small bodies. Also, 

detailed information from some particular 

targets is available through missions such 

as Rosetta, Dawn and New Horizons. 

However, when compared with the number 

of small bodies estimated to be part of the 

Solar System, or even with the ones already 

detected (~700,000 asteroids, ~ 3,000 transFigure 13.30. The Chariklo and Haumea rings systems, to scale, detected by occultations. Pluto 

is shown for comparison.
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Neptunian objects, ~7,000 Jupiter Trojans, 

and ~5,500 comets), actual knowledge of 

the small bodies population is sparse. 

A key program for a space telescope 

such as LUVOIR dedicated to observations 

of comets and minor planets, from the ultraviolet to the infrared wavelengths, would 

revolutionize the actual knowledge of the 

Solar System in a lot of different areas. Below 

I detail some of them

13.19.2 The role of LUVOIR

Size/Shape/Ring detection and 

characterization: In the last years rings 

have been detected around three minor 

bodies (Figure 13.30) (Braga-Ribas et al. 

2014, Ortiz et al. 2015, Ortiz et al. 2017). 

The viewing geometries of these rings 

change secularly due to the movement of 

these bodies around the Sun. High angular 

and spatial resolution would allow direct 

confirmation of the presence of these rings 

and possibly detection of other rings or small 

moons. Spectroscopy of the systems under 

different viewing circumstances would allow 

compositional studies of these rings (Duffard 

et al. 2014).

Direct spatial resolution would also 

enable studies of the shape of minor planets. 

The most effective tool in that regard is radar 

imaging, but this only works for targets that 

pass very close to the Earth. Very recently, a 

survey using Adaptive Optics for the ~100 

brightest asteroids is aiming to determine the 

shapes of these bodies, VLT/SHEPRE (Garufi 

et al. 2017). For the rest of the populations, 

most asteroids, Trojans, Centaur and 

TNOs, only inversion techniques applied 

to lightcurves provide an approach to 

measuring the shapes of these bodies.

Surface Composition: Some solid ices 

( e.g., Cruz-Diaz et al. 2014a,b), and mineral 

spectral features (Cloutis et al. 2008) occur 

at wavelengths accessible with LUVOIR 

but not from the ground (see Figure 13.31

and Figure 13.32). The ultraviolet end of 

the spectrum is particularly interesting. 

Wavelengths below 400 nm have been 

largely unexplored. These wavelengths are 

particularly sensitive to water (either in the 

form of water ice or in the form of hydrated 

materials), ammonia compounds, CO2, 

complex organic materials and Fe-rich 

materials, which result as a product of space 

weathering. They are also more sensitive to 

hydrated minerals than the NIR-wavelegths 

(~3 mm). In particular, the study of small 

Figure 13.31. Spectral ratio of bright terrain 

to average dark terrain on Iapetus. The ratio is 

very similar to pure water ice suggesting that 

the major difference between two terrains is 

the amount of water ice present (Hendrix et al. 

2010).

Figure 13.32. Reflectance spectra of three 

different sets of high-calcium pyroxenes with 

different ratios of F2+ to Fe3+ (Cloutis et al 

2008).
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bodies at the UV wavelengths with LUVOIR 

would provide new insights in tracing the 

water ice and organics, the seeds of life on 

Earth, in the Solar System. Also it would be 

key in studying the nature of ices at different 

penetration depths than VNIR spectroscopy.

Binary detections and characterization: The number of detected and well-characterized binary systems among the TNO 

population is as low as some tens. The great 

LUVOIR improvement in angular resolution 

would open a new field that is dormant now 

because of the lack of adequate technology. The determination of the density of minor 

bodies is a key question in the determination of density and mass that are crucial to 

better understand the structure of the interior 

of these bodies and the physical characteristics of the superficial materials (rubble-pile 

vs. monolithic, porosity etc.). 

At wavelengths below 2 mm JWST will not 

provide any power in excess of HST in terms 

of ability to resolve tightly bound binary TNO 

systems, and even at 2 mm it will be similar to 

WFC3 UVIS (Benecchi et al. 2009). At midIR wavelengths, JWST’s spatial resolution 

(as low as ~1" at the longest wavelengths) 

is insufficient to resolve all but the rare, wide 

binary TNO systems (Parker et al. 2016). 

Additionally, one key distinguishing 

feature of TNO binary systems is the common 

optical colors of the components that would 

give indication of formation of the system 

(Benecchi et al. 2009), in this regards multi 

color imaging of binary systems coupled 

with better angular resolution would enable 

us to explore this subject. 

Activity (comets and active asteroids):

The traditional separation between the rocky 

inactive asteroids and the icy active comets 

has disappeared during the last two decades 

due to the discovery of several objects with 

the dynamics of asteroids displaying activity 

as comets (the active asteroids, AAs Jewitt 

Figure 13.33. Images of active asteroids a) P/2013 P5 multiple tails; b)P/2010 A2(LINEAR) tail 

and nucleus; c) P/2013 R3 (Catalina-PANSTARRS) breaking up (Jewitt et al. 2015).
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et al 2015). These asteroids display activity 

in the form of particle loss at rates ranging 

from 10-2 to 4 kg/s (Jewitt, et al. 2015). For 

the currently known active asteroids, several 

driver processes have been identified, 

including solar radiation sweeping of particles, 

electrostatic effects, ice sublimation, and 

thermal disintegration of surface regolith. 

Understanding the mechanisms that lead 

to the activity on these bodies will have 

implications on both our understanding 

of the origins of the Solar System and the 

future of space exploration, and in particular 

planetary defense and mitigation. 

The activity of these targets can last 

from weeks to months. In some cases it has 

led to the disappearance of the target after 

a complete break-up, while in others the 

activity has been observed recurrently (see 

Figure 13.33). For the cases of a total break 

up, observation of the fragments is crucial 

and has to happen soon after the discovery. 

Using HST limits to the size of fragments has 

been placed to Hv~23.5 and a size of ~35 m 

(Moreno et al. 2017).

13.19.3 The science program

Size/Shape/Ring detection and 

characterization: This program requires 

high spatial and angular resolution for direct 

observations of rings and an observing 

mode allowing multiple visits or a chain 

of observations in order to cover a whole 

rotation for the shape studies (HDI). The 

superb angular resolution provided by 

LUVOIR in the UVIS mode (where there 

happens to be the best balance between 

collected light and spatial resolution) of 2.73 

mas per pixel will allow detection of the ring 

system of Chariklo with a spread of 25 pixels 

(its major projected axis). See Figure 13.30

for the size of the ring systems in comparison 

with dwarf-planet Pluto.

Surface Composition: For this purpose of compositional studies we could use 

LUMOS and HDI. We would need low and 

medium spectroscopy (<30000) of small 

bodies. According to the LUMOS-ETC provided by STScI we can obtain S/N > 10 in 1 

h for all targets with AB mag < 22. Regular 

detailed observations on some targets are 

desirable as this may mean rotational coverage, i.e., multiple visits with time constrain.

Using the HDI we could characterize 

almost all known Centaurs and transNeptunian objects, which are typically fainter 

and more difficult to characterize (as listed 

in the Minor Planets Center). Using HDI ETC 

we find we could get spectrophotometry 

with high S/N in about 1 hr. of exposure per 

filter. Survey mode will be highly desirable. 

Solar System science would extraordinarily benefit from Large Programs (to systematically collect VNIR data from targets 

in particular populations not reachable with 

other telescopes) and Fillers (to increase the 

knowledge on a population without any special requirement in the number of targets observed) in VNIR but also in the UV, where no 

data exist up to date and even a small number of targets would provide groundbreaking 

information.

Binary detections and characterization: This program requires HDI spectro-photometry. The minimum angular distance in TNO binaries observed in Benecchi 

et al. 2009 is comparable to the angular size 

of Chariklo’s ring system (which we have 

showed if would have an excellent coverage 

by LUVOIR). Using the UVIS mode, we could 

easily observe binary systems with half that 

angular size and study the properties of tight 

binary systems. Observations should be carried out as large programs, with multiple visits to determine orbits.

Activity (comets and active asteroids): Exceptional imaging capabilities would 

allow detection of the tiniest fragments. Multiobject spectroscopy would be desirable to 

be able to study the different fragments as 
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well as the coma. A target-of-opportunity 

program would be desirable, with the possibility of activating it in the first 24 hours after 

the discovery of the activity, and subsequent 

follow-up that could be concentrated in one 

week or over several months.

Last but not least, most of these 

observations have to be done with the 

telescope tracking the source, so this 

capability is extremely important.

References

Benecchi et al. 2009, Icarus, 200, 292 

Braga-Ribas et al. 2014, Nature, 508, 72

Cloutis et al. 2008, Icarus, 197, 321

Cruz-Diaz et al. 2014a, A&A, 562, 119 

Cruz-Diaz et al. 2014b, A&A, 562, 120

De Leon et al. 2017, Handbook of 

Exoplanets, Ed. by Hans J. Deeg and 

Juan Antonio Belmonte, Springer Living 

Reference Work, ISBN: 978-3-319-

30648-3, 2017, id.55, 55 

Garufi et al. 2017, The Messenger, 169, 32 

Jewitt et al. 2015, Asteroids IV, 221

Ortiz et al. 2017, Nature, 550, 219 

Duffard et al. 2014, A&A, 576, 18 

Moreno et al. 2017, AJ, 154, 248 

Ortiz et al. 2015, A&A, 576, 18

Parker et al. 2016, PASP, 128, 8010

Program at a Glance

Science goal: Detection and characterization of minor bodies, their rings, and their binarity. 

Measurements of water ice and organics in Centaurs and TNOs. Activity in comets and 

asteroids.

Program details: High spatial and angular resolution imaging. Low and medium 

spectroscopy (<30000). Multi-object spectroscopy.

Instrument(s) + configuration(s): HDI (UVIS mode), LUMOS. 

Key observation requirements: S/N > 10 in 1 h for all targets with AB mag < 22. 

Telescope tracking the source.
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13.20 Detecting liquid surface water on exoplanets

Tyler D. Robinson (Northern Arizona University & Virtual Planetary Laboratory) and Mark S. 

Marley (NASA Ames Research Center)

13.20.1 Introduction

Planetary habitability requires liquid water 

stability at the surface of a terrestrial planet 

(e.g., Kasting et al. 1993). The remote 

characterization of habitability for planets 

detected in reflected light will require either 

(1) information about the near-surface 

atmospheric state for a confirmed terrestrial 

world, or (2) the direct detection of surface 

liquid water via ocean glint and/or polarization 

measurements. This brief report explores 

what telescope and instrument requirements 

would be required to make either of these 

two observations.

Surface liquid water stability depends 

on both temperature and pressure from the 

overlying atmosphere. Unfortunately, we 

know of no studies that investigate how well 

atmospheric pressure and temperature can 

be constrained from visible and near-infrared 

spectral observations of terrestrial planets—

more work here is clearly required. A Rayleigh 

scattering slope, or detection of dimer 

or pressure-induced absorption features 

(Misra et al. 2014b), could indicate pressure. 

Detection of Rayleigh scattering requires 

observations at blue-visible wavelengths, 

and would not place strict requirements 

Figure 13.34. A moderate resolution, near-infrared spectrum of Earth, from the validated models 

of Robinson et al. (2011). Key features are labeled. Note the separate y-axis for wavelengths 

beyond 2.75 um, which helps show the rise in thermal emission beyond this wavelength.
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on spectral resolution. Misra et al. (2014b) 

emphasize dimer detection at 1.06 μm at a 

resolution (λ/Δλ) of 100. 

Temperatures will be difficult to constrain 

in the absence of a detection of planetary 

thermal radiation. However, it may be 

possible to constrain temperature, and 

also habitability, by detecting a water vapor 

profile consistent with condensation (i.e., a 

condensation curve). Detection of clouds 

and weather (e.g., from variability) would also 

indicate condensation, but not necessarily 

at/near the surface. Figure 13.35 shows 

pressure at the τ=1 level for a clear sky 

Earth atmosphere, and this roughly indicates 

the pressure levels sensed by a spectrum. 

Comparison of high- (λ/Δλ=1,000) and 

moderate- (λ/Δλ=100) resolution spectra 

shows that, for water vapor, not much vertical 

resolution is gained by pushing beyond a 

spectral resolution of roughly 100. Notably, 

while the 0.94 μm water band probes the 

near-surface environment, pushing to the 1.4 

μm and 1.9 μm water bands is necessary 

for probing Earth’s middle and upper 

troposphere. Thus, these bands may be 

essential to constraining water vapor profiles 

for potentially habitable exoplanets. 

Direct detection of surface liquid water 

may prove a challenging endeavor, but 

would be complimentary to any atmospheric 

spectral retrieval analysis (like that outlined 

above). The near- infrared is best suited to 

polarization or glint detection, owing to the 

lack of Rayleigh scattering here (Robinson et 

al. 2010; Zugger et al. 2011), and detection 

could be accomplished with photometric 

observations at continuum wavelengths (i.e., 

between strong atmospheric absorption 

bands). Performing observations in the 

near-infrared would also minimize glint false 

positive signatures from polar ices (Cowan 

et al. 2012), whose reflectivity is diminished 

at these wavelengths. For an Earth-twin, 

clouds and hazes would likely obscure any 

polarization signature from surface oceans 

(Zugger et al. 2011). Thus, glint is the best 

option for the direct detection of surface 

liquid water, and, at least for Earth, is most 

pronounced near a star-planet-observer 

(phase) angle of 150°, where a glinting 

planet would be nearly twice as bright as a 

non-glinting planet. The ability to measure 

planetary phase functions to such close 

planet-star separations will depend on the 

inner-working angle (IWA) of the high-contrast 

field-of-view, and cannot be achieved for 

planetary systems with inclinations below 

about 60 degrees. For an Earth-Sun twin 

system at 10 parsecs, the required IWA 

would be 1.8 λ/D (at the 1.33 μm continuum 

and for a 10-meter diameter aperture). As 

another example, taking an Earth-like planet 

near the inner edge of the habitable zone 

for a mid-K dwarf, the requirement shrinks 

to 0.9 λ/D at 10 parsecs. Thus, an IWA 

of 2 λ/D could allow glint detection out to 

almost 10 pc for habitable planets around 

G dwarfs, and almost 5 pc for K dwarfs. 

These distances decrease to 6 pc and 3 pc, 

respectively, for a 3 λ/D IWA. 

We briefly note that the James Webb 

Space Telescope (JWST) will not be capable 

of probing the near-surface environment 

Figure 13.35. The pressure of the t=1 level for 

a clearsky Earth atmosphere at λ/Δλ=100. Note 

the strong water vapor bands at 0.94, 1.1, 1.4, 

and 1.9 mm.
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of habitable zone planets orbiting stellar 

types earlier than M (Misra et al. 2014a). 

Depending on systematics, JWST may be 

able to characterize potentially habitable 

worlds around nearby M dwarf stars, but 

these worlds possess their own unique 

challenges to habitability (e.g., Luger & 

Barnes 2015). Finally, while not discussed 

at length here, the capability of measuring 

profiles of atmospheric water vapor through 

an atmosphere will also be useful for studies 

of gaseous worlds interior to and throughout 

the habitable zone, as water should be a key 

radiatively active species (and greenhouse 

gas) for these planets (Cahoy et al. 2010). 

In summary, R=70 (or greater) spectra 

of Earth-like planets would be suitable for 

constraining water vapor abundance profiles. 

Table 13.2. Observation requirements for detecting key atmospheric features that constrain the 

presence of liquid surface water on rocky exoplanets.
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Future work is necessary to determine the 

minimum signal-to-noise ratios required for 

such an analysis. Broadband detection of 

glint in the phase function of a habitable zone 

exoplanet is possible in the near-infrared with 

a 2 λ/D IWA and becomes restricted for a 3 

λ/D IWA. The tables above contain additional 

information on requirements for detecting 

water vapor and carbon dioxide features (for 

climate modeling constraints). 

Note on near-infrared CO2 features:

Carbon dioxide is a key greenhouse gas for 

Earth, whose greenhouse forcing is essential 

to maintaining Earth’s habitability (Hansen et 

al. 2013). Thus, inferring atmospheric CO2 

concentrations from near-infrared spectra 

of Earth-like planets would be essential to 

running predictive/forward climate models 

for these worlds (thereby helping to inform 

our understanding of the potential habitability 

of these planets). The strongest CO2 features 

in Earth’s near-infrared, reflected-light 

spectrum are a pair of double features at 

1.59 μm and 2.03 μm. Both would require 

a spectral resolution of λ/Δλ=100 to place 

a single spectral resolution element across 

the bands, and double this resolution could 

enable better detections. The features 

at the shorter wavelength are shallower, 

requiring a SNR of order 40 to distinguish. 

The longer wavelength features are deeper, 

requiring an SNR only of order 10, but may 

be more difficult to detect due to the overall 

decrease in stellar brightness at these longer 

wavelengths. These details are represented 

in Table 13.2.

13.20.2 The role of LUVOIR

While the above observations are enabled 

by a direct imaging mission, they are difficult 

and require either long observations to 

increase the signal-to-noise on a spectral 

feature, or multiple visits to a target to 

observe it at multiple phases. Additionally, 

some of these phases will be at small 

inner working angles. All of these problems 

benefit from an observatory with a larger 

aperture, which increases the signal from the 

planet, decreases the amount of time for an 

observation (which in turn enables multiple 

visits), and decreases the inner working angle 

of the observatory. Finally, the wavelength 

range of LUVOIR encompasses multiple 

water vapor features, from 0.94–1.9 μm, will 

allow crude determination of the water vapor 

profile of an exoplanet’s troposphere.

13.20.3 The science program

Detection of glint from liquid surface water on 

rocky exoplanets will require multiple visits of 

ECLIPS coronagraphic imaging to the same 

target. These multiple visits will conduct 

phase-dependent imaging in polarized light, 

and (if time allows) direct spectroscopy to 

measure H2O vapor concentrations.

Program at a Glance

Science goal: Definitively show the presence of liquid water on the surface of rocky 

exoplanets. 

Program details: Coronagraphic imaging at phase angles as close to 150° as possible to 

maximize glint signal. More generally, glint can be observed between gibbous and crescent 

phase. Direct spectroscopy to measure atmospheric water vapor concentrations. Direct 

spectroscopy to measure NIR CO2 features and constrain greenhouse effect.

Instrument(s) + configuration(s): ECLIPS imaging and IFS

Key observation requirements: Contrast < 10-10; Multiple visits; Water vapor features at 

0.94, 1.1, 1.4, and 1.9 μm, R ~ 100; CO2 feature at 1.59 mm, R > 100.
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References

Cahoy, K. L., Marley, M. S., and Fortney, J. 

J. 2010, ApJ, 724, 189. 

Cowan, N. B., Abbot, D. S., and Voigt, A. 

2012, ApJL, 752, L3. 

Hansen, J. M., Sato, M., Russell, G., and 

Kharecha, P. 2013, Phil. Trans. Roy. 

Soc. A, 371, 20120294. 

Kasting, J. F., Whitmire, D. P., and 

Reynolds, R. T. 1993, Icarus, 101, 108-

128. 

Luger, R., and Barnes, R. 2015, 

Astrobiology, 15, 119-143. 

Misra, A., Meadows, V., and Crisp, D. 

2014a, ApJ, 792, 61. 

Misra, A., Meadows, V., Claire, M., and 

Crisp, D. 2014b, Astrobiology, 14, 67-

86. 

Robinson, T. D., Meadows, V. S., and Crisp, 

D. 2010, ApJL,721, L67. 

Zugger, M. E., Kasting, J. F., Williams, D. 

M., Kane, T. J., and Philbrick, C. R. 

2011, ApJ, 739, 12
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13.21 Geology and surface processes in the solar system

Noah Petro (NASA Goddard Space Flight Center)

13.21.1 Introduction

All solid objects in the Solar System undergo 

regular variations due to internal and external 

forces. However, we know now that these 

variations can cause measurable changes to 

the surface and exosphere of small, airless, 

bodies. The large moons of Jupiter and 

Saturn, for example, experience tidal heating 

that can trigger geysers or possibly volcanic 

eruptions. However, in order to measure these 

events requires nearly constant monitoring of 

an object over multiple hours or even days, 

so that the full range of variation in surface 

properties (surface temperature changes, 

albedo and/or compositional changes) can 

be measured. Spectral measurements of 

the erupted material (either volcanic or via 

geyser, see Figure 13.36) and comparison to 

the surface will provide important constraints 

on compositional variations of the source 

regions of these moons. Additionally, 

watching an entire eruption and being able 

to characterize any compositional changes 

that occur will provide insights into the 

mechanism by which these eruptions occur.

13.21.2 The role of LUVOIR

A highly capable observatory in space 

provides an excellent opportunity to make 

high-resolution (spatial, spectral, and 

temporal) observations of planetary bodies. 

Figure 13.36. Galileo (June/28/1997: daylight and eclipse, Nov/8/1997, Sep/19/1997) and HST 

(July/22/1997) observations of the eruptions on Io. Montage by Jason Perry.
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Any of the observations listed above could be 

the focus of one or more dedicated missions 

to those bodies. LUVOIR could fulfill, 

complement, or supplement any mission 

objectives for a mission to those bodies.

13.21.3 The science program

Observations of the moons of Saturn and 

Jupiter would require planning for optimal 

viewing of their transits about their planet. 

Imaging with HDI would be useful for 

characterizing the composition of these 

moons and to identify activity there. Ideally, 

wavelengths should go beyond 3 μm for 

water observations (longer for thermal 

observations).

Program at a Glance

Science goal: Planetary observations to identify surface and environmental changes on 

the moons of Jupiter and Saturn.

Program details: Observing silicate and icy bodies extensively (over days and possibly 

weeks) allows for the identification of changes such as volcanic or eruptive activity on 

outer Solar System moons. Measurements into thermal bands will support observations 

of Io’s volcanic activity.

Instrument(s) + configuration(s): HDI 

Key observation requirements: Imaging at 0.5–3.5 μm; several meter-km spatial 

resolution. Observations of the moons of Jupiter and Saturn require tracking and highprecision pointing.
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13.22 Transit spectroscopy of Earth-sized planets around M-dwarfs

Avi Mandell (NASA GSFC) and Eric Lopez (NASA GSFC)

13.22.1 Introduction

Studying transiting planets is highly complementary to studies of directly imaged planets: (1) we can readily measure the mass 

and radius of transiting planets, linking atmospheric properties to bulk composition and 

formation, (2) many transiting planets are 

strongly irradiated resulting in novel atmospheric physics, and (3) the most common 

temperate terrestrial planets orbit close to 

red dwarf stars (M-dwarfs) and are difficult to 

image directly, but are comparatively likely to 

transit at high signal to noise. The Transiting 

Exoplanet Survey Satellite (TESS) will discover transiting planets orbiting the brightest stars and should discover a small number of temperate terrestrial planets transiting 

nearby early-to-mid M-dwarfs. Furthermore, 

ground-based surveys of very-late M-dwarfs 

and sub-stellar primaries may yield additional 

targets—in fact, one of the best Earth-sized 

HZ targets to date is TRAPPIST-1e (Gillon 

et al. 2017). Follow-up of these discoveries 

should provide the first opportunity to place 

constraints on the atmospheres and habitability of temperate terrestrial planets.

JWST will be a fantastic platform for 

examining larger and brighter planets, 

resulting in a revolution in our understanding 

of hot planets orbiting close to their parent 

stars. However, characterizing the smaller, 

cooler worlds will be incredibly time-intensive: 

JWST will need months of integration time 

to provide tantalizing constraints on the 

presence of an atmosphere. The amplitude 

of spectral features for a temperate terrestrial 

planet transiting in front of a nearby M-dwarf 

is comparable to the single-transit photoncounting precision with JWST; therefore, 

in the absence of a systematic noise floor, 

100 transits of such a planet could yield 

10-σ detections of greenhouse gases—and 

this neglects the effects of cloud opacity in 

damping the signal of spectral absorption 

(Cowan et al. 2014). Spending a total of 

one month of JWST time to characterize the 

atmosphere of a potentially habitable world is 

compelling, but the observations would have 

to be spread out over nearly a decade for a 

planet in a month-long orbit (this scheduling 

problem is somewhat alleviated for planets 

in the habitable zones of later M-dwarfs and 

sub-stellar companions, which have shorter 

orbital periods).

13.22.2 The role of LUVOIR

JWST will most likely make important inroads 

into the exploration of temperate Earth-sized 

planets around M-dwarfs, but it is entirely 

possible that little will be known about the 

atmospheres of these planets by the time 

JWST ends its mission—and further (and 

deeper) study will be left for a future flagship 

mission with equal or greater photongathering power.

LUVOIR will be a capable successor to 

JWST in this regard. LUVOIR Architecture A 

will have a collecting area 178 m2, a factor 

of 7x larger than JWST, and observations 

will reach the same SNR with 2.7x less 

integration time. The primary instrument for 

transit spectroscopy with LUVOIR will be 

the High Definition Imager (HDI), due to the 

broad simultaneous wavelength coverage 

(200 nm–2.5 μm) and the ability to spatially 

scan the spectra of bright stars across the 

large focal plane detectors. HDI will have 

sets of grisms and will operate similarly to 

the Wide Field Camera 3 (WFC3) instrument 

currently on HST. It will have the capability 

of full-throughput observations in either 

short (200–900 nm) or long (800 nm–2.5 

μm) wavelength channels, or simultaneous 
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observations with both bands but at half 

the throughput. Spectral resolution will 

be R~500, enabling full characterization 

of spectral bands of molecular as well as 

atomic species. 

LUVOIR will improve on JWST 

measurements between 0.8 and 2.5 μm, a 

band which covers molecular features of H2O 

and CH4 and therefore provides constraints on 

the water vapor content and oxidation state 

of the atmosphere. At shorter wavelengths 

(0.2–0.8 μm), LUVOIR’s capabilities will 

be unique. In particular, measurements of 

Figure 13.37. Simulated transit spectrum of TRAPPIST-1e with LUVOIR-A (15-m), assuming 50 

transits combined. The effective resolution of the spectrum is R=150 at l > 1.2 μm, R = 30 at 700 

nm < l < 1.2 μm, and R = 10 at l < 700 nm. Spectral bands of multiple key atmospheric species 

are visible by eye, enabling constraints on habitability. Credit: Planetary Spectrum Generator 

Tool

Rayleigh scattering and possibly O3 at 200–

300 nm will be the first searches for these 

key biomarkers of photosynthetic life.

13.22.3 The science program

The TRAPPIST-1 system will be a high priority 

for any future transit spectroscopy science 

program. Figure 13.37 illustrates what 

could be accomplished with 50 transits on 

the potentially habitable Earth-sized planet 

TRAPPIST-1e. A number of molecules are 

visible even by eye, and it is clear that strong 

constraints on the atmospheric chemistry 

Program at a Glance

Science goal: Characterizing the atmospheres of potentially habitable planets to constrain 

the chemical compositions and search for signs of life.

Program details: Transit spectroscopy of Earth-sized planets in the habitable zones of 

nearby M-dwarf and brown dwarf primaries.

Instrument(s) + configuration(s): HDI grism spectroscopy

Key observation requirements: 0.4 nm–2.5 μm; R~150; ultra-high precision 

spectroscopy of bright sources (photometric uncertainty < 3 ppm).
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and even on biomarker species such as O2 

will be possible.

Sullivan et al. (2015) modeled the yield 

of planets discovered by the TESS mission, 

determining that the mission would discover 

between 2 and 7 temperate Earth-sized 

planets orbiting M-dwarfs with K < 9. These 

stellar hosts will be larger than TRAPPIST-1, 

but will also be brighter, and may therefore 

provide a similar SNR with the same amount 

of observing time.

References

Cowan, N., et al. 2015, PASP, 127, 311

Gillon, M., et al. 2017, Nature, 542, 456
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13.23 The transmission spectra of rock atmospheres on magma worlds

Eric Lopez (NASA GSFC) and Avi Mandell (NASA GSFC)

13.23.1 Introduction

One of the most exciting revelations from 

planetary transit surveys has been the 

discovery of a new population of extremely 

irradiated rocky planets on ultra-shortperiod (USP) orbits (e.g., Charbonneau et al. 

2009; Léger et al. 2009; Batalha et al. 2010; 

Sanchis-Ojeda et al. 2014). The irradiation 

on these planets is so intense, with dayside 

temperatures reaching over 2500 K, that they 

are expected to have dayside surfaces that 

are partially or completely molten (Kite et al. 

2017). Although completely uninhabitable, 

these planets have the potential to teach us 

a great deal about geophysical processes 

under extreme conditions. Indeed, models 

predict that these planets should partially 

vaporize their rocky mantles and outgas 

significant but highly refractory atmospheres 

dominated by silicates and heavy metals, 

potentially allowing us to directly probe their 

bulk mantle compositions (Miguel et al. 

2011). Moreover, models also predict that 

alkali metals in this outgassed material may 

be escaping at significant rates, producing 

highly extended metallic exospheres possibly 

reaching all the way to the Roche lobe (Kite et 

al. 2017). However, these exospheres would 

be highly time-variable, driving the need to 

obtain higher S/N over a single transit. Indeed, 

recently there was a claimed detection of 

significant thermal variability in the dayside 

emission from the USP 55 Cancri e (Demory 

et al. 2016) along with the possible detection 

of highly extended exospheric Na and Ca+ 

absorption (Ridden-Harper et al. 2016).

Given their short periods and extremely 

high temperatures, JWST is extremely well 

suited for characterizing the overall thermal 

emission from these planets (Samuel et al. 

2014) and to detect emission from SiO at 4 

and 10 microns (Ito et al. 2015). However, 

due to its wavelength coverage JWST 

will be limited in its ability characterize the 

atmospheric compositions of USP planets 

with transmission or emission spectroscopy 

since many of the primary lines for the 

relevant metal species aside from SiO are 

found in the optical and near UV between 

300 and 800 nm (e.g., Na, K, Ca, Mg, Fe, 

TiO, VO), and therefore are inaccessible with 

JWST. At the same time, the intrinsic rarity 

of these USP planets means that even after 

the NASA’s TESS mission concludes, most 

of the known USPs will be around relatively 

faint FGK stars (V-mag >11), making them 

difficult to characterize with HST due to its 

smaller aperture.

13.23.2 The role of LUVOIR

With its combination of large aperture and 

broad UV and optical wavelength coverage, 

LUVOIR using will be uniquely suited to 

probe the compositions of vaporized 

atmospheres on USP planets. Between 300 

and 800 nm, a range covered by the UVIS 

Figure 13.38. Predicted compositions of outgassed atmospheres for a Bulk Silicate Earth 

composition from Miguel et al. (2011), black 

points show transitions where the dominant 

species changes.
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channel on HDI, are a series major spectral 

features for the key metal species predicted 

by outgassing models. In particular, features 

like sodium D at 590 nm, calcium H & K 

at 390 nm, potassium at 770nm, and the 

TiO from 600 to 800 nm, should all be 

detectable in either transmission or emission 

spectroscopy along with a wide range of iron 

and magnesium lines between 300 and 600 

nm. For example, modeling the emission 

spectra of several known USPs, Ito et al. 

(2015) predicted that both the 590 nm Na 

and 770 nm K features should be observable 

with moderate spectral resolution (R~500) in 

emission at ~9 ppm for a 2500 K USP planet 

and ~36 ppm for a 3000 K planet.

Additionally, if these planets do intend 

possess extended exospheres then these 

should be detectable in transmission at 

S/N in single transit, which is important 

given the expected variability in both the 

planetary atmospheric escape rates and 

the stellar spectrum. The possible detection 

of exospheric Na & Ca+ on 55 Cancri e by 

Ridden-Harper et al. (2016) claimed a 7% 

transit in the 590 nm sodium line extending 

all the way to the planet’s Roche lobe at ~5 

Earth radii.

Given the extremely short orbital periods, 

typical transit durations are just 1.5–2 hours. 

However, the 15-m LUVOIR Architecture A 

should be able to reach a precision of ~16 

ppm at a R~500 for a 12th magnitude G-star 

at ~150 pc in a single 1.5-hour integration. 

This will allow the detection of any exospheric 

metals at high S/N in single transit, while for 

a planet with dayside temperature of 3000 

K, sodium should be detectable in emission 

from the lower atmosphere at ~5s with just 5 

transits or ~10 hours of integration.

13.23.3 The science program

Given the relatively short integration times 

needed, it should be possible to obtain multiepoch observations for multiple spectral 

features on multiple exoplanets, especially 

in transmission. Currently there are already 

five well-studied USP planets (55 Cancri e, 

CoRoT-7b, Kepler-10b, Kepler-78b, and K2-

141b), which would be well suited to these 

observations, with the likelihood that at 

least a few more will be found by upcoming 

surveys like TESS and PLATO. With just ~50 

hours of observation time per planet it will 

be possible to obtain ~10 transits and 5–20 

eclipses per planet, which will allow us to 

search for extended exospheres in multiple 

species, including all the common alkalis, 

characterize those species in emission close 

to the planet’s surface, and study their time 

variability.

References

Charbonneau, D., Berta, Z. K., Irwin, J., et 

al. 2009, Nature, 462, 891

Léger, A., Rouan, D., Schneider, J., et al., 

2009, A&A, 506, 287

Batalha, N.M., Borucki, W.J., Bryson, S.T., 

et al. 2011, ApJ, 729, 27

Program at a Glance

Science goal: Characterizing the compositions of vaporized silicate and metal atmospheres 

from extremely hot molten rocky planets.

Program details: ~250 hours for multi-wavelength and multi-epoch NUV and optical 

transmission and emission spectroscopy for the best ~5 ultra-short-period exoplanets.

Instrument(s) + configuration(s): HDI UVIS spectroscopy

Key observation requirements: 300–800 nm; R=500; ultra-high precision spectroscopy 

of bright sources (photometric uncertainty ~ 2–7 ppm)
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Miguel, Y., Kaltenegger, L., Fegley, B., & 

Scaefer, L., 2011, ApJ, 742, 19

Sanchis-Ojeda, R., Rappaport, S., Winn, J., 

N., et al., 2014, ApJ, 787, 47

Samuel, B., Leconte, J., Rouan, D., et al. 

2014, A&A, 563, 103

Ito, Y., Ikoma, M., Kawahara, H., et al., 

2015, ApJ, 801, 144

Demory, B., Gillon, M., Madhusudhan, N., & 

Queloz, D., 2016, MNRAS, 455, 2018

Kite, E. S., Fegley, B., Schaefer, L., & 

Gaidos, E., 2016, ApJ, 828, 80

Ridden-Harper, A.R., Snellen, I.A.G., Keller, 

C.U., et al., 2016, A&A, 593, 129
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13.24 A statistical search for global habitability and biospheres beyond 

Earth

Shawn Domagal-Goldman (NASA GSFC), Jacob Bean (University of Chicago), Eliza Kempton 

(Grinnell College, University of Maryland), Sara Walker (Arizona State University)

13.24.1 Introduction

Here, we propose a statistical search for 

habitability and life. Such a search would 

provide tests for our understanding of the toplevel factors controlling planetary habitability 

and raise the overall confidence that we are 

not alone. This search would be conducted 

by searching for signs of habitability and of 

life on multiple planets, both in and beyond 

the habitable zone.

The statistical search for habitability 

would turn the concept of the habitable zone 

into a testable hypothesis. According to the 

habitable zone hypothesis, there is a region 

around the star for which feedback processes 

in the climate system work to stabilize surface 

temperatures to allow for sustained, global 

reservoirs of liquid surface water. Beyond this 

zone, the climate would stabilize in waterpoor regimes, with vastly different surface 

temperatures. Because of all the feedbacks 

in the climate system, multiple aspects of 

a planet’s spectral signatures should also 

change beyond the habitable zone: CO2 

concentrations (and their features) should 

increase on either side of the habitable zone 

(due to lack of a CO2 weathering feedback); 

water-soluble gases and aerosols should 

increase in concentration; and water vapor 

and water cloud features themselves should 

disappear. For a more thorough discussion 

of this concept, and how it can begin with 

near-term observations, see Bean, Abbot, & 

Kempton (2017). 

A statistical search for life should raise 

the overall confidence in our detection of life 

beyond Earth compared to what is achievable 

by the search for life on a single exoplanet 

(Walker et al., 2018). For example, consider 

an exoplanet for which LUVOIR detects the 

presence of water (H2O), molecular oxygen 

(O2) and ozone (O3), but not methane (CH4). 

This is a plausible scenario, given the 

long integration times required to detect 

CH4 in LUVOIR’s wavelength range. The 

combination of these measurements, as well 

as constraints on UV fluxes from observations 

of the host star, would produce a scenario 

for which the most likely explanation is life 

on that planet. However, the lack of CH4 

on that planet may be enough to prevent a 

conclusion as bold as “We are not alone.” 

As an alternative, consider a scenario where 

LUVOIR detects this same set of features 

multiple times on worlds in different planetary 

systems. This would suggest that either our 

understanding of photochemical processes 

is woefully incomplete and under-predicting 

some O2-producing mechanism, or that life 

is present on at least some subset of these 

worlds.

This example would also apply to other 

biosignatures. An organic haze in the 

presence of a CO2-rich atmosphere has 

been recently proposed as a biosignature 

(Arney et al., 2018). While consideration 

to false positives has been given, a hazebiosignature has not undergone the same 

amount of thorough scrutiny as O2/O3. 

Thus, one might place a lower amount of 

confidence in this biosignature than the 

detection of O2/O3. To quantify this example, 

if we are to assume that ~50% of CO2-rich, 

haze-bearing habitable zone worlds have life, 

the detection of multiple such planets would 

increase the likelihood that at least one of 

these planets has a biosphere (whereas a 
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search focused on a single target would not 

be able to draw such a strong conclusion).

This search does not necessarily need to 

focus on the same signature being present 

on multiple worlds. It would also apply to any 

combination of potential biosignature across 

multiple planets, and thereby allow the mission 

to conduct both a broad survey for easy-todetect biosignatures on multiple planets and 

a detailed assessment of the biosignatures 

on LUVOIR’s best targets. The approach 

could therefore by optimized so that detailed 

assessments (such as the measurement 

of longitudinal-dependent colors/spectra) 

are only conducted on relatively close 

targets, for which such assessments are 

capable of being conducted with reasonable 

investments of telescope time.

A statistical search will also be useful 

in the case of no (or very poor) signs of life 

being detected by LUVOIR. Instead of just 

a full null result, the process of quantifying 

the likelihood of life on each planet, with a 

statistical combination of those likelihoods, 

will yield an overall confidence level that 

sets an upper-bound on the frequency of 

life. Such a conclusion is not really possible 

unless we conduct a statistical survey of 

multiple worlds rather than a deep dive on 

a single planet. Further, the more planets we 

assess, the stronger our conclusions will be. 

Detecting no biosignatures on 55 habitablezone planets would give a 2-s upper limit 

on the frequency of life of 6%. Thus, our 

uncertainty on “etalife” would decrease by an 

order of magnitude, from a range of 0–1.0 to 

a range of 0–0.06.

Again, it is useful to consider an example: 

if LUVOIR finds multiple Earth-sized worlds 

in the habitable zone of other stars and 

determines that many of them have “antibiosignatures” (such as high atmospheric 

concentrations of H2 and either CO or CO2), 

the confidence level that life is rare might be 

high.

There are two requirements to such an 

approach to the search for life:

1) The science community must be able to 

ascribe a quantitative estimate a given 

data set resulted from a biosphere on any 

particular planet; and

2) We must be able to collect spectroscopic 

data on many worlds.

The former requirement demands 

development in the research tools utilized by 

the astrobiology and exoplanet communities. 

Options for quantifying our assessment that 

a given planet has a biosphere are discussed 

and reviewed by Walker et al. (2018, in press), 

and leverage similar efforts to quantify signs 

of life in our own Solar System. The latter 

requirement inevitably leads to a LUVOIRsized telescope.

13.24.2 The role of LUVOIR

A statistical approach to habitability and the 

habitable zone does not require LUVOIR, as 

it could begin with ground- and space-based 

surveys for H2O and CO2 features via transit 

spectroscopy (Bean et al., 2017). LUVOIR 

would extend this statistical approach to new 

targets, with new observational techniques. 

Specifically, LUVOIR would expand the 

search to planets around Sun-like (F, G, K) 

stars. It would also provide new information 

on clouds and aerosols on planets around 

Sun-like and M-type stars, via a wavelength 

range that is complementary to currentlyplanned transit spectroscopy observatories.

While other missions might attempt to 

search for signs of life, and perhaps detect 

them on one world, a statistical approach 

requires that this search be conducted on a 

large number of worlds. This, in turn, demands 

a large-aperture telescope, to drive up the 

yield of planets that the mission can discover 

and characterize. LUVOIR will be able to 

detect over 50 such planets. This is a large 

enough sample size to conduct a statistical 
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search for signs of life on exoplanets. If 

detectable biospheres exist on at least 10% 

of potentially habitable worlds, LUVOIR will 

find at least one such biosphere. But if life is 

relatively common, LUVOIR has the potential 

to find signs of life on many worlds. If it is 

rare, we will be able to set upper bounds on 

the frequency of life.

13.24.3 The science program

A statistical search for habitability and life 

would be consistent with the main exoplanet 

observing program outlined in Chapter 3. 

It would be conducted almost exclusively 

via ECLIPS observations of exoplanets in 

the habitable zones of their host stars, and 

LUMOS observations would obtain the UV 

spectral energy distribution of those host 

stars. It would use different bands within the 

ECLIPS wavelength range, depending on the 

distance to the target and the spectral feature 

in question, and likely would run ECLIPS 

to the point where integration on individual 

bands becomes time-limited. Details of this 

approach are below.

The main difference of the approach 

outlined here to the main exoplanet 

characterization program is in how optimized 

the biosignature search becomes, and how 

the data from that search are processed. 

Figure 13.39. An example of the observing strategy for a statistical approach to the search for life 

on biosignatures. Credit: S. Domagal-Goldman (NASA GSFC)
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This approach would likely require an 

optimized search for biosignatures, spending 

as little time as possible confirming/

rejecting biosignatures on each planet. This 

search would likely focus on planets with 

atmospheric H2O, which would be found 

during LUVOIR’s planet detection phase. 

In addition to weeding out dry planets, this 

initial phase would also include a search for 

H2O beyond the habitable zone. This would 

help test the habitable zone concept by 

mapping the presence of H2O water vapor 

as a function of stellar irradiation (Bean et al. 

2017).

Then, the search would turn towards easyto-detect, but low-confidence biosignatures, 

such as O3 and atmospheric organic haze, 

both of which can be detected in the UV 

channel of ECLIPS. This initial biosignature 

search would include some planets near—

but not in—the habitable zone. This would 

provide a basis for comparison of presumed 

“dead” planets, which could lead to more 

robust conclusions about the detection of 

life and help inform the science community 

if something about our understanding of 

photochemical and geological processes is 

dramatically incomplete.

For planets with O3 or an organic haze, 

LUVOIR would proceed to the detection of 

O2 (for O3-bearing worlds) and CO2 (for hazebearing worlds). However, it would only do 

so if the detection of this second feature was 

permissible in a reasonable integration time. 

(The definition of “reasonable” would be up 

to a future time allocation committee, but 

history suggests  100 hours.) If O2 or CO2 

is detected, LUVOIR would then proceed 

to further spectral measurements to rule 

out false positives, search for secondary 

biosignature gases (such as CH4) or attempt 

to characterize the surface. Again, this would 

be limited on a target-by-target bases based 

on required integration time. A full decision 

tree for such a search is shown in Figure 

13.39. 

The totality of these observations, at 

varying degrees of detail depending on the 

target, would maximize LUVOIR’s ability to 

test our ideas of planetary habitability and 

search for life across the set of exoplanets it 

characterizes.

References

Arney, G. A. et al. 2018, AsBio, 18,4

Bean, J., Abbot, D., & Kempton, E. 2017, 

ApJ, 841, L24

Program at a Glance

Science goal: To find signs of life—or conclusively rule out life in our Sun’s local 

neighborhood.

Program details: This requires an optimized search for biosignatures

Instrument(s) + configuration(s): This would require all of the bandpasses and 

spectroscopy modes of ECLIPS.

Key observation requirements: The main requirement here is a broad wavelength range 

for ECLIPS, running from ~0.2 mm to ~2 mm, so that different signatures with different 

integration times can be detected. This also requires a large-aperture telescope, in order 

to detect and characterize dozens of potentially habitable exoplanets).
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14 Appendix B: Science calculation and simulation details

14.1 Estimating LUVOIR’s exoplanet yield

Christopher C. Stark (STScI)

14.1.1 Methodology

To estimate the yield of directly imaged 

planets, we assume that LUVOIR must 

conduct a blind survey to search for and 

characterize potentially Earth-like exoplanets. 

While the efficiency of the LUVOIR exoplanet 

survey and the quality of its data products 

would benefit from a precursor survey 

identifying potentially Earth-like planets, we 

conservatively assume such a survey does 

not exist at the time of launch. Thus, the 

yield of such a blind survey is a probabilistic 

quantity, which depends on LUVOIR’s 

coronagraphic capabilities, the occurrence 

rate of planets of various types, their 

detectability, and the unknown distribution of 

planets around individual nearby stars. 

To calculate expected exoplanet yields, 

we used the Altruistic Yield Optimization 

(AYO) yield code of Stark et al. (2014), which 

employs the completeness techniques 

introduced by Brown (2005). Briefly, for each 

star in our master target list, we randomly 

distribute a large number of synthetic planets 

of a given type, forming a “cloud” of synthetic 

planets around each star, as shown in Figure 

14.1. Planet types are defined by a range 

of radii, albedo, and orbital elements. We 

calculate the flux from each synthetic planet 

in reflected light given its properties, orbit, 

and phase, and then determine the exposure 

time required to detect it at SNR=7. Based 

on these detection times and the exposure 

time of a given observation, we can calculate 

the fraction of the synthetic planets that 

are detectable, i.e., the completeness, as a 

function of exposure time. The completeness 

simply expresses the probability of detecting 

that planet type, if such a planet exists. 

The average yield of an observation is the 

Figure 14.1. The completeness of an observation is the fraction of detectable planets to total 

planets and is a function of the exposure time. The yield of an observation is the product of 

completeness and the probability that such a planet actually exists (the occurrence rate).
LUVOIR The Large UV Optical Infrared Surveyor

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product of the completeness and the 

occurrence rate of a given planet type. We 

repeat this process for every observation 

until the total program lifetime is exceeded, 

arriving at an average total mission yield. 

In reality, yields may vary from this average 

due to the random distribution of planets 

around individual stars; we incorporate this 

source of uncertainty in our yield calculations 

by accounting for the Poisson probability 

distribution of planets for each star. 

We employ the techniques of Stark 

et al. (2015) and Stark et al. (2016), which 

optimize the observation plan to maximize 

the yield of potentially Earth-like planets. 

For a coronagraph-based search, this 

involves optimizing the targets selected 

for observation, the exposure time of each 

observation, the delay time between each 

observation of a given star, the number of 

observations of each star, and the planet 

phase for spectral characterization (Stark 

et al. 2015). We do not explicitly schedule 

the observations. We expect the ability 

to schedule the observations will have a 

negligible impact on the exoplanet yield given 

LUVOIR’s extremely large field of regard and 

rapid retargeting capabilities.

14.1.2 Inputs and assumptions

Yield estimates require simulating the 

execution of a mission at a high level. They 

are therefore dependent on a large number 

of assumptions about the target stars, 

the planetary systems they host, and the 

capabilities of the mission. Given the inherent 

uncertainties in many of these assumptions, 

consistency between yield analyses is of 

primary importance. We adopt inputs and 

assumptions that are consistent with the 

choices made by the Exoplanets Standard 

Definitions and Analysis Team and those 

made by the HabEx STDT. We now review 

and justify our fiducial assumptions about 

the parameters that affect the yield.

14.1.2.1 Astrophysical assumptions

14.1.2.1.1 Planet types & occurrence 

rates

We followed the planet categorization 

scheme of Kopparapu et al. (2018), which 

consists of a 3 by 5 grid of planets binned 

by temperature (hot, warm, and cold) and 

planet radius (“rocky,” “super-Earths,” “subNeptunes,” “Neptunes,” and “Jupiters”), 

as shown in Figure 14.2. Each planet was 

assigned a single albedo defined by its radius 

(listed in Figure 14.2), a Lambertian phase 

function, and all planets were assumed to 

be on circular orbits. We assume that the 

semi-major axis boundaries that define the 

temperature bins of each planet type scale 

with the bolometric stellar insolation, such 

that they scale with the square root of the 

bolometric stellar luminosity.

For exoEarth candidates we adopted the 

green outlined region in Figure 14.2. By this 

definition, exoEarth candidates are on circular 

orbits and reside within the conservative HZ, 

spanning 0.95–1.67 AU for a solar-twin star 

(Kopparapu et al. 2013). We only include 

planets with radii smaller than 1.4 Earth radii 

and radii larger than or equal to 0.8a–0.5, 

where a is semi-major axis for a solar-twin 

star. The lower limit on our definition of the 

radius of exoEarth candidate is derived from 

an empirical atmospheric loss relationship 

derived from solar system bodies (Zahnle 

& Catling 2017). The upper limit on planet 

radius is a conservative interpretation of an 

empirically measured transition between 

rocky and gaseous planets at smaller semimajor axes (Rogers 2015). All exoEarth 

candidates were assigned Earth’s geometric 

albedo of 0.2, assumed to be valid at all 

wavelengths of interest.
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We adopted the exoplanet occurrence 

rate values from the NASA ExoPAG SAG13 

meta-analysis (Kopparapu et al. 2018), given 

by

where N(R,P) is the number of planets per 

star in a bin centered on radius R and period 

P, R is in Earth radii and P is in years, and 

[G,a,b] = [0.38,-0.19,0.26] for R < 3.4 REarth

and [G,a,b] = [0.73,-1.18,0.59] for R ≥ 3.4 

REarth. Figure 14.2 lists the occurrence rates 

when integrating over the boundaries of 

each planet type. Within each planet type, 

we adopted the SAG13 radius and period 

distribution above. With this distribution, 

within a given planet temperature bin, small 

planets outnumber large planets. 

The SAG13 meta-analysis is a crowdsourced average of published and 

unpublished occurrence rates, averaged 

over FGK spectral types. Uncertainties 

on the SAG13 occurrence rates are not 

well understood and are simply set to the 

standard deviation of the crowd-sourced 

values. Because of the large uncertainties 

in the SAG13 occurrence rates, we have 

weak constraints on how occurrence rates 

change with spectral type. Thus, we simply 

assume that the occurrence rates for each 

planet type bin are independent of spectral 

type. Table 14.1 summarizes the key 

astrophysical assumptions underlying our 

exoEarth candidate yield calculations.

Figure 14.2. Planet classifications for a solar twin used for yield modeling, including bin-integrated 

occurrence rates (h) and geometric albedos (AG). Planets are binned into hot (red), warm (blue), 

and cold (ice blue) temperature bins and rocky, super-Earth, sub-Neptune, Neptune, and Jupiter 

size bins. The green outline indicates the boundaries of exoEarth candidates. The semi-major axis 

boundaries shown are for a solar-twin star; for other types of stars, semi-major axis boundaries 

are scaled to maintain a constant bolometric stellar insolation.

0.1 1.0 10.0

a (AU)

1

10

R (Earth radii)

AG=0.5

AG=0.2

η=0.68 +1.37

−0.45

η=0.30 +0.74

−0.21

η=1.61 +4.66

−1.18

η=0.47 +0.57

−0.26

η=0.21 +0.32

−0.13

η=1.16 +2.15

−0.74

η=0.48 +0.30

−0.19 η=0.22 +0.19−0.10 η=1.33 +1.48−0.69

η=0.08 +0.05

−0.03 η=0.07 +0.06−0.03 η=0.95 +1.23−0.54

η=0.05 +0.05

−0.02 η=0.05 +0.06−0.03 η=0.58 +1.07−0.38

η⊕=0.24+0.46

−0.16

0.1 1.0 10.0

a (AU)

1

10

R (Earth radii)
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14.1.2.1.2 Exozodiacal & zodiacal 

dust

Exozodiacal dust adds background noise, 

thereby reducing the SNR of a planet detection 

relative to the case of no exozodiacal dust. 

Unfortunately, we currently have only weak 

constraints on the amount of zodiacal dust 

around our target stars. Upcoming Large 

Binocular Telescope Interferometer (LBTI) 

measurements will better inform this current 

working assumption in the near term, but at 

the time of this report are unavailable.

We therefore simply adopted a baseline 

exozodi level of 3 zodis. Our definition of 1 

zodi is a uniform (optically-thin) optical depth 

producing a V band surface brightness of 22 

mag arcsec-2 at a projected separation of 1 

AU around a solar twin. Thus, the exozodi 

surface brightness drops off as the inverse 

square of the projected separation (Stark et 

al. 2014). Because the HZ boundaries scale 

by the bolometric stellar insolation, the V 

band surface brightness of 1 zodi of exozodi 

varies with spectral type (Stark et al. 2014). 

The solar system’s zodiacal brightness 

varies with ecliptic latitude and longitude; 

the closer one observes toward the Sun, the 

brighter the zodiacal cloud will appear. We 

calculated the zodiacal brightness for each 

target star by making simple assumptions 

about typical telescope pointing (Leinert 

et al. 1998). We assume the coronagraph 

can observe near where the local zodi is 

minimized and adopted a solar longitude of 

135 degrees for all targets.

14.1.2.1.3 Target catalog

Our input star catalog was formed using the 

methods of Stark et al. (in prep). Briefly, the 

target list is equivalent to the union of the 

original Hipparcos catalog and the GAIA 

TGAS catalog. For each star, we adopted the 

most recent measured parallax value from the 

original Hipparcos, updated Hipparcos, and 

GAIA TGAS catalogs, then down-selected 

to stars within 50 pc. BVI photometry and 

spectral types were obtained from the 

Hipparcos catalog. Additional bands and 

Table 14.1. Summary of astrophysical assumptions.
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missing spectral types were supplemented 

using SIMBAD. We filtered out all stars 

identified as luminosity class I–III, leaving 

only main sequence stars, sub-giants, and 

unclassified luminosity classes.

We note that while the accuracy of any 

individual star’s parameters may be important 

when planning actual observations, yield 

estimates can be very robust to these 

inaccuracies, as their effects average out 

when considering a large target sample. 

Given LUVOIR’s extremely large sample 

size of hundreds of stars, inaccuracies in 

the target catalog should have a negligible 

impact on yield.

14.1.2.2 Binary stars

Detecting exoplanets in binary star 

systems presents additional challenges. 

Light from companion stars outside of the 

coronagraph’s field of view, but within the 

field of view of the telescope, will reflect off 

the primary and secondary mirrors. Due to 

high-frequency surface figure errors and 

contamination, some of this light is scattered 

into the coronagraph’s field of view. For some 

binary systems, this stray light can become 

brighter than an exoEarth.

We directly calculate the stray light from 

binary stars in the final image plane. We 

adopt the numerical stray light models of 

Sirbu et al. (2018, in prep). These models 

predict the power in the wings of the PSF 

at large separations assuming a l/20 RMS 

surface roughness and an f-3 envelope, 

where f is the spatial frequency of optical 

aberrations. We assume that the stray light 

can be measured or modeled and include it 

simply as an additional source of background 

noise. We make no artificial cuts to the target 

list based on binarity and allow the benefitto-cost optimization in the AYO yield code 

to determine whether or not stray light noise 

makes a target unobservable. In practice, 

the AYO prioritization does reject a number of 

binary systems with contrast ratios close to 

unity and/or close separations. We note that 

including the full amount of light scattered by 

the companion is actually conservative, as 

the companion scattered starlight could be 

actively reduced with specialized observation 

methods. For example, LUVOIR could use 

multi-star or super-Nyquist wavefront control 

coronagraphic techniques (Thomas et al. 

2015; Sirbu et al. 2017).

14.1.2.3 Mission parameters

We adopted a total of 2 years for the 

exoEarth survey time (including overheads). 

Coronagraph observations were assessed 

a 5 hour overhead on all exposures for 

wavefront control, a reasonable assumption 

based on the WFIRST CGI operations 

concept. Total exposure time and overheads 

were required to fit within the exoplanet 

science time budget.

For planet detections, we required an 

SNR=7 evaluated over the full bandpass of the 

detection instrument, where both signal and 

noise are evaluated in a simple photometric 

aperture of 0.7 l/D in radius (where in this 

case, D is the diameter of the outer edge 

of the Lyot stop). The SNR was evaluated 

according to Eq. 7 in Stark et al. (2014), 

which includes a conservative factor of 2 on 

the background Poisson noise to account for 

a simple background subtraction. We also 

included a background term for detector 

noise, discussed in Section 14.1.2.4.2. For 

spectral characterizations, we required a 

spectrum with R=70 and SNR=5 per spectral 

channel which we evaluated at a wavelength 

of 1 micron, to search for water vapor in all 

detected exoEarth candidates.

14.1.2.4 Instrument performance 

assumptions

14.1.2.4.1 Coronagraph assumptions

Coronagraph performance was estimated 

via a wave propagation model, assuming an 
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idealized optical system and perfect wavefront control. We adopted two coronagraphs 

designed for the LUVOIR Architecture A segmented, on-axis primary: an apodized pupil 

Lyot coronagraph (APLC) and a charge 6 

vortex coronagraph (VC). The APLC coronagraph consisted of 3 masks: a small-IWA 

mask (10% bandwidth, 3.8–12 l/D working 

angle), a medium-IWA mask (15% bandwidth, 6–20 l/D working angle), and a largeIWA mask (11–33 lD working angle). To 

each star we independently assigned either 

one of the APLC masks or the VC coronagraph, based on the wavelength of observation and a cost-to-benefit ratio. We simulated the leaked starlight as a function of stellar 

diameter and the off-axis PSFs as a function 

of angular separation, providing inputs to the 

yield calculations according to the standards 

of Stark & Krist (2017).

The wave propagation model does 

not include some known systematic noise 

sources, such as residual spatial speckle 

noise caused by dynamic wavefront errors. 

Realistic estimates would require full end-toend simulations of a well-defined telescope, 

instrument, and observing procedure. These 

effects will impact the coronagraph noise 

floor, the properties and frequency of false 

positives, and the final yield.

The APLC coronagraph design consists of 

a binary apodizer mask in the entrance pupil, 

followed by an image plane coronagraph 

mask, followed by a Lyot stop (N’Diaye 

et al. 2016). The apodizer is optimized to 

maximize coronagraph throughput while 

achieving 10-10 raw contrast over a desired 

range of working angles (e.g., Zimmerman 

et al. 2016). In the end, we designed three 

separate masks with overlapping working 

angles, allowing us to maximize the 

throughput for any size habitable zone. The 

resulting APLC designs are robust to stellar 

diameter and jitter (tolerance ~1 mas stellar 

diameter and ~0.5 mas jitter respectively for 

the LUVOIR Architecture A primary at 600 

nm).

The apodized vortex coronagraph design 

(Mawet et al. 2013) consists of a grayscale 

apodizer mask in the entrance pupil, followed 

by a vortex phase mask in the focal plane 

(Mawet et al. 2009), and an annular Lyot 

stop. We chose a charge 6 vortex mask to 

balance sensitivity to low order aberrations 

and throughput at small angular separations 

(Ruane et al. 2017). We used the Auxiliary 

Table 14.2. Summary of adopted coronagraph performance. Listed contrast is for a theoretical 

point source; contrasts used in simulations included the effects of finite stellar diameter. While only 

the spatially averaged raw contrast and coronagraph throughput are indicated, AYO simulations 

used their actual values at the planet angular separation.
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Field Optimization algorithm (Jewell et al. 

2017) to optimize the grayscale apodizer 

and reduce the diffracted starlight due to the 

central obscuration, the secondary mirror 

support structures, and gap between mirror 

segments in an annular region about the star. 

Table 14.2 summarizes the coronagraph 

performance that we adopted. We note that 

although these metrics may provide a useful 

high-level understanding of coronagraph 

performance, some metrics should be 

interpreted with caution. For example, the 

inner working angle (IWA) estimates where 

the planet’s throughput reaches 50% of the 

maximum value, but this does not mean that 

there is no planet signal interior to the IWA. 

On the contrary, the vortex coronagraph 

provides useful (albeit lower) throughput 

down to ~2 l/D, such that bright, shortperiod planets may be detectable interior to 

the quoted 3.7 l/D IWA.

The bandpass of the APLC designs is 

limited by the design of the apodized masks 

and was chosen based on a throughputbandwidth tradeoff analysis. To work with 

a segmented on-axis telescope, the vortex 

coronagraph design also uses an apodized 

mask, which was designed assuming 20% 

bandwidth. All of these bandwidths are less 

than or equal to the expected simultaneous 

bandwidth of the wavefront control system. 

High Contrast Imaging Testbed results 

indicate that surpassing a bandwidth of Δλ/

λ=0.2 is challenging with a conventional dual 

DM coronagraph layout, thereby justifying 

our adopted maximum bandwidth of 20%. 

The total throughput of the system in 

Table 14.2 is evaluated at visible wavelengths and includes the reflectivity of all optical surfaces, the detector quantum efficiency 

(QE), detector readout ineffeciencies in photon-counting mode, IFS throughput, and a 

5% contamination budget. Detector parameters are discussed below. This throughput 

metric does not include the core throughput 

of the coronagraph, which was taken into 

account separately via the off-axis PSF simulations discussed above.

14.1.2.4.2 Detector & other 

performance assumptions

Table 14.3lists the detector noise parameters 

that we adopted for yield calculations. We 

calculated the total detector noise count rate 

in the photometric aperture as

where f is the photon counting rate and 

npix is the number of pixels contributing to 

the signal and noise. We tuned f to each 

individual target, such that our photoncounting detector time-resolves photons 

from sources 10 times a bright as an Earthtwin at quadrature.

Table 14.3. Photon-counting CCD noise parameters adopted for yield modeling.
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We assumed the IFS splits the core of 

the PSF into 4 lenslets at the shortest wavelength, each of which are dispersed into 6 

pixels per spectral channel for a total of 24 

pixels per spectral channel at the shortest 

wavelength. For broadband coronagraphic 

detections using the imager, we adopted 4 

pixels for the core of the planet. We note that 

the assumed detector noise is sufficiently 

low that small changes to the number of pixels have a negligible impact on yield.

14.1.3 Operations concepts

Yield is commonly thought of as the number 

of planets detected and/or characterized. 

As shown by Stark et al. (2016), the yield 

of a mission is very sensitive to precisely 

what measurements are required for 

“characterization,” and how the mission goes 

about making those measurements. Thus, 

the yield depends on the science products 

desired and how the mission conducts the 

observations.

14.1.3.1 Desired science products

LUVOIR is designed to be capable of obtaining 

many data products on exoplanets. For the 

exoplanet yield analysis, we considered three 

primary data products on planets identified 

as exoEarth candidates:

1. Photometry: to detect planets and 

measure brightness and color

2. Spectra: to assess chemical composition of atmospheres

3. Orbit measurement: to determine 

if planet resides in HZ and measure spectro-photometric phase variations

In the following sections, we describe 

how LUVOIR will obtain these data products 

in an efficient manner to maximize the yield 

of the mission.

14.1.3.2 Dealing with confusion

Upon initial detection of a possible companion, 

the nature of the source may be unclear. We 

will have only photometry, possibly one color, 

and a stellocentric separation to determine 

the nature of the object. Color, brightness, 

and the fact the source is unresolved may 

allow us to discriminate between many 

background galaxies and exoplanets. 

However, recent work has shown that other 

planets can mimic the color of exoEarth 

candidates (e.g., Krissansen-Totton et al. 

2016). Further, planets that most easily 

mimic Earth are small, hot terrestrial planets, 

which have even higher occurrence rates 

than exoEarth candidates (van Gorkom & 

Stark, in prep); planet-planet confusion may 

be common. However, performing costly 

characterizations on all planets mimicking 

an Earth could decrease the efficiency of 

the exoplanet survey and reduce the yield 

of exoEarth candidates; we may need to 

disambiguate point sources to identify high 

priority planets. LUVOIR is capable of dealing 

with these expected sources of confusion 

without significantly impacting the yield.

14.1.3.3 Order of operations

The order in which observations are 

conducted will impact the final yield of the 

mission. For example, taking spectra of every 

object consistent with an exoEarth candidate 

immediately upon discovery would be costly, 

as spectral characterization times can be long 

and we are likely to find many other planets at 

coincidental phases that mimic exoEarths. A 

more efficient order of operations would play 

to the strengths of the coronagraph, e.g., 

by first following up initial detections with 

orbit measurements, followed by spectra of 

interesting systems when planets are known 

to be at advantageous phases.

Ultimately these decisions will depend 

on uncertain quantities, like hEarth for nearby 

FGK stars and the rate of confusion with 

background objects. A precise operations 

concept will require further detailed study 
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and will surely be adapted “on the fly” during 

mission operations.

14.1.3.4 Simulating operations 

concepts

To simulate a given operations concept, we 

would need to generate a fictitious universe 

and simulate the execution of the mission 

one observation at a time, adapting to 

the detections, non-detections, and false 

positives as we go using decision-making 

logic. While current yield codes are capable 

of doing most of this (Savransky & Garrett 

2016), the critical decision-making logic 

step is extremely complex and in its infancy. 

Realistic decision-making processes require 

simulating the precision of multiple types of 

measurements, estimating the likelihood that 

a planet is an exoEarth versus a background 

object or another planet, and determining 

the optimum criteria for decision-making.

In light of these complexities, we relied 

on the findings of Stark et al. (2016), wherein 

the impact of different operations concepts 

on yield was estimated by adopting general 

rules that define the observation plan. For 

example, to include orbit determination, Stark 

et al. (2016) required each planetary system 

be observed at least six times to a depth 

consistent with detecting an exoEarth. Using 

these methods, Stark et al. (2016) found 

that for coronagraph-based missions like 

LUVOIR, orbit determination is not particularly 

costly while spectral characterization can be 

very costly.

Therefore, we adopt the following 

operations scenario:

1. Detect planets using two bands within 

the UV and VIS coronagraph channels 

simultaneously (~450–500 and 500–550 

nm), likely providing color information for 

the majority of detections.

2. Revisit all systems as necessary with 

the coronagraph until the orbits of highpriority planets are sufficiently constrained 

(likely more than 6 times each over the 

course of months to years)

3. Based on the color, orbit, brightness, and 

phase variations, identify high-priority 

targets for spectral characterization.

4. Schedule and conduct spectral 

characterization observations on each 

exoEarth candidate at an optimized 

orbital phase to search for the presence 

of water vapor in the planet’s atmosphere.

This operations scenario is both realistic 

and robust to error. By requiring orbit 

measurement regardless of what is detected, 

the operations concept is straightforward, 

does not rely on any confusion mitigation 

immediately after a detection, and proper 

motion will be established for free for all 

detected planets. Because the LUVOIR 

coronagraph’s field of regard is greater than a 

hemisphere at any given time, we expect that 

the revisit schedule for each star can easily be 

optimized to maximize detections, constrain 

orbits, and minimize characterization time 

without detailed consideration of whether or 

not the targets are inaccessible.

14.1.4 Exoplanet yield-informed 

trades

Exoplanet yield was considered continuously 

during the LUVOIR Architecture A design 

study. Four major design trades were made 

based at least in part on yield studies:

14.1.4.1 Optimizing the primary 

geometry

Concurrent with the LUVOIR Architecture A 

study, the Segmented Coronagraph Design 

Analysis (SCDA) study concluded that two 

major factors greatly impacted the yield of 

coronagraphs for segmented apertures: the 

obscuration ratio of the secondary mirror 

and the inscribed diameter of the primary. 
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The LUVOIR design team and the SCDA 

team at STScI (PI: R. Soummer) studied 

10 possible apertures for LUVOIR, ranging 

from engineering-optimized designs to 

coronagraph-optimized designs. The studied 

apertures produced a range of yields that 

varied by a factor of two. The final aperture 

was selected based on its high yield and 

scalability.

14.1.4.2 IFS vs fiber-fed 

spectrograph

Two proposed options were considered to 

obtain spectra of exoplanets: an integral 

field spectrograph (IFS) and a fiber-fed 

spectrograph. Both of these instruments 

have design strengths and weaknesses. The 

former has heritage from the WFIRST CGI, 

but packaging considerations and detector 

size place limitations on the field of view and 

maximum resolution. The latter is compact 

and provides potentially higher resolution, 

but only provides spectra of one source at 

a time and requires a potentially complex 

operations concept. In the end, a yield 

study revealed that the throughput reduction 

required for the fiber-red spectrograph 

significantly reduced exoplanet yield to an 

unacceptable level.

14.1.4.3 NUV coronagraph channel

In addition to introducing additional engineering challenges, the presence of a NUV 

coronagraph channel reduces the throughput of all coronagraph channels by ~30%, as 

it requires all optical surfaces after the telescope and prior to the dichroics to be aluminum. In addition, the end-to-end throughput 

of the NUV channel coronagraph is roughly 

half that of the visible channel. We considered replacing the NUV coronagraph channel 

with a second visible channel and co-adding 

the coronagraph channels to double the detection bandwidth. We found that this trade 

increased yield by ~15%. The STDT determined that NUV coronagraphic capability 

was a higher priority than a 15% increase in 

exoplanet yield.

14.1.4.4 TMA vs Cassegrain design

The current LUVOIR Architecture A is a TMA 

design with 4 telescope optics (primary, 

secondary, tertiary, and FSM). By switching 

to a Cassegrain design, potentially two 

aluminum surfaces could be removed, 

increasing the throughput of all instruments 

by ~18%. We determined that an 18% 

increase in throughput would increase 

yield by ~6%. The STDT and design team 

determined that a 6% increase in yield did 

not warrant the increased complexity of the 

Cassegrain design given the timeline of the 

study.

References

Brown, R. A. (2005). ApJ, 624, 1010

Jewell, J., Ruane, G., Shaklan, S., et al. 

(2017). SPIE, 10400, 104000H

Kopparapu, R. K., Ramirez, R., Kasting, J. 

F., et al. (2013). ApJ, 765, 131

Kopparapu, R. K., Hebrard, E., Belikov, 

R., et al. (2018). ApJ in press, arXiv:1802:09602

Krissansen-Totton, J., Schwieterman, E., 

Charnay, B., et al. (2016). ApJ, 817, 31

Leinert, Ch., Bowyer, D., Haikala, L. K., et 

al. (1998). A&AS, 127, 1

N’Diaye, M., Soummer, R., Pueyo, L., et al. 

(2016). ApJ, 818, 163

Mawet, D., Trauger, J., Serabyn, E. et al. 

(2009). SPIE, 7440, 74400X

Mawet, D., Pueyo, L., Carlotti, A., et al. 

(2013). ApJS, 209, 7

Rogers, L. A. (2015). ApJ, 801, 41

Ruane, G., Mawet, D., Jewell, J., & Shaklan, 

S. (2017). SPIE, 10400, 104000J

Savransky, D. & Garrett, D. (2016). JATIS, 2, 

011006

Sirbu, D., Thomas, S., Belikov, R., & 

Bendek, E. (2017). ApJ, 849, 142
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Stark, C. C., Roberge, A., Mandell, A., & 

Robinson, T. (2014). ApJ, 795, 122

Stark, C., C. Roberge, A., Mandell, A., et al. 

(2015). ApJ, 808, 149

Stark, C. C., Cady, E. J., Clampin, M., et al. 

(2016), SPIE, 9904, 99041U

Thomas, S., Belikov, R., & Bendek, E. 

(2015). ApJ, 810, 81

Zahnle, K. J. & Catling, D. C. (2017). ApJ, 

843, 122

Zimmerman, N. T., N’Diaye, M., St. Laurent, 

K., et al. (2016). SPIE, 9904, 99041Y
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14.2 The LUVOIR science simulation tools

The LUVOIR team has produced a set of online software tools to enable quick, accurate 

performance modeling and to facilitate team and community input to the LUVOIR concept 

study. These tools were developed by the STDT’s Simulations Working Group, led by Jason 

Tumlinson (STScI), with substantial contributions from Giada Arney (NASA GSFC), Tyler 

Robinson (NAU), and Graham Kanarek (STScI). 

The online tools are hosted by STScI (at http://luvoir.stsci.edu) and are accessible 

though the LUVOIR website at https://asd.gsfc.nasa.gov/luvoir/. For all tools, the aim has 

been to keep their usage and results simple and easy to grasp for first-time users. Each tool 

includes an “Info” tab that describes its usage and assumptions. The user may save plots 

and other results. These tools will continue to evolve during the LUVOIR study and additional 

new tools may be added.

The tools are written in python and make extensive use of the bokeh python library 

(bokeh.pydata.org). The underlying code is all open-source and available at github.com/

tumlinson/luvoir_simtools. The github repository includes instructions for how to use the 

Figure 14.3. Front page for online LUVOIR Science Simulation Tools at https://asd.gsfc.nasa.

gov/luvoir/tools/
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code in local mode and its dependencies, as well as an ipython notebook with basic usage 

of the tools that can be adapted to many purposes.
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14.2.1 Multiplanet yields visualization

This tool visualizes multi-planet yield calculations based on the work in Stark et al. (2014, 

2015, 2016). These yields assume one year of science exposure time (1.5 to 2 years total 

time including overheads) and the planet mass/orbit bins shown under the “Planets” tab. 

The bar chart at left shows the expected numbers of different kinds of planets observed.

In the main panel, the planets colored in gold are shaded according to the fractional 

“yield” for that star (aka. completeness) in the altruistic yield calculation. These estimated 

(probabilistic) yields are then sampled with random draws to highlight a hypothetical sample 

of detected “warm rocky” planets, which are marked in light blue. This sample is randomly 

drawn again according to the “probability of life” slider, which specifies the fraction of warm 

rocky planets with remotely detectable biosignature gases, which are marked in flashing 

green. This tool demonstrates why dozens of habitable planet candidates are needed to 

perform a survey for habitable conditions and life in the nearby galaxy that can provide a 

statistically meaningful answer even if it is a null result.

Figure 14.4. Multiplanet yields visualization tool.
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14.2.2 Coronagraphic spectra of varied planets

This richly featured tool is based upon the coronagraph noise model from Robinson et al. 

(2016) and calculates realistic, noisy spectra of many kinds of planets for an ECLIPS-like 

coronagraph. Many model planet spectra are available, and both LUVOIR and ground-based 

spectra can be simulated. The ground-based mode includes thermal radiation from the 

atmosphere and the wavelength-dependent Earth atmosphere transmissivity. The library of 

planetary spectra users can select includes a range of planet types such as Earth-like worlds 

at different periods of Earth history, other solar system planets, false positive biosignature 

planets, mini-Neptunes, and warm Jupiters.

Noise terms include thermal radiation from the telescope, dark current, read noise, 

zodiacal light, exozodiacal light, and stellar light leakage. Users can adjust these noise terms 

and can also adjust basic observational parameters such as the size of the planet, the 

observer-system separation distance, the telescope diameter, the inner and outer working 

angles, the exposure time, the telescope temperature, throughput terms, and spectral 

resolution. Users may also specify a desired signal-to-noise ratio (SNR), which the tool uses 

to calculate the exposure time required as a function of wavelength to achieve this SNR. 

Figure 14.5. Simulated coronagraphic planet spectra. The “Spectrum” tab shows the spectrum 

of the selected planet with added noise. The “Exposure Time” tab shows the wavelengthdependent exposure time required to obtain a user-specified SNR. The “Count Rates” tab shows 

the wavelength-dependent planet flux and noise terms in counts/second.
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14.2.3 The High-Definition Imager exposure time calculator

This tool computes S/N limits and exposure times for imaging with HDI and LUVOIR-A-like 

telescopes. A range of input spectral energy distributions are available. This is one of the 

tools that permits the user to save and restore their specific calculations. By supplying a 

unique string, the user names a file that will be stored on the STScI server.

Figure 14.6. HDI exposure time calculator results. For a flat continuum source normalized to AB 

= 33 mag, the S/N returned in the V band is S/N = 5. The “ExpTime” and “Magnitude” tabs show 

alternate results.
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14.2.4 The LUMOS exposure time calculator

This tool uses UV-band input spectra provided by the STScI pysynphot package to calculate 

S/N values for the FUV modes of LUMOS paired with LUVOIR-A-like telescopes. Various 

pysynphot spectral templates are available, and the normalization is done in the GALEX FUV 

band. S/N is always given per resolution element.

Figure 14.7. LUMOS exposure time calculator.
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14.2.5 LUVOIR imaging comparisons

This basic tool compares images at Hubble and LUVOIR spatial resolutions to illustrate the 

gains in image quality going from a 2.4 to a 12-m UV/optical telescope.

Figure 14.8. Gallery of imaging simulations to visualize LUVOIR’s spatial resolution.
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15 Appendix C: Complete science traceability matrix for LUVOIR-A

Characterize 

exoplanet host 

stars

Survey for dozens 

of rocky planets 

around F,G,K,M 

stars 

Establish habitable 

zone orbits of 

rocky planets

Search for water 

vapor in habitable 

zone rocky planet 

atmospheres

Measure UV 

spectrum and 

activity of host 

stars

Constrain 

abundances of 

major 

atmospheric 

constituents (e.g. 

H2O, CO2, CH4)

Detect surface 

liquid water and 

constrain 

atmospheric water 

vapor profile

Determine 

atmospheric 

properties 

(Rayleigh 

scattering, haze)

Observe changes 

in surface 

morphology

Monitor plume 

activity and study 

plume 

morphology and 

dynamics

Determine moon 

atmospheric 

properties

Multi‐band high‐

contrast 

photometry near 

500 nm 

(SNR=5 in 10% 

bands)

Multi‐epoch 

(6 visits) 

high‐contrast 

photometry near 

500 nm

(SNR=5 in 10% 

bands)

High‐contrast 

spectroscopy near 

900 nm

 (SNR=5, R70)

UV spectroscopy 

(SNR=50, R > 

10,000 at Lyman 

alpha; SNR=10, R 

>100 in NUV); time‐

resolved

High‐contrast 

spectroscopy 

between 500 nm 

and 2.0 μm

(SNR=10, R)

High‐contrast 

spectroscopy and 

photometry 

between 1.0 μm  ‐ 

1.5 μm at phase 

angles > 100°  at  

inner‐working 

angles  3 λ/D 

(SNR>10, R70)

Multi‐band high‐

contrast 

photometry 

between 200 nm ‐ 

500 nm (SNR=5 in 

>10% bands)

High‐contrast 

spectroscopy at 

760 nm for O2

(SNR>10, R=140)

High‐contrast 

photometry 

between 200 nm ‐ 

700 nm for O3 

(SNR>5 in 10% 

bands)

High‐constrast 

spectroscopy 

between 1.0 μm ‐ 

1.8 μm for CH4, 

CO2, O4

(SNR>10, R=70, 

200)

 Multi‐epoch 

imaging between 

500 nm ‐ 2.0 μm 

(SNR5)

Multi‐epoch, 

spatially resolved 

spectroscopy at 

Lyman‐alpha 

(SNR>5, R=30,000)

Multi‐epoch, 

spatially resolved  

spectroscopy from 

1.0 μm ‐ 2.0 μm 

(SNR>10, R=70, 

200)

R Coronagraph contrast

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass

T 10%

G 15%

R Coronagraph UV bandpass

T 200 nm ‐ 525 nm

G 200 nm ‐ 525 nm

R Coronagraph Inner Working 

Angle in NUV (λ/D)

T 4

G 3

R Coronagraph contrast 

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass 

T 10%

G 15%

R Coronagraph VIS bandpass

T 515 nm ‐ 1.03 μm

G 515 nm ‐ 1.03 μm

R Coronagraph Inner Working 

Angle in visible (λ/D)

T 4

G 3

R

Coronagraph visible spatially‐

resolved  spectroscopic 

resolution

T 140

G 140

R Coronagraph contrast 

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass

T 10%

G 15%

R Coronagraph NIR bandpass

T 1.0 μm ‐ 2.0 μm

G 1.0 μm ‐ 2.0 μm

R Coronagraph Inner Working 

Angle in NIR (λ/D)

T 2

G 2

R

Coronagraph NIR spatially‐

resolved spectroscopic 

resolution

T 70 or 200

G 70 or 200

R Spectrograph FUV Bandpass

T 1000 Å ‐ 2000 Å

G 1000 Å ‐ 2000 Å

R Spectrograph Low Resolution 

Resolving Power

T 300

G 500

R Spectrograph High Resolution 

Resolving Power

T 30,000

G 60,000

R Spectrograph MOS shutter size

T 100 μm x 200 μm

G 100 μm x 200 μm

R Spectrograph MOS FOV

T 2' x 2'

G 3' x 3'

R Spectrograph NUV Bandpass

T 2000 Å ‐ 4000 Å

G 2000 Å ‐ 4000 Å

R Spectrograph High Resolution 

Resolving Power

T 30,000

G 40,000

R Spectrograph MOS 

shutter size

T 100 μm x 200 μm

G 100 μm x 200 μm

R Spectrograph MOS FOV

T 2' x 2'

G 3' x 3'

R  FUV imaging bandpass

T 1150 Å ‐ 2000 Å

G 1000 Å ‐ 2000 Å

R  FUV imaging angular 

resolution

T 25 mas

G 15 mas

R UVIS imaging wavelength 

coverage

T 220 nm ‐ 900 nm

G 200 nm ‐ 950 nm

R UVIS imaging angular 

resolution

T Nyquist sampled at 400 nm

G Nyquist sampled at 400 nm

R UVIS imaging astrometric 

precision

T

Detect relative shifts of 0.0002 

pixel within 96 hours of 

geometry calibration

G

Detect relative shifts of 0.0001 

pixel within 48 hours of 

geometry calibration

R NIR imaging wavelength 

coverage

T 950 nm ‐ 1.8 μm

G 850 nm ‐ 2.5 μm

R NIR imaging angular resolution

T Nyquist sampled at 1.2μm

G Nyquist sampled at 1.2μm

R Total collecting area

T 135 m^2

G 155 m^2

R Aperture Diameter

T 15 m

G 15 m

R

Obscuration Ratio

(SM circumscribed diameter / 

PM inscribed diameter)

T 15%

G 10%

R Diffraction‐limited imaging at 

the specified wavelength

T 500 nm

G 500 nm

R Solar Elongation Viewing Angle

T 45°

G 30°

R Pointing stability (1 per axis 

over an observation)

T 0.30 mas

G 0.25 mas

R Repoint anywhere in anti‐sun 

hemisphere within timeframe

T 45 min

G 30 min

R Fast object tracking

T 60 mas/s

G 60 mas/s

R Capability for operating 

instruments in parallel.

T ‐

G ‐





  



Science

  

  

Question / Report Chapter

Objectives

Measurement

Observations



   

  

   

 

   

Observatory / OTE

   







NIR Channel





HDI

UVIS Far‐UV Imager



Near‐UV MOS











LUMOS

Far‐UV MOS







   







NIR

 

 

    

    

    

VIS

    

  





Requirements / Implementation                                        Requirements / Implementation                                        Requirements / Implementation                                        Requirements / Implementationn

ECLIPS

UV

 

Is there life elsewhere?

Chapter 3

Frequency of habitable exoplanet candidates Search for signs of life on exoplanets Investigate habitability of Solar System ocean moons

Constrain abundances of atmospheric biosignature gases  

(e.g. O2, O3, CH4), and false positive indicators (e.g. high‐

CO2, O4)

 

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 

Confirmation of exoplanet habitable conditions



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 



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  

  

  
LUVOIR The Large UV Optical Infrared Surveyor

C-2 The LUVOIR Interim Report

Constrain 

exoplanet 

atmospheric 

composition 

Detect and 

characterize 

atmospheric hazes 

and clouds

Determine 

morphology and 

architecture of 

debris disks

Determine bulk 

composition of 

extrasolar 

planetesimals

Constrain the 

physics of 

atmospheric 

escape

Measuring the 

baryons over 

cosmic time from 

10‐10^7 K

Characterize 

nearby galaxy 

inflows and 

outflows in detail

Explore star 

formation 

histories across 

the Hubble 

sequence

Constrain 

abundances of 

gases (e.g. H2O, 

CO2, CH4, O2)

Constrain haze 

and cloud 

abundances

Determine planet 

mass and orbital 

paramteters

Constrain 

atmospheric 

composition

Spatially map dust 

distribution

Measure 

abundances of 

multiple atomic 

and molecular 

species in debris 

disk gas

Measure size, 

distribution, 

orbital parameters

Characterize 

surface 

composition and 

volatile outgassing

Detect, map, and 

characterize 

outflows

Absorption line 

measures: Column 

density, velocity 

width, redshift for 

multiple ions 

Absorption line 

measures: Column 

density, velocity 

width, redshift for 

multiple ions

Emission line maps 

of multiple ions 

Spatially map 

inflowing/outflowi

ng gas in 

absorption 

Color‐Magnitude 

diagrams for stars 

in galaxies out to 

tens of Mpc across 

multiple galaxy 

types

High‐contrast 

spectroscopy 

between 500 nm ‐ 

2.0 μm (SNR>10, 

R=70‐200)

High‐contrast 

photometry 

bewteen 200 nm ‐ 

1 μm (SNR>5 in 

10% bands)

Multi‐epoch 

astrometric 

imaging 

(~0.1 μas 

precision) 

High‐contrast 

spectroscopy 

between 500 nm ‐ 

2.0 μm (SNR>10, 

R=70‐200)

High‐contrast 

imaging bewteen 

500 nm ‐ 1.0 μm

High resolution 

spectroscopy from 

1000 Å ‐ 4000 Å

(SNR10, 

R60,000)

Multi‐epoch 

imaging between 

500 nm ‐ 2.5 μm 

Spatially resolved 

spectroscopy 

between 1.0 μm ‐ 

2.0 μm (R=70,200) 

and between 1000 

Å ‐ 2000 Å 

(R=30,000)

Multi‐object 

spectroscopy from 

1000 Å ‐ 4000 Å 

(SNR>20, 

R=30,000)

Multi‐object 

spectroscopy from 

1000 Å ‐ 4000 Å 

(SNR>20, 

R=30,000)

Multi‐object 

spectroscopy from 

1000 Å ‐ 3000 Å 

(SNR10, 

R=30,000)

Multi‐object 

spectroscopy and 

imaging from 1000 

Å ‐ 3000 Å (SNR5, 

R=30,000)

Multi‐object 

spectroscopy 

between 1000 Å ‐ 

4000 Å (SNR30, 

R=30,000)

g‐ and r‐band 

imaging

(SNR>5) 

R Coronagraph contrast

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass

T 10%

G 15%

R Coronagraph UV bandpass

T 200 nm ‐ 525 nm

G 200 nm ‐ 525 nm

R Coronagraph Inner Working 

Angle in NUV (λ/D)

T 4

G 3

R Coronagraph contrast 

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass 

T 10%

G 15%

R Coronagraph VIS bandpass

T 515 nm ‐ 1.03 μm

G 515 nm ‐ 1.03 μm

R Coronagraph Inner Working 

Angle in visible (λ/D)

T 4

G 3

R

Coronagraph visible spatially‐

resolved  spectroscopic 

resolution

T 140

G 140

R Coronagraph contrast 

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass

T 10%

G 15%

R Coronagraph NIR bandpass

T 1.0 μm ‐ 2.0 μm

G 1.0 μm ‐ 2.0 μm

R Coronagraph Inner Working 

Angle in NIR (λ/D)

T 2

G 2

R

Coronagraph NIR spatially‐

resolved spectroscopic 

resolution

T 70 or 200

G 70 or 200

R Spectrograph FUV Bandpass

T 1000 Å ‐ 2000 Å

G 1000 Å ‐ 2000 Å

R Spectrograph Low Resolution 

Resolving Power

T 300

G 500

R Spectrograph High Resolution 

Resolving Power

T 30,000

G 60,000

R Spectrograph MOS shutter size

T 100 μm x 200 μm

G 100 μm x 200 μm

R Spectrograph MOS FOV

T 2' x 2'

G 3' x 3'

R Spectrograph NUV Bandpass

T 2000 Å ‐ 4000 Å

G 2000 Å ‐ 4000 Å

R Spectrograph High Resolution 

Resolving Power

T 30,000

G 40,000

R Spectrograph MOS 

shutter size

T 100 μm x 200 μm

G 100 μm x 200 μm

R Spectrograph MOS FOV

T 2' x 2'

G 3' x 3'

R  FUV imaging bandpass

T 1150 Å ‐ 2000 Å

G 1000 Å ‐ 2000 Å

R  FUV imaging angular 

resolution

T 25 mas

G 15 mas

R UVIS imaging wavelength 

coverage

T 220 nm ‐ 900 nm

G 200 nm ‐ 950 nm

R UVIS imaging angular 

resolution

T Nyquist sampled at 400 nm

G Nyquist sampled at 400 nm

R UVIS imaging astrometric 

precision

T

Detect relative shifts of 0.0002 

pixel within 96 hours of 

geometry calibration

G

Detect relative shifts of 0.0001 

pixel within 48 hours of 

geometry calibration

R NIR imaging wavelength 

coverage

T 950 nm ‐ 1.8 μm

G 850 nm ‐ 2.5 μm

R NIR imaging angular resolution

T Nyquist sampled at 1.2μm

G Nyquist sampled at 1.2μm

R Total collecting area

T 135 m^2

G 155 m^2

R Aperture Diameter

T 15 m

G 15 m

R

Obscuration Ratio

(SM circumscribed diameter / 

PM inscribed diameter)

T 15%

G 10%

R Diffraction‐limited imaging at 

the specified wavelength

T 500 nm

G 500 nm

R Solar Elongation Viewing Angle

T 45°

G 30°

R Pointing stability (1 per axis 

over an observation)

T 0.30 mas

G 0.25 mas

R Repoint anywhere in anti‐sun 

hemisphere within timeframe

T 45 min

G 30 min

R Fast object tracking

T 60 mas/s

G 60 mas/s

R Capability for operating 

instruments in parallel.

T ‐

G ‐

Characterize full population of Solar 

System minor bodies



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  

 

Science

Question / Report Chapter

Objectives

Measurement

Observations

  

      

 



      



   

         

Observatory / OTE



  



NIR Channel



   

   

HDI

UVIS





Far‐UV Imager

    

    

     

     

Near‐UV MOS

    

    

     

   

LUMOS

Far‐UV MOS

 

 

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



NIR

 

   

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 

 

 

VIS





Requirements / Implementation                                        Requirements / Implementation                                        Requirements / Implementation                                        Requirements / Implementationn

ECLIPS

UV

How do we fit in?

Chapter 4

Determine planetary system 

architectures

How do galaxies evolve?

Chapter 5

High‐definition exploration of the 

CGM 





 



 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report C-3

Determine the 

turnover in the 

high redshift 

galaxy luminosity 

function

Determine the 

escape fraction of 

ionizing radiation 

out to z~1

Characterize the 

composition of 

planet forming 

material

 Incidence 

frequency of z~7 

galaxies at faint 

magnitudes 

 Matter power 

spectrum on small 

scales via 

observations of 

dwarf galaxies 

near Milky Way 

analogues 

 Stellar proper 

motions in Local 

Group galaxies 

Amount of flux 

below

 the Lyman break 

for N>5000 

galaxies from 0.2 < 

z < 1.2 

Imaging of 

individual stars 

and star clusters

Spectroscopy of 

individual stars 

and star clusters

Spectroscopy of 

atoms and 

molecules in 

protoplanetary 

disks

Spectroscopy of 

extremely metal‐

poor stars in the 

Milky Way halo

Spectroscopy of 

late‐type stars in 

the Milky Way 

halo

I‐, J‐, and H‐band 

imaging 

(SNR>=5 to J>33.5)

V‐ and R‐band 

imaging 

(SNR>5 to V>31)

Multi‐epoch 

astrometric 

imaging (~0.5 μas 

precision) 

Multi‐object 

spectroscopy 

between 1000 Å ‐ 

2000 Å 

(SNR5, R500)

U‐, B‐, V‐, R‐, I‐

band imaging 

(SNR 10‐300)

Multi‐object 

spectroscopy from 

1000 Å ‐ 4000 Å 

(SNR>10, 

R=30,000)

Multi‐object 

spectroscopy from 

1000 Å ‐ 4000 Å 

(SNR>10, 

R=30,000)

Multi‐object 

spectroscopy from 

1000 Å ‐ 4000 Å 

(SNR80, 

R=30,000)

Multi‐object 

spectroscopy from 

1000 Å ‐ 4000 Å 

(SNR80, 

R=30,000)

R Coronagraph contrast

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass

T 10%

G 15%

R Coronagraph UV bandpass

T 200 nm ‐ 525 nm

G 200 nm ‐ 525 nm

R Coronagraph Inner Working 

Angle in NUV (λ/D)

T 4

G 3

R Coronagraph contrast 

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass 

T 10%

G 15%

R Coronagraph VIS bandpass

T 515 nm ‐ 1.03 μm

G 515 nm ‐ 1.03 μm

R Coronagraph Inner Working 

Angle in visible (λ/D)

T 4

G 3

R

Coronagraph visible spatially‐

resolved  spectroscopic 

resolution

T 140

G 140

R Coronagraph contrast 

T 1E‐10

G 1E‐10

R Coronagraph instantaneous 

bandpass

T 10%

G 15%

R Coronagraph NIR bandpass

T 1.0 μm ‐ 2.0 μm

G 1.0 μm ‐ 2.0 μm

R Coronagraph Inner Working 

Angle in NIR (λ/D)

T 2

G 2

R

Coronagraph NIR spatially‐

resolved spectroscopic 

resolution

T 70 or 200

G 70 or 200

R Spectrograph FUV Bandpass

T 1000 Å ‐ 2000 Å

G 1000 Å ‐ 2000 Å

R Spectrograph Low Resolution 

Resolving Power

T 300

G 500

R Spectrograph High Resolution 

Resolving Power

T 30,000

G 60,000

R Spectrograph MOS shutter size

T 100 μm x 200 μm

G 100 μm x 200 μm

R Spectrograph MOS FOV

T 2' x 2'

G 3' x 3'

R Spectrograph NUV Bandpass

T 2000 Å ‐ 4000 Å

G 2000 Å ‐ 4000 Å

R Spectrograph High Resolution 

Resolving Power

T 30,000

G 40,000

R Spectrograph MOS 

shutter size

T 100 μm x 200 μm

G 100 μm x 200 μm

R Spectrograph MOS FOV

T 2' x 2'

G 3' x 3'

R  FUV imaging bandpass

T 1150 Å ‐ 2000 Å

G 1000 Å ‐ 2000 Å

R  FUV imaging angular 

resolution

T 25 mas

G 15 mas

R UVIS imaging wavelength 

coverage

T 220 nm ‐ 900 nm

G 200 nm ‐ 950 nm

R UVIS imaging angular 

resolution

T Nyquist sampled at 400 nm

G Nyquist sampled at 400 nm

R UVIS imaging astrometric 

precision

T

Detect relative shifts of 0.0002 

pixel within 96 hours of 

geometry calibration

G

Detect relative shifts of 0.0001 

pixel within 48 hours of 

geometry calibration

R NIR imaging wavelength 

coverage

T 950 nm ‐ 1.8 μm

G 850 nm ‐ 2.5 μm

R NIR imaging angular resolution

T Nyquist sampled at 1.2μm

G Nyquist sampled at 1.2μm

R Total collecting area

T 135 m^2

G 155 m^2

R Aperture Diameter

T 15 m

G 15 m

R

Obscuration Ratio

(SM circumscribed diameter / 

PM inscribed diameter)

T 15%

G 10%

R Diffraction‐limited imaging at 

the specified wavelength

T 500 nm

G 500 nm

R Solar Elongation Viewing Angle

T 45°

G 30°

R Pointing stability (1 per axis 

over an observation)

T 0.30 mas

G 0.25 mas

R Repoint anywhere in anti‐sun 

hemisphere within timeframe

T 45 min

G 30 min

R Fast object tracking

T 60 mas/s

G 60 mas/s

R Capability for operating 

instruments in parallel.

T ‐

G ‐

     



  

Science

Question / Report Chapter

Objectives

Measurement

Observations

  





  



        

Observatory / OTE





NIR Channel





   

   

HDI

UVIS

   

Far‐UV Imager

   

 

   

   

Near‐UV MOS



   

 

   



  



LUMOS

Far‐UV MOS

 

NIR VIS

What are the building blocks of cosmic structure?

Chapter 6

Constrain the nature of dark matter

How do stars and planets form?

Chapter 7

Constrain massive star formation and 

determine the stellar IMF Follow the rise of the periodic table

Requirements / Implementation                                        Requirements / Implementation                                        Requirements / Implementation                                        Requirements / Implementationn

ECLIPS

UV
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report D-1

16 Appendix D: Further technical information

In this appendix, we present whitepapers with additional information on key technological 

issues that drove LUVOIR design trades.
LUVOIR The Large UV Optical Infrared Surveyor

D-2 The LUVOIR Interim Report

16.1 LUVOIR telescope temperature considerations

Lee Feinberg (NASA GSFC)

16.1.1 Introduction

At first glance, an obvious goal for an 

observatory such as LUVOIR that operates in 

the near infrared is that it should be as cold as 

possible to reduce the thermal background 

emission. However, there are number of 

technical and programmatic considerations 

that compete with this goal. This whitepaper 

discusses the issues associated with 

operating LUVOIR at temperatures below a 

nominal room temperature of 293 K.

The difficulties of designing and validating 

a large space telescope that operates at 

temperatures below room temperature have 

been explored in great detail through the 

development of JWST, and primarily relate to 

three engineering challenges:

1. The need to fabricate and assemble a 

highly stable system that is guaranteed 

to function properly at cold temperatures.

2. The need to demonstrate the full end-toend optical and mechanical performance 

of the telescope before launch.

3. The need to eliminate contamination 

prior to and during operations in order to 

achieve peak performance.

All of these challenges have engineering 

solutions, but the question is whether the 

gain in science capability justifies the cost 

and difficulty associated with those solutions.

16.1.2 Temperature vs. thermal 

background radiation

The first question is what temperature a 

telescope needs to operate at to enable the 

desired long-wavelength science. To assess 

this, one must consider the temperature of 

everything in the optical chain (primary mirror, 

secondary mirror and struts, instrument 

optics and components, etc.). For simplicity, 

consider the radiance vs. wavelength of the 

telescope, including coatings, for varying 

temperatures as shown in Figure 16.1. This 

analysis assumes Aluminum-coated mirrors, 

which has worse emissivity in the NIR, but is 

the necessary coating to enable observations 

with LUVOIR in the UV.

Figure 16.1. Telescope background radiance as a function of temperature. Credit: P. Lightsey / 

Ball Aerospace Technology Center.

Background Radiance

min zodi

12 X min

300 K

250 K

200 K

150 K

100 K

50 K

wavelength (μm)

0 5 10 15 20

Radiance (MJy/sr)

1.E+09

1.E+08

1.E+07

1.E+06

1.E+05

1.E+04

1.E-02

1.E+03

1.E+02

1.E+01

1.E+00

1.E-01
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report D-3

Figure 16.1 plots the thermal emission 

at different telescope temperatures 

compared the limiting Solar System zodiacal 

light background (“1 Zodi”). Also shown for 

comparison is 1.2× Zodi. Figure 16.1 shows 

that the telescope temperature must be 200 

K to achieve faint-source observations that 

are Zodi background-limited at 2.5 mm. 

The figure also shows that reducing the 

telescope temperature only 50 K from room 

temperature increases the background limit 

by only ~0.3 mm. However, achieving a Zodilimited background flux is not necessary for 

bright-target applications such as stellar, 

planetary, or exoplanetary applications, and 

reductions in the thermal background at long 

wavelengths will still lead to improvements in 

sensitivity. We therefore address the impacts 

of decreasing the telescope temperature on 

the overall telescope engineering design.

16.1.3 Impacts

There are a number of issues associated 

with operating a telescope at cold temperatures and the impacts vary depending 

on temperature and system architecture. A 

summary of these issues and the level of 

complexity they add at different temperature 

regimes is shown in Figure 16.2. The temperatures chosen were natural break points, 

which derive from both from basic physics 

and extensive experience. For example, 

we know that quartz crystal microbalance 

(QCM) monitors during JWST testing start 

to show large depositions at 260 K (a point 

where molecular deposition starts to happen), 200 K is above where water begins to 

condense on optical surfaces (roughly 140 

K to 170 K), 140 K is where some materials have become CTE-stable (coefficient of 

thermal expansion), and 50 K is the JWST 

heritage where virtually all contaminants are 

frozen and Beryllium and JWST M55J laminates are stable. 

The remainder of the white paper 

summarizes of each of the considerations 

from Figure 16.2. The topics are grouped 

by whether they are a significant factor in the 

trade space or not.

Figure 16.2. Material and system issue risks by temperature region. Red is considered high risk, 

yellow is moderate risk, green is low risk.
LUVOIR The Large UV Optical Infrared Surveyor

D-4 The LUVOIR Interim Report

16.1.3.1 Significant factors

Molecular Sticking vs. UV Sensitivity—

UV systems are extremely sensitive to even 

monolayers of absorbing molecules. As contaminants build up, the shortest wavelengths 

will first see reductions in throughput for layers as thin as a few angstroms. First order 

effects are due to absorption, specific to the 

type of molecular contamination. Quarterwave effects will also come into play for 

thicker layers of contamination. 

To deal with molecular absorption, we 

must carefully select, bake, and certify all 

materials to minimize molecular adherence. 

There is a long heritage of doing this at room 

temperature where the sticking coefficients 

in vacuum are low. But even with extreme 

care, epoxies, plasticizers used in cables, 

hydrocarbons from lubricants, and many 

other small residuals will exist and will stick 

to mirrors at colder temperatures. Water also 

desorbs and begins to stick to surfaces in 

the 140–170 K range. For cryogenic optics or 

components, it is very possible that frequent 

bakeouts would be needed where baked off 

products are carefully managed (venting of 

cold fingers). There is not a lot of heritage 

of doing this for UV systems because of the 

complexity.

Given these challenges, is any reduction 

in the system operating temperature 

acceptable? During JWST instrument testing, 

it was routinely found that at 243 K the QCM 

sensor would show significant depositions, 

while at 258 K it did not. This suggests that 

258 K is an approximate breakpoint where 

molecules begin depositing (how many are 

UV absorbing will depend on the species). An 

advantage of operating at a temperature of 

260 K is that mirror bakeouts of contaminants 

would not be as time consuming or technically 

challenging (the power required to bake out 

mirrors would be a small addition compared 

to what would already be needed to maintain 

nominal operations, and the time to achieve 

a mirror bakeout would be reduced).

One possible strategy with cold systems 

is to operate above where water condenses in 

vacuum (170K) and is frozen in structures by 

operating at 200K. However, other molecular 

species will have already deposited. One 

could also use a warm window to isolate 

an instrument, but at shorter near-UV 

wavelengths the availability of wide-band, 

highly transmissive windows is limited

Take-home Message: While it is not 

physically impossible to engineer solutions 

to achieve high UV sensitivities at cryogenic 

temperatures, it would add a high degree of 

complexity with many unknowns and with no 

true heritage, especially below 260 K where 

molecular deposition begins to significantly 

increase.

Mirror Stability vs. Temperature—

Mirror stability is driven by the architecture 

of the mirror subsystem, which combines 

the material stability of the mirror substrate, 

with local thermal controls designed 

to keep the temperature of the mirrors 

constant in the operational environment. 

However, mirror materials have different 

stability characteristics at room temperature 

compared with lower temperatures, and 

the engineering solution for mirror segment 

control will be dominated by what materials 

are used and the thermal conditions of the 

segments. Exoplanet coronagraphy with 

LUVOIR requires individual mirror segment 

stability of a few picometers RMS, which is a 

very significant technological challenge.

Material options for LUVOIR include 

ULE®, Zerodur®, and Silicon Carbide (SiC) 

for room temperature operation, and the 

same materials plus beryllium and silicon for 

colder temperatures. The current LUVOIR 

architecture is based on the high material 

stability of ULE® or Zerodur® with low thermal 

and optical control authority, operating near 

room temperature at 270 K. To date, ULE® 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report D-5

CTE and modeling has been demonstrated 

to have the best thermal stability 

performance at room temperature to meet 

the challenging stability requirements. ULE®

at room temperature has heritage for high 

stability studies dating back to the Terrestrial 

Planet Finder-Coronagraph (TPF-C) and 

Exo-C Probe studies. A study conducted by 

the Smithsonian Astrophysical Observatory 

(Eisenhower et al. 2015), performed 

detailed modeling of ULE® segment at room 

temperature with CTE values measured from 

a real representative mirror boule. Eisenhower 

et al. shows wavefront stability changes as 

small as 0.5 pm for a 1.2 meter flat-to-flat 

hexagonal mirror (approximately LUVOIR 

size) controlled with a 1 mK-stable heater 

plate (deemed feasible but challenging). This 

is a key result for demonstrating the feasibility 

of LUVOIR.

Another consideration for LUVOIR mirrors 

is the degree of optical figure actuation that 

will be needed to meet stringent wavefront 

performance goals. ULE® mirrors can be 

equipped with mechanical actuators, to 

permit a certain degree of correctability. It 

remains to be shown how this will impact the 

thermal stability of the segment subsystem, 

however.

Other approaches are possible, with 

substrates with high thermal controllability 

and very precise thermal sensing and 

control, allowing use of stronger materials 

such as SiC and operation at a wider 

range of temperatures. SiC mirrors have 

been demonstrated with high levels of 

correctability by incorporating electrostrictive 

or piezoelectric actuators into the substrate 

structure. This gives SiC active mirrors the 

ability to operate at the required optical 

performance level both at 1-g and 0-g. It 

provides correctability for wavefront errors 

that can easily arise (and have often arisen 

in the past) during fabrication, test, assembly 

or launch. SiC mirrors have lower passive 

thermal stability at room temperature, but 

higher thermal controllability than ULE®. SiC 

at room temperature is a viable approach for 

LUVOIR mirrors provided that very precise 

thermal controls (<< 0.1 mK) are used. If 

temperatures below 150 K are desired, 

SiC stability improves to be better than 

conventional ULE®.

While additional details on mirror CTE 

and stability are controlled by International 

Trafficking and Arms Regulations (ITAR), 

the key point is that a ULE®-based, roomtemperature approach appears to be feasible, 

while offering the least departure from 

traditional practice. While ULE® and Zerodur® 

can be tailored to cryogenic temperatures, 

picometer stability performance has not 

been assessed. At a minimum, going away 

from room temperature would risk the best 

possible stability performance that has been 

demonstrated and that builds on a large 

database and history including heritage back 

to the TPF-C design. While it is true other 

materials like SiC could have advantages 

for optical control, thermal control, mass 

efficiency, or dynamics stability, the need for 

these advantages is yet to be determined.

Other materials like SiC, silicon, and 

beryllium have high CTE at room temperature 

but are thermally conductive and may offer 

stable solutions at colder temperatures (e.g., 

at 150 K). While these solutions could offer 

stable solutions at cryogenic temperatures, 

more work would need to be done with 

substrate CTE measurements and modeling 

equivalent to the Eisenhower analysis. 

Note that while some mirror manufacturers 

will consider thermal conductivity when 

assessing stability, the LUVOIR architecture 

is not driven by thermal conductivity but 

rather is driven primarily by CTE performance 

and thermal inertia.

UV and high contrast systems require 

very tight controls on both mid-frequency 

wavefront and surface roughness errors. To 
LUVOIR The Large UV Optical Infrared Surveyor

D-6 The LUVOIR Interim Report

this end, glass mirrors like ULE®, Fused Silica, 

and Zerodur® have been used on most UV 

and EUV telescopes including Hubble and 

Fuse. Polishing of Si-clad or chemical-vapor 

deposition (CVD)-clad SiC can also achieve 

good surface roughness, although CVD SiC 

is harder to polish. Another solution for SiC 

is to embed actuators for higher control 

authority.

Take-home Message: The mirror materials 

for ultra-precise mirror stability with the best 

demonstrated performance and highest 

heritage are ULE® and Zerodur®, but both 

of these materials are designed to operate 

at or close to room temperature. Operating 

at a different temperature will necessitate a 

significant mirror technology demonstration 

effort.

Damping of Dynamic Disturbances—

Thermal damping affects dynamic stability 

and the change in damping is as much as 10× 

from room temperature to 50 K. In general, 

warmer is better for dynamic damping, 

regardless of material. Less damping would 

impact wavefront error and line-of-sight 

dynamic stability of the system.

Coronagraph Instrument 

Technologies vs. Temperature—To 

achieve longer wavelength performance for 

coronagraphy, not only does the telescope 

need to be cold, so does the entire coronagraph instrument, including the deformable 

mirrors, Lyot stops, occulting mask, and optical train. All of these coronagraph technologies have an extensive technological history 

for picometer stability and high contrast at 

room temperature. While some actuators 

can work at colder temperatures, a whole 

technology development program would 

be needed including the picometer stability 

performance of such systems at colder temperatures. This essentially would restart the 

coronagraph technology effort and would 

prevent using WFIRST heritage directly. Of 

course, this is only applicable for coronagraphic measurements, and would not apply 

to other infrared instruments.

Cryo/Cold Figuring and Polishing 

Complexity—The wavefront budget for 

LUVOIR is tight for a 500 nm diffractionlimited wavefront performance. A single 

primary mirror segment needs to have <10 

nm RMS wavefront (5 nm RMS surface), 

which is roughly 4× better than what was 

done on JWST. This includes gravity backout 

and metrology uncertainties and is already 

considered a very challenging requirement 

at room temperature (gravity effects and 

the need to match radius of curvature are 

the dominating terms). Due to CTE effects, 

mirrors distort as they go cold by many 

nanometers. Cold optics will need to be 

tested at temperature and go through a 

cryo-figuring/cryo-polishing iteration, adding 

metrology uncertainty, and requiring more 

time. A single cryogenic test for a JWST 

mirror would take anywhere from 3–6 

months when all of the logistics and pre- and 

post-integration and testing are considered. 

Testing many segments could add many 

years to the critical path of the telescope.

Material Stress and Strength 

Properties —Any material mismatch (mirror 

to mounts, mounts to flexures, etc.) will have 

temperature-induced stresses. This can 

impact strength margins and every material 

mismatch will need to be analyzed and likely 

tested at temperature via pull tests. Also, 

other material properties like stiffness can vary 

with temperature and material testing may 

be needed. These issues required complex 

flexures to be included on JWST that required 

a significant amount of time and testing in 

the design phase and additional time in the 

production phase. Just the additional time to 

design the very complex flexures for JWST 

likely added more than a year to the design 

phase of the mirror segments.

System Alignment at Room 

Temperature—It is highly desirable to be 
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report D-7

able to align the system at room temperature 

and know that it will still be aligned at operating temperature. Otherwise accurate models 

will be needed to predict alignment changes 

at temperature. Those models must be verified with tests at temperature. Alternatively, 

cryo-actuators can be used to compensate 

for misalignments, as was done on JWST 

segments. This will introduce additional requirements and complexities. Operating at 

or near room temperature (as cold as 260 K 

where alignment shifts are small) would allow 

for alignment and even some optical testing 

in a cleanroom environment, which is much 

more efficient and lower cost.

Epoxy and Bonds—Acrylics used to 

hold multi-layer insulation (MLI) and epoxies 

used to bond interfaces have glass transition 

temperatures that can impact their strength 

or cause other problems like contamination 

release. These transition temperatures are 

typically in the 240–220 K range. To deal 

with this, cryo-strength and contamination 

testing is needed. In addition, every epoxy 

bond and joint will not only need to be 

analyzed for room temperature launch loads, 

but also for cryo-strength margins. Likely this 

means considerable testing for cryo-material 

strength at the proposed temperature, which 

was a cause of cost and complexity on 

JWST.

Heater Power and Thermal 

Management—The biggest engineering 

advantage for cold operation of the telescope 

is reducing the required heater power. A 

well-insulated mirror would need about 

20–30 Watts to maintain room temperature 

at a Sun-Earth L2 orbit. Total power just for 

the mirrors on a 12-meter telescope will be 

approximately 1.5 kW and the backplane 

could require as much or more, though 

efficient use of insulation can help reduce the 

total power requirement. While this is a large 

power requirement, it is within engineering 

capabilities. In addition, strides are being 

made in solar array efficiency and it may 

be that the mass and cost of the arrays in 

this timeframe are no larger than other large 

observatories.

Shock—At cold temperatures, shock 

is not absorbed as well. For JWST, shock 

has played a key role in driving the launch 

restraint mechanism selection and drove 

extensive cryo-shock testing. Leveraging 

this experience from JWST, the impact of 

the issue could be reduced but will still be a 

complexity driver depending on how cold an 

operating temperature is chosen.

16.1.3.2 Less significant factors

Composite Stability and Complexity—

For totally passive systems, backplane 

CTE for stability is a similar issue to that for 

mirrors. However, one not only needs to 

consider stability but also cryo-stress and 

cryo-distortion of the backplane itself. On 

JWST, cryogenic temperatures drove the 

backplane schedule in a large way. Every 

joint type and bond had to be assessed for 

cool-down stresses (in addition to launch 

loads). Both stability and thermal distortion 

had to be modeled in the design and then 

verified through cryogenic testing. The CTE 

of every tube needed to be measured and 

statistical studies done to assess backplane 

stability. A whole technology was needed to 

develop the 50 K-stable backplane material. 

There is a huge database of experience with 

room temperature composites. Going cold, 

especially to a temperature that is not 50 K, 

could require extensive material testing and 

possibly even new laminate design.

One way to mitigate backplane stability 

(and stability only) is to use edge sensors or 

laser metrology in a loop with a segmented 

DM to mitigate segment jitter. Segment 

motions due to thermal changes can 

be controlled with feedback to segment 

actuators. If a metrology solution is 

developed, thermal stability will prove less 
LUVOIR The Large UV Optical Infrared Surveyor

D-8 The LUVOIR Interim Report

important than dynamic stability, which 

would favor a SiC backplane solution. The 

likely edge sensor technologies also have a 

room temperature heritage (capacitive and 

laser metrology technologies) so going cold 

might complicate this problem.

Environmental Cryo/Thermal 

Testing—Everything will need to be tested 

at its nominal operating temperature, which 

for cryogenic systems will take extra time 

and add complexity. This includes thermal 

balance testing and verification of all electrical connections where impedances and 

phase vary with temperature. 

System Optical Testing and 

Verification—It is important to test an optical system at operating temperature in order 

to verify system wavefront error, alignment, 

wavefront sensing and control, and other 

optical performance metrics. If the telescope 

is significantly colder than room temperature, then room temperature optical testing 

will likely not be sufficient. The system testing 

of JWST was a major cost and complexity 

driver and just cooling down and warming up 

can take months and require costs to plan 

and practice.

Use of Lubricants—At colder 

temperatures, typical warm lubricants will 

need to be replaced. This can also mean 

mechanism bearings may need to be 

redesigned. While cryo-lubricants do exist, 

cryo-actuators are more complex and 

generally more expensive.

References

Eisenhower, M. et al. (2015). SPIE, 9602, 

96020A
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16.2 Getting to orbit: launch vehicles

Norman Rioux & Matthew R. Bolcar (NASA GSFC)

The LUVOIR mission has the potential to 

enable revolutionary scientific breakthroughs 

with the largest telescope aperture ever 

deployed in space. While in the more distant 

future, telescopes might be assembled in 

space, the most economical and immediate 

path forward is to put a large aperture in 

space with a single launch. In the 2030s when 

LUVOIR is expected to fly, the launch vehicle 

industry will not be exactly the same as it is 

now. Here we summarize our understanding 

of the current and future launch capabilities.

The LUVOIR observatory will be in orbit 

at the Sun-Earth L2 (SEL2) point, which 

provides a stable thermal environment and 

excellent field of regard. Constraints on 

the size of the telescope aperture and the 

instrument suite include the mass-to-orbit 

and the size of the fairing that the launcher can 

provide. A mission with the ambitious goals 

of LUVOIR will surely need a heavy lift launch 

vehicle; however, as with all technology, 

the near-term landscape for launch vehicle 

options includes some mature options with 

well-known characteristics and some lessmature options whose characteristics are 

still somewhat in flux and therefore are more 

difficult to evaluate with certainty. In order to 

mitigate the risk (and its associated costs) 

inherent in each different launch vehicle 

option, it is prudent to adopt telescope 

designs with the flexibility to use a variety of 

different launch vehicles and fairings. 

Below we outline the current options 

for heavy-lift vehicles and describe their 

various attributes. Figure 16.3 depicts 

representative values of lift capabilities for 

a variety of launch vehicle configurations 

Figure 16.3. Launch mass to Sun-Earth L2 orbit for a variety of current (blue) and future (red) 

launch vehicles. These are representative values subject to refinement of designs in development 

and evolution of existing vehicles. Estimates for the Vulcan Centaur and Big Falcon Rocket are 

not available.

2.9

6.6

9.8

14.0 15.1

37.0

44.3

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

Falcon 9 Ariane V Delta IV

Heavy

Vulcan

Centaur

New

Glenn

Falcon

Heavy

SLS

Block 1B

SLS

Block 2

Big

Falcon

Rocket

metric tons

Lift Capacity to Sun-Earth L2
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D-10 The LUVOIR Interim Report

Figure 16.4. Fairing outer diameters for a variety of current (blue) and future (red) launch vehicles. 

These outer diameters correspond to the exterior physical extent of the fairing. Inner diameters 

are developed through coupled loads analyses with particular payloads and are roughly 0.5 m to 

1 m smaller.

Figure 16.5. Fairing heights for a variety of current (blue) and future (red) launch vehicles. These 

outer heights correspond to the exterior physical extent of the fairing. Useable inner heights vary 

depending on the fairing shape and dynamic envelopes.

?

5.2 5.4 5.1

7.4

5.2

8.4 8.4

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

Falcon 9 Ariane V Delta IV

Heavy

Vulcan

Centaur

New

Glenn

Falcon

Heavy

SLS

Block 1B

SLS

Block 2

Big

Falcon

Rocket

meters

Fairing Outer Diameter

13.9

18.0 19.1 19.1

13.9

27.4 27.4

0.0

5.0

10.0

15.0

20.0

25.0

30.0

Falcon 9 Ariane V Delta IV

Heavy

Vulcan

Centaur

New

Glenn

Falcon

Heavy

SLS

Block 1B

SLS

Block 2

Big

Falcon

Rocket

meters

Fairing Outer Height
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report D-11

to achieve Sun-Earth L2 orbit. Figure 16.4

depicts the outer diameters of the associated 

fairings, and Figure 16.5 depicts the height 

of the associated fairings. Not all of this space 

will be available to the payload; margins of 

roughly 0.5 to 1 m around the edges are 

needed to allow for payload motions during 

launch. These launch vehicles range in 

maturity from existing vehicles with proven 

flight records to vehicles that are undergoing 

development. Below we highlight a few of the 

launch providers most relevant for LUVOIR.

United Launch Alliance (ULA)

ULA’s Delta IV Heavy is an existing 

heavy lift launch vehicle with a proven track 

record. It supports a fairing with a 5-m outer 

diameter and a 4.6-m inner diameter, and 

~10,000 kg lift capacity to SEL2. United 

Launch Alliance (ULA) has announced 

contracts for Delta IV (both “Medium” and 

“Heavy” variants) launches through 2023. 

ULA is also developing the Vulcan Centaur 

as their next generation medium-to-heavy 

lift vehicle. Little detail is currently available 

on the Vulcan performance, other than that 

it will be of a similar class to other next-gen 

vehicles such as the Falcon Heavy and New 

Glenn.

SpaceX

In Feb. 2018, SpaceX successfully 

launched of the first Falcon Heavy

vehicle. It offers lift capability in excess of 

the Delta IV Heavy, ~15,000 kg to SEL2. 

Currently, SpaceX plans to use the same 

5.2 m-diameter, 13.9 m-tall fairing for both 

Falcon 9 and Falcon Heavy variants; there 

are no plans to develop a larger fairing for the 

Falcon Heavy. SpaceX has also announced 

plans to develop the Big Falcon Rocket (BFR), 

with capabilities exceeding that of the Saturn 

V. Space X has announced an “aspirational 

goal” of a first BFR cargo mission to Mars in 

2022.

Blue Origin

Similar to SpaceX, Blue Origin is also 

developing a series of reuseable launch 

vehicles intended to increase the cadence 

of commercial spaceflight launches while 

reducing costs. Blue Origin’s heavy-lift 

variant, the New Glenn, is expected to be 

able to lift ~14,000 kg to SEL2, but with a 

7.4 m-diameter, 18.5 m-tall fairing.

NASA

NASA is also currently developing 

the Space Launch System (SLS) in three 

stages (Block 1, 1B, and 2) with increasing 

lift and fairing capacities. Options are under 

study for fairings with 5 m, 8.4 m, and 10 m 

outer diameters. The first SLS Block 1 launch 

is expected by 2020 with Block 1B variant 

coming online by 2022. A Block 2 launch is 

not expected until ~2030.

Acknowledgements

Launch vehicle and fairing capacity data 

presented here were largely provided via 

private communications with representatives 

from SpaceX, Blue Origin, and NASA’s 

Launch Services Program.
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D-12 The LUVOIR Interim Report

16.3 Starlight suppression with coronagraphs

Stuart Shaklan (JPL)

Stellar coronagraphs are instruments 

designed to suppress the veiling glare of 

starlight so that faint planets can be seen 

adjacent to their parent stars. The glare is 

caused by both diffraction (sidelobes) from 

the aperture boundaries including the outer 

limit of the pupil, the secondary obscuration, 

secondary support structures, mirror segment 

gaps, and scatter from the imperfect optical 

surfaces in the telescope and instrument.

Generally speaking, coronagraphs are easier 

to design for filled, off-axis apertures than 

for partially obscured, segmented, on-axis 

apertures. 

The diffraction problem can be addressed 

in three ways: 

• Pupil Apodization: The first solution is to 

apodize the pupil. Apodization, like a lowpass electronic filter, reduces sidelobes 

by cancelling or removing diffracted 

light from the beam and allowing for 

the use of a small central obscuration 

to block the undiffracted starlight. This 

is accomplished either by placing a 

specially designed mask at a pupil image, 

or by using optics to concentrate the 

beam in an advantageous way. Figure 

16.6 shows a grayscale apodization 

mask function for a segmented aperture 

telescope, while Figure 16.7 2 shows an 

optical remapping solution. Examples of 

this strategy include the phase-induced 

amplitude apodization (PIAA) or the 

apodized pupil lyot coronagraph (APLC). 

• Lyot Mask: The second approach is to 

use masks in the image plane designed 

to diffract most of the on-axis light to the 

outer edges of the beam. This is then 

followed by another mask at the reimaged 

pupil, called the Lyot plane, to block the 

outer edges of the beam and remove the 

diffracted starlight (Figure 16.8). Off-axis 

light from a nearby star passes around 

the first mask and then through the gap 

in the Lyot stop. Examples of this strategy 

include the hybrid lyot coronagraph (HLC) 

and vector vortex coronagraph (VVC).

Figure 16.6. Pupil apodization on a segmented 

aperture. Credit: N’Diaye et al. (2016). ApJ, 

818, 163

Figure 16.7. Pupil remapping. Credit: Guyon et al. (2005). ApJ, 622, 744
The Large UV Optical Infrared Surveyor LUVOIR

The LUVOIR Interim Report D-13

• Nulling Interferometry: The third solution, 

the Visible Nulling Coronagraph (VNC), 

is to split the pupil light into two beams 

and then recombine them using a phase 

shift and beam shear to cancel the onaxis starlight but not off-axis source flux 

(Figure 16.9).

Coronagraph designers for space-based 

corongraphy are gravitating toward hybrid 

approaches combining pupil apodization and 

image plane masking to deal with segmented 

apertures, but other solutions are still being 

explored.

With diffracted light eliminated from 

the system, scattered light originating 

from aberrations, coating defects, and 

contamination remains and must be 

removed. This is achieved by flattening the 

wavefront using one or more deformable 

mirrors (DMs), with typically >1000 actuators 

within the pupil (Figure 16.10). To control 

the wavefront, it must first be sensed. 

This is done by adjusting the DM surface 

several times while recording the change 

in image plane illumination. An algorithm 

then determines the required wavefront 

Figure 16.8. Hybrid Lyot image plane mask (left) and Lyot mask (right) for the WFIRST coronagraph. 

Credit: Trauger et al. (2013). SPIE, 8864, 886412

Figure 16.9. Nulling coronagraph. Credit: Shao 

et al. (2006). SPIE, 6265, 626517

Figure 16.10. 64 x 64 element DM with a fused 

silica facesheet. Credit: Trauger et al. (2011). 

SPIE, 8151, 81510G
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D-14 The LUVOIR Interim Report

correction and commands the DMs to form 

new surface shapes. This is repeated until a 

“dark hole” is formed with a level of glare low 

enough to expose a planet. Figure 16.11

shows a dark hole achieved in the laboratory 

in a 10% wavelength bandpass. 

The desired level of suppression is 10-10 

for imaging of an Earth-like planet around 

a Sun-like star; that is, the residual scatter 

in the image plane after diffraction and 

wavefront control is 10 billion times below the 

level of the incident starlight. Amazingly, this 

can be achieved using optics of equivalent 

quality to those currently used on the Hubble 

Space Telescope, and is limited mainly by 

the ability to accurately set the DM and to 

hold the system stable. The stability issue 

is perhaps the most challenging, with subnanometer requirements imposed on the 

wavefront. This is particularly challenging 

when it comes to low-order aberrations such 

as pointing, focus, coma, and astigmatism. 

To measure and control these terms, loworder wavefront sensors with fast response 

times using the starlight rejected by the 

coronagraphic masks are being developed.

The effectiveness of a coronagraph is 

evaluated based on several performance 

metrics: the inner working angle (IWA; i.e., 

how close to the star you can observe), 

wavelength bandwidth over which it can 

operate effectively, the throughput of 

the system, and the raw contrast (i.e., 

the level of glare suppression). Typically, 

achieving high performance at small IWA 

leads to either lower system throughput 

or increased sensitivity to aberrations and 

the finite stellar diameter. The challenge of 

suppression increases with bandwidth; the 

broader the band (more signal photons), 

the more background appears, and the 

more challenging the wavefront sensing and 

control becomes. Perhaps most importantly, 

the IWA scales proportionally with wavelength 

(∝ l/D, where D is the telescope diameter); 

the same coronagraph that works at 50 

milliarcsec (mas) at a wavelength of 500 nm 

will be limited to about 100 mas at 1 mm. This 

is an important factor when characterizing 

exoplanet spectra in the near-infrared.

Figure 16.11. Dark hole in broadband light, demonstrated at the JPL High Contrast Imaging 

Testbed. Credit: Trauger et al. (2011). SPIE, 8151, 81510G
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The LUVOIR Interim Report D-15

16.4 Impacts of optical coatings on polarization and coronagraphy

Matthew R. Bolcar (NASA GSFC)

The LUVOIR mission’s exoplanet science 

objectives require high-contrast imaging with 

an internal coronagraph. A coronagraph’s 

wavefront control system uses deformable 

mirrors to correct amplitude and wavefront 

aberrations in the optical system, allowing the 

diffracted starlight to be suppressed so that 

orbiting exoplanets can be directly observed. 

Metallic coatings that are typically used on 

mirror surfaces present a challenge for a 

coronagraph’s wavefront control system. 

Specifically, the orthogonal polarization 

states of light (often denoted by “s” and “p”) 

are affected differently by the coating, as a 

function of wavelength, angle of incidence, 

and coating properties.

There are two effects of concern. The 

first effect is polarization aberration, where 

each polarization state sees a different 

amplitude (diattenuation) and phase change 

(retardance) upon reflecting from a metallic 

surface (see Figure 16.12). The second effect 

is cross-polarization leakage, where some 

portion of a polarization state is converted 

into the orthogonal state upon reflection. 

The result of these two effects is that when 

unpolarized light (such as starlight) reflects 

from a metallic surface, four independent 

(or incoherent) electric fields are created: 

s-incident light reflected in the s-state (“ss”), 

s-incident light reflected in the p-state (“sp”), 

p-incident light reflected in the p-state (“pp”), 

and p-incident light reflected in the s-state 

(“ps”). Since these fields are incoherent 

(i.e., they have uncorrelated amplitude and 

spatial variations), a coronagraph cannot 

sense and correct each one individually. 

Instead, the coronagraph wavefront control 

system will sense and correct the average 

of the four fields. The remaining uncorrected 

portion will contribute to leaked starlight that 

could potentially obscure an exoplanet. It is 

important to note that the cross-polarization 

terms (sp and ps) are orders of magnitude 

Figure 16.12. Left: Amplitude reflection coefficient for each polarization state as a function of 

angle of incidence for a bare aluminum mirror (solid lines) and an aluminum mirror coated with a 

quarter-wave of MgF2 (dashed lines) at a wavelength of 550 nm. Right: The reflected phase shift 

for the same two cases. In each case, the orthogonal polarization states experience a different 

amplitude and phase change upon reflection. Angles of incidence at the primary and secondary 

mirrors for an on-axis, ~12-m-class telescope would typically be less than 15 degrees.
LUVOIR The Large UV Optical Infrared Surveyor

D-16 The LUVOIR Interim Report

smaller than the primary terms (ss and pp) 

(Balasubramanian et al. 2005).

There are several ways to address the 

polarization issues:

Telescope Design Considerations:

Both polarization effects are strongly 

dependent on the angle-of-incidence (AOI) 

of the light at the optical surface. Slower 

(high-F/#) optics will have lower AOIs across 

the surface of the mirrors, thus minimizing 

the polarization effects. Flat mirrors that are 

used to fold the optical path at large angles 

should also be avoided or used in crossedpairs such that the effects are cancelled out. 

Slower optics, however, can lead to longer 

optical systems with smaller fields-of-view. 

The impacts of polarization aberration and 

cross-polarization leakage must therefore 

be traded against volume constraints and 

science objectives.

Coating Properties: The polarization 

effects are also dependent on the coating 

properties, specifically the index of refraction; 

choosing appropriate materials can help 

minimize the effects. To enable the LUVOIR 

mission’s ultraviolet (UV) science objectives 

requires a protected aluminum coating on at 

least the primary and secondary mirrors. It is 

expected that there is little that can be done 

from a coating perspective to further reduce 

the polarization effects, aside from ensuring 

that the protective overcoat material does 

not significantly increase the effects over the 

base aluminum layer (Balasubramanian et al. 

2011).

Coronagraph Architecture: Another 

effective way to deal with polarization aberrations is to split the light at the coronagraph 

and only observe one polarization at a time. 

This can be done serially by a single instrument: a polarized filter would select a single 

polarization state for which the aberrations 

would be sensed and observed, allowing for 

exoplanet detection and correction in that 

polarization state; the orthogonal polarization state could then be selected and the observation repeated. The obvious drawback is 

requiring twice the amount of time to capture 

all of the exoplanet photons. Alternatively, the 

polarization states can be split with a polarizing beam-splitter, with each being sent to 

a separate coronagraph instrument. This allows for simultaneous observation of all of the 

exoplanet photons, at the expense of requiring two coronagraph instruments, each with 

its own focal plane, filter wheels, deformable 

mirrors, and associated electronics. An additional avenue of research includes introducing polarization-controlling diffractive optical 

elements to correct the effects. 

Regardless of the approach taken, 

there are two questions that remain to 

be answered. The first question is: how 

effective are polarizing filters or polarizing 

beam-splitters at separating the orthogonal 

states? Achieving 10-10 raw contrast may 

require polarizing optical components 

that are beyond the state-of-the-art. 

The second question regards the crosspolarization leakage term. When a single 

polarization state is selected (say, s), both 

the ss and ps components are transmitted. 

If the coronagraph wavefront control system 

senses and corrects the ss component, then 

the ps component will contribute a low-level 

static speckle background that may obscure 

an exoplanet. Modeling is being performed on 

the LUVOIR architectures to fully understand 

the magnitude of these polarization effects 

and determine if they are significant enough 

to be of concern.

It is important to note that coatinginduced polarization aberration and crosspolarization leakage will be generated by any 

metallic mirror coating. Figure 16.13 shows 

the diattenuation and retardance for both 

a bare aluminum-coated and bare silvercoated mirror. Both aluminum and silver 

have similar order-of-magnitude effects. It is 

therefore a false assumption to believe that 
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The LUVOIR Interim Report D-17

high-contrast coronagraphy performance 

can be significantly improved by switching 

from an aluminum coating to a silver coating. 

References

Balasubramanian, K., Hoppe, D. J., 

Mouroulis, P. Z., et al. (2005). SPIE, 

5905, 59050H

Balasubramanian, K., Shaklan, S., Give’on, 

A., et al. (2011). SPIE, 8151, 81511G1

Figure 16.13. Left: The diattenuation (normalized reflected amplitude difference between 

polarization states) as a function of angle of incidence for bare aluminum and bare silver coatings 

at a wavelength of 550 nm. Right: The retardance (absolute reflected phase difference between 

polarization states) as a function of angle of incidence for the same two metals. Angles of incidence 

at the primary and secondary mirrors for an on-axis, 12-m-class telescope would typically be less 

than 15 degrees.
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16.5 UV coatings and short-wavelength cutoff

Matthew R. Bolcar (NASA GSFC)

Enabling the LUVOIR mission’s compelling 

ultraviolet (UV) science goals, while also 

maintaining broadband capabilities to 

support the exoplanet science mission and 

other general astrophysics observations, 

will require a high-performance broadband 

reflective coating. Of the common metallic 

mirror coatings (gold, silver, aluminum), 

only aluminum is capable of providing high 

reflectivity into the UV at wavelengths as short 

as 100 nm. However, almost immediately 

upon exposure to air (even at very low 

pressures), an oxidation layer forms on the 

surface of aluminum-coated mirrors that 

dramatically reduces its UV performance 

(Balasubramanian et al. 2017). Protective 

overcoats (usually a fluoride such as LiF, MgF2, 

or AlF3) are used to arrest the oxidation once 

it has begun and protect the aluminum layer 

from further oxidation. These overcoat layers 

themselves can also impact the reflectivity 

of the mirror at the shortest wavelengths. 

Current technology development efforts 

are focused on improving deposition 

Figure 16.14. Theoretical and demonstrated performance for various aluminum coatings. The 

theoretical, un-oxidized bare Al performance is shown in black. Demonstrated performance for 

MgF2 and LiF overcoats are shown in red and green, respectively. A LiF overcoat protected by a 

thin layer of AlF3 is shown in blue. Finally, theoretical performance for an AlF3 overcoat is shown 

in purple for two scenarios: without an interstitial oxide layer (dashed) and with a 3 Å interstitial 

oxide layer (dot-dashed). This figure was adapted with permission from Hennessy et al. (2016), 

with additional contributions from M. Quijada (GSFC) and K. Balasubramanian (JPL).
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The LUVOIR Interim Report D-19

processes to maximize protected aluminum 

coating reflectivity at wavelengths below 

150 nm, while maintaining high reflectivity at 

longer wavelengths through the visible and 

near-infrared. Figure 16.14 shows a number 

of reflectivity curves for demonstrated and 

theoretical protected aluminum coatings 

(Hennessy et al. 2016).

Typical protected aluminum coatings use 

a single, thin layer of MgF2 as the protectant. 

This is the coating used on the Hubble Space 

Telescope and provides excellent reflectivity 

(>85%) at wavelengths longer than ~120 nm. 

Shortward of 120 nm, the reflectivity sharply 

drops to about 20%. Current research efforts 

are developing Atomic Layer Deposition 

(ALD) processes for depositing a very thin 

(~few nanometers) layer of MgF2, which 

may provide protection from oxidation while 

maintaining the high-reflectivity of bare 

aluminum. 

Aluminum protected by LiF provides 

reflectivity greater than 80% at wavelengths 

as short as 110 nm and depends critically 

on the coating process employed. Its cutoff 

wavelength is around 100 nm. It is important 

to also note that LiF is a hygroscopic material 

that deteriorates when exposed to water 

vapor. Mirrors coated with LiF would need to 

be held under a constant dry purge during the 

entire integration and test phase, as well as 

launch. This will be extremely challenging for 

a system as large and complex as LUVOIR. 

Alternatively, protecting the LiF layer with a 

thin layer of AlF3 or MgF2 can prevent the 

absorption of water vapor and stabilize the 

performance of the coating over long periods 

of time (see Figure 16.15).

Figure 16.15. Demonstrated stability for an aluminum coating protected by LiF, and further 

overcoated with a thin layer of AlF3. The AlF3 helps prevent the LiF coating from absorbing water 

vapor from the ambient environment, thus preserving the as-deposited performance of the 

coating. Data provided courtesy K. Balasubramanian (JPL).
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Figure 16.14 also shows two theoretical 

curves for an aluminum mirror protected by 

AlF3 only (purple curves). The first curve is a 

theoretical best case in which no oxide layer 

has formed on the Al undercoat. The second 

curve shows the theoretical performance 

assuming a 3 Å layer of oxide has formed 

between the Al and the AlF3 overcoat. Layer 

thicknesses and process conditions affect 

the resulting performance and may allow 

further optimization.

Several technology development efforts 

are currently underway to optimize these 

coatings and the deposition processes, 

including both physical vapor deposition 

(PVD) and atomic layer deposition (ALD). 

New techniques that allow the overcoat to be 

deposited immediately after the deposition 

of the base Al layer will reduce the thickness 

of the oxide layer that forms or prevent it all 

together. Process improvements will also 

help increase the reflectivity, as well as its 

uniformity across the mirror surface.

References

Balasubramanian, K., Hennessy, J., Nikzad, 

S., et al. (2017). SPIE, 10298, 102980X

Hennessy, J., Balasubramanian, K., Moore, 

C. S., et al. (2016). JATIS, 2, 41206
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16.6 Ultraviolet detectors for cosmic origins and exoplanet science with 

LUVOIR

Kevin France (University of Colorado – Boulder) & David Schiminovich (Columbia University)

The LUVOIR Surveyor is envisioned as the 

`Space Observatory of the 21st Century’. 

With up to 40 times the geometric collecting 

area of the Hubble Space Telescope and 

highly sensitive, multiplexed instruments, 

LUVOIR is poised to provide transformative 

scientific measurements of a broad range of 

astrophysical objects. Ultraviolet imaging and 

spectroscopic capabilities are central to the 

majority of the key scientific goals of LUVOIR, 

from quantifying the flows of matter between 

galaxies and the intergalactic medium, to 

understanding how host star’s UV radiation 

regulates the atmospheric photochemistry 

on habitable planets. 

Carrying out high-precision ultraviolet 

astronomy across such a wide range of 

sources requires detector systems working 

below the atmospheric cut-off (100–400 

nm) with low-noise and/or photon-counting 

capability, high quantum detection efficiency, 

large format size, and high temporal 

resolution. Ideally, all of these characteristics 

would be encompassed in a single detector, 

but multiple technologies may be required 

to accomplish LUVOIR’s suite of science 

investigations. We present an overview of 

UV detector technologies so the LUVOIR 

STDT and NASA can make informed 

recommendations for directed technology 

investment to support first generation 

UV instrumentation for LUVOIR. We note 

that with the goal of serviceability, some 

technologies that are less mature today may 

be optimized by the time second generation 

LUVOIR instruments are proposed, and as 

such, a long-term but adaptable technology 

maturation plan would be desirable for UV 

detectors. 

Microchannel Plate Devices: Microchannel plates (MCPs) with “solar-blind” 

photocathodes, low dark noise rates, and 

zero read noise have a rich flight heritage 

on astronomy, heliophysics, and planetary 

science missions. 

Advantages: MCP-based detectors are 

inherently photon-counting, operate at room 

temperature, can be ruggedized for 10+ 

year lifetimes in space, are scalable to large 

formats, and offer relatively high quantum 

efficiency at short UV wavelengths (λ < 130 

nm). 

Disadvantages: MCPs have limited 

dynamic range for bright objects that 

require instrument safety protocols, do not 

regularly support signal-to-noise (S/N) > 100 

observations owing to fixed pattern noise, 

and experience issues with long-term “gainsag” (burn-in at locations of prolonged high 

illumination). 

CCD and sCMOS: Charge coupled 

devices (CCDs) have significant heritage in 

space and on the ground, can be δ-doped 

to improve UV performance, and antireflection coatings can be further optimized 

to offer higher QE over a selected bandpass. 

Research is ongoing to extend the range of 

QE enhancements. 

Advantages: UV-optimized CCDs have 

flight-heritage on astrophysics suborbital 

missions and on solar missions at shorter 

UV wavelengths. The large dynamic range 

and flat-field characteristics of these CCDs 

are well suited for high S/N observations. 

CCDs are moving to larger formats, and 

advancements in electron-multiplying CCDs 

(EMCCDs) offer the prospect of sub-electron 

read noise photon-counting operation. Other 

silicon-based technologies such as lownoise scientific complementary metal-oxidesemiconductor (sCMOS) are now being 
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D-22 The LUVOIR Interim Report

optimized for the UV, using similar processing 

techniques to those being used for CCDs.

Disadvantages: CCDs are not inherently 

“solar-blind,” due to the low band-gap of 

silicon, so care must be taken to mitigate 

sensitivity to scattered visible-band radiation 

in UV applications. Testing and optimization 

of these devices for radiation hardness in 

an L2-like environment is also an active 

area of research. Initial results indicate that 

EMCCDs in particular suffer degrade charge 

transfer efficiency due to extended periods of 

radiation exposure. Hole-multiplying CCDs 

(HMCCD), a p-type version of EMCCDs, are 

currently under development and would be 

inherently radiation hard. sCMOS devices 

require similar investigation into radiation 

hardness and low-temperature operation.

Advanced Concepts: Less mature UV 

detector technologies, such as microwave 

kinetic inductance detectors (MKIDs), offer 

the possibility of energy resolution at the 

Table 16.1. UV detector targets for LUVOIR and a comparison with the state-of-the-art in 

laboratory, flight and commercial technology (adapted from Bolcar et al. 2016). 

Parameter Goal State-of-the-Art

Operational Bandpass 90 nm–400 nm

< 90 nm–300 nm MCP

< 90 nm–400 nm EMCCD

~200 nm–400 nm sCMOS

Read Noise 0

0 MCP 

N/A for multi. mode EMCCD

0.8–1.0 e– sCMOS

Dark Current and/or 

Spurious Count Rate

≤0.1–1 counts/cm2/s 

≤1–10-3 e–/resol/hr 

0.05–0.5 counts/cm2/s MCP

> 0.005 e–/resol/hr EMCCD

> 0.005 e–/resol/hr sCMOS

Quantum Efficiency 

(Peak) 75% (Far UV–Near UV)

45–20% FUV–NUV MCP

30–50% FUV–NUV EMCCD

20–50% NUV sCMOS

Resol Size ≤ 10 mm

20 mm MCP

8–13 mm EMCCD

~5–11 mm sCMOS

Dynamic Range (Max. 

Count Rate)

≥ 104 Hz / resol 

(as needed)

2kHz / resol 5 MHz 

global

 MCP

Readout dependent EMCCD

105 Hz / resol sCMOS

Time Resolution ≤ 1 ms (as needed)

<< 1 ms MCP

< 10 ms EMCCD

< 10 ms sCMOS

Format ≥ 8–16k pixels per side 

with high fill factor

8k × 8k MCP

~1k × 2k (4k x 4k proto) EMCCD

~4k × 4k sCMOS

Radiation Tolerance Good

Good MCP

TBD EMCCD

Good sCMOS
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The LUVOIR Interim Report D-23

pixel level. Uncertainties in the scalability to 

significant pixel/spaxel counts and cryogenic 

operation currently limit the utility of these 

devices for LUVOIR, but these issues may 

be quantified and possibly overcome with 

additional technology investment. 

Challenges unique to UV: Key 

technology challenges that remain unique 

to the UV involve boosting efficiency and 

reducing noise, along with several other 

issues linked to these goals:

Quantum Efficiency: UV sensitive 

detectors have quantum efficiencies at or 

below 20–50% within the band, leading to 

the possibility that future technologies may 

provide factors of up to 2–5× improvement 

in overall UV sensitivity. Some UV sensor 

technologies have a comparable or higher 

QE in the visible, raising potential “red leak” 

issues that may require additional filtering. 

A related challenge is the low transmittance 

of UV bandpass filters, which limits options 

for efficient red-blocking or band selection. 

Multi-layer filter advancements offer a 

promising avenue of research.

Noise: The sky background is several 

magnitudes fainter in the UV than in the 

visible, which presents both an opportunity 

and a challenge. The opportunity is the 

exploitation of a low-background window 

for the study of faint objects. For example, 

for broadband FUV observations, the sky 

background is 28.5 mag/sq. arcsec, or 

fewer than ~1 photon per resol for 10–30 min 

exposure times on a 10-m class telescope, 

even less for narrowband observations or 

spectroscopy. The challenge is that taking 

full advantage of this low background 

requires detectors that do not themselves 

limit faint observations, motivating low noise 

or photon-counting technologies, with read 

noise and/or dark current typically lower 

than required for standard broadband visible 

observations. Borosilicate MCPs with anticoincidence shielding should mitigate the 

higher particle backgrounds inherent to 

operation at L2.

Dynamic Range: These same photoncounting detectors may also have limited 

dynamic range, particularly in the large 

formats required for LUVOIR, and this is 

a challenge that needs to be addressed 

by all technologies being considered. A 

detailed comparison of the dynamic range 

performance for each of the leading detector 

technologies, given realistic instrumentation 

and observing programs, would be a valuable 

exercise for the LUVOIR study.

Improving performance in these areas 

can significantly impact science return. The 

required exposure time to reach a given S/N 

for a particular target in a sky-backgroundlimited observation scales as t ~ QE and 

increases to t ~ QE2 in the detector dark 

current-limited regime, motivating efficiency 

gains. Similarly, the required exposure times 

decrease linearly with improvement in dark 

current and/or read noise, down to the 

very low sky-background limit. No single 

technology leads performance in all three 

areas (efficiency, read noise, and dark current). 

At this point, the optimal UV detector is likely 

to be application or instrument-specific.
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17 Appendix E: Additional LUVOIR instrument concepts

Here we present information on some possible alternate and/or second-generation 

instrument concepts for LUVOIR.
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17.1 Starshades for LUVOIR

Eric Cady (JPL) & Aki Roberge (NASA GSFC)

17.1.1 Introduction

Starshades are a relatively newer idea for 

providing the extreme high-contrast needed 

for exoplanet direct observations. They 

have strengths and weaknesses that are 

complimentary to those of coronagraphs. 

A starshade is a independent spacecraft 

flying in formation with the telescope (Figure 

17.1). The goal is to keep the telescope in 

the shadow cast by the starshade, and 

keep both spacecraft aligned with the target 

star. The larger the telescope, the larger 

the starshade needs to be. The edges of a 

starshade have a very particular shape to 

control diffraction and deepen the shadow 

at the location of the telescope. An example 

of a small starshade mission concept can 

be seen in this video: https://exoplanets.

nasa.gov/resources/1015/. A lecture on 

the Theory and Development of Starshades 

given at the 2014 Sagan Summer 

Workshop is available here: https://www.

youtube.com/watch?feature=player_

detailpage&v=h5w6z0jow1Q#t=0. 

A starshade blocks the unwanted bright 

light from an exoplanet host star before it 

enters the telescope, while allowing light from 

nearby planets to pass nearly unattenuated. 

Therefore, internally scattered light reaching 

the detector is minimized. Telescope 

segments and obstructions do not need 

to be masked out and the wavefront does 

not need to be corrected with deformable 

mirrors. A starshade can provide a very small 

inner working angle (IWA), approximately 

given by the starshade radius (R) divided by 

the starshade-telescope separation (z), down 

to around the diffraction limit of the telescope 

(~ 1.2 l/D). For planets in habitable zones 

of nearby sun-like stars, the starshadetelescope separations are tens to hundreds 

of thousands of km. 

Starshades have no intrinsic outer 

working angle (OWA). They can be designed 

Figure 17.1. Artist’s conception of the Exo-S mission concept, a starshade paired with a small 

telescope. The image captures a moment just after starshade deployment. Credit: NASA / JPL / 

Caltech.
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to operate over large bandpasses (a few 

times larger than today’s coronagraph 

bandpasses) and to provide small IWAs at 

virtually any wavelength. For a fixed IWA 

and contrast level, the required starshade 

size increases with wavelength. Since the 

starshade is not part of the optics train, 

internal reflections are kept to a minimum and 

total throughput is high. This makes them 

excellent for deep spectroscopy, especially 

in the NIR where coronagraphs struggle to 

provide small IWAs.

On the weaknesses side, the need to 

slew the starshade over huge arcs to realign 

it with different target stars means there are 

long intervals (days to weeks) between the 

high-contrast observations and the total 

number of observations is fuel-limited. The 

telescope can do other kinds of astronomical 

observations in the intervals, but starshades 

are relatively inefficient for high-contrast 

imaging surveys. The starshade and 

telescope must be precisely aligned during 

observations to maintain high contrast. 

Keeping the telescope in the darkest part of 

the starshade shadow generally translates to 

lateral position precision of about a meter (the 

separation precision is much less stringent).

Full-scale end-to-end system tests on 

the ground are not possible, although subscale tests are being be done in the lab and 

in the field (e.g., Cady et al. 2010; Smith et al. 

2016). The large sizes of starshades means 

that they must be folded up for launch and 

deployed in space. The exact shape of the 

optical edge must be accurate (on the order 

of 100 μm tolerance for contrast in the 10-10

range) after deployment. Further, the thin 

edge of the starshade must be engineered 

to minimize sunlight scattering back into the 

telescope. More information on starshade 

technology deleveopment may be found at 

https://exoplanets.nasa.gov/exep/technology/technology-overview/

17.1.2 Scope of LUVOIR starshade 

study

Starshades have several attractive features, 

and their strengths complement those of coronagraphs. Therefore, a series of starshade 

designs for LUVOIR were investigated over 

the May/June 2017 period. Here we discuss 

the design parameters and constraints, the 

optimization methodology, and the results.

Performance Constraints: Starshade 

designs were created for 4 architectures with 

the following constraints:

Case 1. 9-m LUVOIR aperture, long 

wavelength cutoff at 1000 nm

Case 2. 15-m LUVOIR aperture, long 

wavelength cutoff at 1000 nm

Case 3. 9-m LUVOIR aperture, long 

wavelength cutoff at 2500 nm

Case 4. 15-m LUVOIR aperture, long 

wavelength cutoff at 2500 nm

After consultation with the LUVOIR 

team, the short wavelength cutoff for the 4 

architectures was set to a goal of 250 nm. 

The starshade inner working angle (IWA) for 

each architecture was chosen to match that 

of a coronagraph on the same architecture 

(3.5 l/D at 500 nm). For 9-m designs, the 

IWA was 40 milliarcsec (mas); for 15-m 

designs, the IWA was 24 mas.

For the LUVOIR designs, we set a 

requirement that the mean contrast in a 1.0 

l/D annulus centered at the IWA in the final 

telescope focal plane be no greater than 

1 × 10-12. The same metric was used for HabEx 

starshade designs in separate work and is 

consistent with producing 10-10 contrast after 

running through an error budgeting process 

with achievable tolerances.

Engineering Constraints: Each 

starshade was optimized for a diameter 

2-m larger than the telescope aperture, to 

allow ~1-m formation-flying offset between 
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starshade and telescope in all directions. 

Petal tip sizes and inter-petal gaps were 

bounded at ≥1 mm, and the number of petals was fixed at 28, which had been previously preferred in other contexts for mechanical reasons. Starshade diameter and petal 

length were permitted to float as part of the 

overall optimization.

Other: Candidate designs were checked 

with a representation of the preliminary 15-m 

LUVOIR aperture provided by M. Bolcar. In 

the absence of a finalized LUVOIR-B design 

at the time of this analysis, a 9/15 scale 

version of the 15-m design was used instead 

for evaluating the 9-m cases. This preliminary 

aperture is shown in Figure 17.2. The 

resulting starshade designs are not sensitive 

to the exact telescope aperture geometry.

17.1.3 Starshade size and shape 

optimization

The optimization procedure used for the 

LUVOIR starshades has been covered in 

other works (e.g., Cady et al. 2009). Very 

briefly, a smoothly varying radial apodization 

function, A(r), is approximated with a 

binary shape that has transmission of 1 or 

0 everywhere (Figure 17.3). To select an 

appropriate A(r) for a LUVOIR starshade, we 

choose scientific constraints (IWA, bounds 

of science bandpass, telescope diameter) 

and engineering constraints (total starshade 

diameter, petal length, number of petals, 

minimum feature sizes, edge smoothness, 

formation-flying tolerance). These parameters 

are sufficient to populate a constrained linear 

optimization with a min-max cost function. 

We select the science IWA as the geometric 

IWA, the radius physically subtended by the 

starshade on the sky. The geometric IWA is 

defined as θ = arctan(R/z) ≈ R⁄z, where R is 

the starshade radius and z is the starshadetelescope separation. We choose θ based 

on science goals for a particular starshade, 

and let the distance z float as R/θ.

Other choices of science IWA are 

possible, such as angular radius that permits 

some fractional exoplanet throughput, but 

there are two strong advantages of using 

the geometric IWA. First, the parameter is 

independent of A(r), and so is known prior 

to beginning the optimization. Second, since 

the starshade has no structure beyond the 

IWA, errors in starshade manufacturing 

will only appear as sources from inside the 

angular extent of the starshade, and so using 

the geometric IWA as the science IWA during 

design means the ultimate performance will 

be much less sensitive to manufacturing 

errors.

Implicit in these designs is an assumption 

that the apodization is solid in the center 

and tapers monotonically outward, with 

that taper occurring over a sub-region of 

the radial extent. Leaving a solid central 

disk is a practical choice, as the spacecraft 

bus can be located there without interfering 

with the apodization profile. Manufacturing 

considerations require that A(r) cannot go 

smoothly to 0 at R or 1 at the radius of the 

Figure 17.2. Preliminary LUVOIR 15-m pupil 

used to verify performance of 15-m designs. A 

version scaled by 9/15 was used for the 9-m 

designs.
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central disk, as the petal tips or gaps between 

petals would become impossibly narrow. We 

prevent this by constraining feature sizes 

directly in the optimization.

Linear optimizations provide globally 

optimal solutions for their constraints, so 

in practice we iterate on the scientific and 

engineering parameters and rerun linear 

optimizations until an apodization function 

consistent with the science case is produced. 

Once a promising apodization is identified, it 

is propagated through the target telescope to 

the focal plane using a propagator that runs 

on the exact locus of edge points, rather than 

the azimuthally symmetric approximation 

used for the optimization. Final evaluation 

of the starshade performance is done in the 

focal plane.

17.1.4 Results

In the first round of design, results were 

found for the two 250–1000 nm cases:

Case 1. Starshade is a 40 mas IWA design 

for the 250–1000 nm band with the 

9-m telescope (labeled OH22). It 

has a diameter of 95 m with 27.3 

m petals. The telescope-starshade 

separation distance is 244,939 km.

Case 2. Starshade is a 24 mas IWA design 

for the 250–1000 nm band with the 

15-m telescope (labeled OM11). 

It is 160 m in diameter with 46 m 

petals. The telescope-starshade 

separation distance is 687,549 km.

The two shades, with the telescope 

apertures for scale, are shown in Figure 

17.4. For the 250–2500 nm bandpass 

(Cases 3 and 4), no designs that satisfied the 

science constraints were found for starshade 

diameters < 200 m. 

Given the large sizes of these designs, a 

second round of investigation was done to 

look at the effect on IWA of fixing the Case 1 

diameter to 70 m; this raised the geometric 

IWA to 57 mas. From this 70 m case, loosening 

the science constraints only produced weak 

improvement in IWA; raising the permitted 

contrast by 10x only decreased the IWA to 

53 mas, while separately raising the lower 

wavelength cutoff to 500 nm decreased the 

IWA to 54 mas.

Figure 17.3. An apodizer is converted into a binary starshade by replacing the tapering outer 

annulus with a series of N binary structures (“petals’”).
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We conclude that a NUV/visible starshade 

for a 9-m telescope may be a viable starlight 

suppression option, albeit one that surpasses 

the current “nominal” maximum starshade 

size of ~70 m. However, a high performance 

starshade paired with a 15-m telescope is 

likely unfeasible.

References

Cady, E., Balasubramanian, K., Carr, M., et 

al. (2010). SPIE, 7731, 77312F

Cady, E., Belikov, R., Dumont, P., et al. 

(2009). arXiv:0912.2938

Smith, D., Warwick, S., Glassman, T. M., et 

al. (2016). SPIE, 9904, 99043K

Figure 17.4. The two starshades for a 250–1000 nm bandpass, to scale, along with the telescope 

apertures and shadows they were designed to.
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17.2 Optical/Near-Infrared Spectrograph (ONIRS)

17.2.1 Overview

There are a variety of science cases that would 

be enhanced or enabled by an Optical/NearInfrared Spectrograph (ONIRS) with different 

capabilities than are already offered within 

the ECLIPS and HDI instruments. Some of 

those capabilities are:

1. Optical spectroscopy at resolutions 

greater than R~140 (ECLIPS IFS) or 

R~600 (HDI grism).

2. NIR spectroscopy at resolutions greater 

than R~70 (ECLIPS IFS), R~200 (ECLIPS 

point-source spectroscopy), or R~600 

(HDI grism).

3. NIR bandpass longer than 2.0 mm 

(ECLIPS) or 2.5 mm (HDI), for use on bright 

targets where the telescope thermal 

background does not limit performance. 

These additional capabilities would 

enhance a wide range of scientific 

investigations, including probes of the 

chemical evolution of high-redshift galaxies, 

studies of the dynamics and composition 

of the circumgalactic medium, and 

measurements of black hole dynamical 

masses. Closer to home, ONIRS would 

also improve our understanding of star 

formation and the initial mass function, 

protostellar outflows and jets, winds from 

protoplanetary disks, the composition of 

exoplanet atmospheres, and the surfaces 

of solar system bodies. Very high spectral 

resolution in the optical/NIR could also 

enable exoplanet mass measurements via 

the radial velocity technique.

These observing capabilities could be 

achieved in multiple ways, and likely are not 

all compatible with a single instrument. For 

example, solar system surface studies will 

demand spatially resolved spectroscopy, 

while the ultra-high spectral resolution 

(R~105) and stable wavelength calibration 

needed for radial velocity work are more 

easily achieved with a fiber-fed pointsource spectrograph. High resolution optical 

spectroscopy (Capability #1) could likely be 

added to the LUMOS instrument, a design 

option we will be considering in the coming 

months. An instrument concept that has 

Capability #3 appears in Section 17.3.

The LUVOIR STDT considered ONIRS 

for a first-generation instrument study. 

However, the STDT chose to defer detailed 

study of ONIRS, due to the limited number 

of instrument bays, some uncertainty about 

the performance of optical/NIR instruments 

planned for the future ground-based 

Extremely Large Telescopes (ELTs), and 

time/resource constraints within the present 

mission concept study.

17.2.2 Preliminary design drivers

We note that work on ONIRS did not 

progress to the point of a clear and detailed 

set of science goals and observation needs, 

or an actual instrument concept. However, 

we here present some information on 

particular science applications that would 

drive requirements on spectral resolution 

and wavelength range.

Exoplanet masses with radial velocity. 

Space-based radial velocity observations 

may present some advantages over groundbased ones, thanks to higher PSF stability 

and the absence of telluric features. A 

discussion of ultra-precise exoplanet RV 

from space appears in Plavchan et al. 

(2018). To summarize, measuring the 

masses of exoplanets down to Earth-mass 

planets around Sun-like stars (signal ~ 9 

cm/s), radial velocity precision on the order 

of 1 cm/s will be required. Achieving this 

will demand very high spectral resolution to 

combat stellar noise (R~300,000), extremely 
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precise wavelength stability and calibration, 

and sufficient sensitivity to keep individual 

exposure times short at such high spectral 

resolution. A laser frequency comb is a likely 

candidate for a high-precision wavelength 

calibration source. 

Exoplanet spectroscopy. Particular enhancements in exoplanet transit spectroscopy could be achieved by extending the 

bandpass for low-resolution point-source 

spectroscopy beyond ~2 mm. A much fuller 

discussion of this science case appears in 

Section 17.3.

New high-dispersion spectroscopy 

methods are being used to probe exoplanet 

atmospheres with ground-based telescopes. 

This technique employs R~100,000 

spectroscopy to resolve the individual lines 

within molecular absorption bands and 

cross-correlates many spectra taken over the 

orbital period of the planet against a highly 

accurate molecular absorption template. The 

technique is well-suited to measuring the spin 

of giant planets (e.g., Snellen et al. 2014) and 

detecting molecules in non-transiting giant 

planets (e.g., Birkby et al. 2017). Coupled 

with a high-contrast instrument, it may allow 

detection of molecules in the atmospheres 

of Earth-size planets around M dwarf stars 

at relaxed contrast levels (e.g., Snellen et al. 

2015). Such high-dispersion + high-contrast 

instruments are under consideration for 

the future ground-based ELTs. However, 

there is some uncertainty about the optimal 

spectral resolution for using this technique 

on a space-based platform and the limits 

of its performance (e.g., Wang et al. 2017). 

Furthermore, it is not clear how to obtain 

absolute molecular abundances with this 

technique, as the planet’s continuum flux 

level is not measured, and systematic 

uncertainties and degeneracies have not 

been thoroughly studied. With experience 

gained from employing this technique on the 

ELTs, its value on a space-based platform 

will become clearer and may lead to a desire 

for a second-generation LUVOIR instrument.

Composition of solar system bodies. Important spectral features for characterization of solar system atmospheres 

and surfaces lie at wavelengths longward 

of ~2 mm, for example, N2 ices at 2.15 mm 

and organics like ethane (C2H6) at 3.4 mm. 

Such observations will demand spatially resolved spectroscopy, either with an IFS or a 

linear-variable filter camera. An example of 

the latter is the Ralph instrument on the New 

Horizons spacecraft. As many solar system 

bodies are bright, it may be possible to usefully observe them beyond ~2 mm even in the 

presence of the thermal background from a 

warm telescope like LUVOIR. Furthermore, 

a large space-based telescope like LUVOIR 

will provide unprecedented spatial resolution 

for such observations, as extreme adaptive 

optics systems on the ELTs may struggle 

to provide high spatial resolution over large 

fields of view. 

References

Birkby, J. L., de Kok, R. J., Brogi, M., et al. 

(2017). AJ, 153, 138

Plavchan, P., Cale, B., Newman, P., et al. 

(2018). arXiv:1803.03960

Snellen, I., Brandl, B., de Kok, R., et al. 

(2014). Nature, 509, 63

Snellen, I., de Kok, R., Birkby, J. L., et al. 

(2015). A&A, 576, A59

Wang, J., Mawet, D., Ruane, G., et al. 

(2017). AJ, 153, 183
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17.3 Extension of LUVOIR’s capabilities to 5 mm or beyond

Michael Werner (Caltech-JPL)

We present the scientific rationale and 

technical implications for the use of LUVOIR 

out to at least 5 mm. We consider primarily 

the benefits of such an extension to the study 

of exoplanets, which is a pillar of the LUVOIR 

scientific case. We discuss two possible 

realizations of LUVOIR (9.2-m, T = 293 K) 

and (12-m, T = 273 K) and also present a 

simple instrument concept that could achieve 

our scientific objectives with limited impact 

on the rest of the facility. This white paper 

is based on Werner et al. (2016) (hereafter 

W16), which provides considerably greater 

detail. The examples below are based on a 

9.2-m LUVOIR and thus underestimate the 

power of the 15-m version under study.

17.3.1 Introduction

The rationale for operating LUVOIR out to at 

least 5 mm may be summarized as follows. 

One can be confident that, whenever LUVOIR 

launches, the study of exoplanet atmospheres 

will be an exciting and crucial scientific 

discipline. Numerous molecular species 

likely to be important probes of exoplanet 

atmospheres have bands in the infrared 

beyond ~2 mm including H2O, CO, CO2, CH4, 

NH3, and O3. Many of these same species 

are abundant in the Earth’s atmosphere and 

are difficult to access from the ground but 

are easily studied by a space telescope such 

as LUVOIR. In addition, recent developments 

suggest that atmospheric hazes or clouds 

may be common in exoplanet atmospheres 

and that they can interfere with our ability 

to determine molecular abundances using 

optical and near-infrared measurements only 

(e.g., Sing et al. 2016). Observations further 

into the infrared have the potential to improve 

constraints on molecular abundances by 

either punching through the haze or better 

defining its properties and extent. As a 

general principle, the study of exoplanet 

atmospheres will benefit greatly from 

inclusion of the widest range of molecules 

over the widest wavelength range. LUVOIR’s 

infrared capabilities would inform our overall 

understanding of exoplanet atmospheres 

and, in ways which may be difficult to foresee 

at present, support our quest to identify 

habitable or potentially habitable planets 

through atmospheric studies. Of course, an 

infrared-capable LUVOIR could also carry 

out exciting studies of targets other than 

exoplanets.

17.3.2 Use of a warm telescope for 

infrared exoplanet studies from space

Sensitivity is not the only rationale for an 

infrared space telescope. Here we consider 

instead the benefits of a 273–300K space 

telescope, in comparison to a comparable- or 

even larger-sized telescope at a mountaintop 

observatory. For present purposes, these 

benefits come in four categories:

Access to the entire infrared spectrum. 

From outside the Earth’s atmosphere, we 

can access the entire infrared spectrum and 

all infrared-active molecular species. This 

may include important species with critical 

diagnostic features longward of 5 mm, as 

well as the prominent species with spectral 

features in the 2–5 mm band discussed here.

Clear skies and long observations. Many 

exoplanet observations—for example, transits, eclipses, and phase curves—will be of 

long duration and have timing constraints. 

A space telescope, operating always under 

cloudless skies and easily able to observe a 

particular target for days rather than hours, 

provides great practical advantages for this 

type of study.

Stability. The study of exoplanet spectra 

using combined light spectroscopy (i.e., 
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transit spectroscopy) requires achieving 

a measurement precision on the order of 

parts per million (ppm) for the most difficult 

cases. Achieving such photometric precision 

on a large, warm telescope in space would 

certainly be easier than on a similar telescope 

on the ground, looking through the warm 

and turbulent atmosphere. 

Sensitivity. A large space telescope should 

achieve higher sensitivity than a comparably 

sized telescope on the ground because of 

the absence of atmospheric absorption and 

emission and the routine achievement of 

diffraction-limited performance.

The scientific, technical, and functional 

considerations outlined above motivated us 

to explore extending the wavelength range 

of LUVOIR to 5 mm, without imposing a 

constraint on the telescope temperature, 

but by adding an instrument. We begin 

by exploring the scientific benefits of 

this extended wavelength coverage. We 

also present a strawman design for an 

infrared spectrometer, covering the 1–5 

mm wavelength range with resolving power 

R = 200, which could execute the science 

described below with minimal system 

impact. The 5-micron cutoff reflects current 

detector technology. A more thorough study 

should explore longer wavelengths, noting, 

for example, that the 5–8 mm spectral interval 

is largely inaccessible from the ground. It 

could also include exploration of eclipse 

spectroscopy; here we consider only transit 

spectroscopy.

17.3.3 Exoplanet science: 

atmospheric characterization with 

transit spectroscopy

17.3.3.1 Telescope size can trump 

background noise

Transit spectroscopy thrives on observations 

of the brightest stars, for which the main 

noise source, even with a warm telescope, 

can be the photon noise of the star itself. For 

these observations of bright stars there is 

nothing gained, in principle, by cooling the 

telescope. In Figure 17.5, we consider the 

time taken (or inverse sensitivity) for three 

telescope configurations to detect a similar 

small (10 ppm at R=200) and narrow feature 

at 4 mm, as a function of the stellar magnitude 

at that wavelength. The configurations are 

two versions of LUVOIR and JWST (6.5-

Figure 17.5. Exposure time (or inverse 

sensitivity) as a function of stellar magnitude for 

detection of a small and narrow feature (10 ppm 

at R=200) at 4 mm due to a transiting exoplanet. 

Three telescope configurations are shown: 

9.2-m and 12-m LUVOIRs [called ATLAST in 

W16] and JWST. LUVOIR is more sensitive 

than JWST for bright host stars, with the 12-m 

LUVOIR telescope taking ~4× less time to make 

the same observation, as is to be expected in 

the stellar photon limited case. For the LUVOIR 

configurations, a total photon throughput of 

0.35 and a (single mode) background with 

emissivity 0.2 is assumed; these parameters 

are commensurate with those of the instrument 

described below. For JWST, the actual 

expected performance of the NIRCAM grism 

(Greene et al. 2007) is adopted. The specific 

example adopted here is quite stressing in 

terms of photometric precision requirement, 

but the gain of the warm telescope would be 

preserved in less demanding observations as 

well.
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m, cryogenic telescope). LUVOIR is more 

sensitive than JWST for bright host stars, 

with the 12-m LUVOIR telescope taking ~4× 

less time to make the same observation for 

stars brighter than [L]~9, as is to be expected 

in the stellar photon limited case. The 15-m 

LUVOIR would be about 40% faster than 

the 12-m version. For transit spectroscopy, 

a large warm telescope becomes more 

powerful than a smaller but cooler one.

17.3.3.2 There will be targets

Exoplanet direct imaging is a major driver 

of the LUVOIR concept, which includes 

coronagraphy to obtain direct images and 

low-resolution spectra of exoplanets. The 

infrared functionality discussed here would 

provide a powerful complement to this 

direct imaging work through the use of 

combined light studies, greatly increasing 

the scope and wavelength coverage of 

LUVOIR exoplanet studies. Observations 

of transiting exoplanets with LUVOIR can 

yield an enormous scientific return due to 

the combination of a large aperture and 

thousands or tens of thousands of targets, 

each presenting a different scientific puzzle.

In addition to Kepler, K2, and TESS 

[scheduled for launch in 2018], the PLATO 

mission, to be launched by ESA in 2028 for a 

six+ year mission, will yield planets for study 

by LUVOIR. Based on information provided 

by David Brown of the PLATO team, PLATO 

should find over 3400 transiting planets 

around stars brighter than 11th magnitude 

in the visible. The predicted S/N per hour 

vs. planet size for the some of the most 

readily studied exoplanets which PLATO 

might discover is shown in Figure 17.6. 

We take S/N > 100 as our target, because 

only a portion of the total transit signal is 

contributed by the atmosphere. This is 

readily achieved in a single transit for Jovian 

planets with R~10 REarth. A handful of smaller 

planets will orbit stars of ~5th mag, and 

their spectra can be readily obtained. Coadding transits will reach still smaller planets. 

The 5th magnitude star can be observed 

with LUVOIR and the R=200 spectrometer 

described below without saturating the 

detectors, provided that frame rates faster 

than 1 Hz are achievable. 

17.3.3.3 Abundance constraints: an 

example

A critical performance metric for characterizing exoplanet atmospheres via spectroscopy 

is the degree of constraint the measurements 

provide on the abundance, also termed the 

atmospheric mixing ratio. W16 evaluated the 

effects of the LUVOIR wavelength range on 

the constraints that LUVOIR transit spectroscopy might provide. We note a recent 

paper by Greene et al. (2016), which, for the 

specific case of JWST, shows the importance of observations longward of 2.5 mm 

for characterizing exoplanet atmospheres. 

As an illustrative example, W16 considered 

the case of a 2.6 REarth, 6.1 MEarth transitional 

super-Earth/sub-Neptune planet with a 600 

K atmosphere orbiting a K star, assuming solar metallicity, thermal equilibrium chemistry, 

Figure 17.6. Signal/noise on the transit 

depth for each resolution element in one 

hour for planetary transits at 4 mm. A 9.2-m, 

293 K telescope and the R=200 fiber-fed 

spectrograph described below were adopted 

for the calculation.
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and a clear atmosphere, observed with the 

9.2-m LUVOIR. A number of independent 

observations were simulated and the results 

analyzed with a multi-parameter MCMC retrieval using the CHIMERA code.

The results of these experiments are 

presented in Table 17.1, which shows that 

with 0.4–5.0 mm spectral coverage, LUVOIR 

has the potential to determine the abundance 

of water and methane in many TESS/K2/

PLATO targets. If the spectral coverage were 

limited to 2.5 mm, brighter stars are required, 

so substantially fewer targets would become 

available for study. This demonstrates the 

benefits of the enhanced wavelength range 

we are proposing, particularly for the high 

mean molecular weight atmosphere, which 

approximately corresponds to an Earthlike atmosphere. In this case, a very bright 

star ([H] < 6.3) is required for abundance 

determinations with less than an order of 

magnitude uncertainty; stars this bright 

that harbor transiting planets may be rare 

statistically. As expected, the lower mean 

molecular weight atmosphere, with its larger 

scale height, would be much more readily 

observed. Several recent papers (Misra, 

Meadows, & Crisp 2014; Betremieux & 

Kaltenneger 2014) have highlighted the 

effects of refraction in the lower part of an 

exoplanet’s atmosphere on our ability to study 

abundances using transit measurements. 

The importance of this effect depends 

on both the size of the star and the starplanet distance, and it will have to be taken 

into consideration in more sophisticated 

predictions.

17.3.3.4 Studying a true Earth analog

A major thrust of exoplanet studies is, of 

course, the push to identify and study 

potentially habitable Earth-like planets. 

Recent work has shown that simply identifying 

O2 and O3 in a particular atmosphere is not 

adequate. Other molecular species, notably 

CH4 but also N2, CO, and CO2, may have 

to be studied in order to rule out or confirm 

the potential habitability of a particular planet 

which shows O2 and O3. Understanding the 

prevalence of these species (all of which 

have strong spectral features longward of 

2.5 mm) in exoplanet atmospheres, thus 

takes on special interest, even if it may not in 

general be possible to search for all of these 

molecules in the atmosphere of the same 

planet.

The detection of biomarkers in the 

atmosphere of a true Earth analog presents 

major challenges. This is a difficult detection, 

so we present it as a limiting case; 

studying larger planets with more extended 

atmospheres should be much easier. The 

average methane volume mixing ratio in the 

Earth’s atmosphere is quite low (1.7 × 10-6 

mol/mol), a factor of 200 less than CO2, 

but it is the major source of departure from 

thermodynamic equilibrium if the atmosphere 

is considered as a closed system (without 

oceans and rocks). The strong methane 

3.4 mm feature will be blended with a 

water absorption at low R, but nonetheless 

Table 17.1. Limiting magnitude for determination of molecular abundances with an overall 

uncertainty of less than one order of magnitude for a 2.6 REarth super-Earth/sub-Neptune (T~600 

K) orbiting a K star, assuming 24 hours of observation time spread over a number of transits.

Molecule 0.4–2.5 mm 0.4–5 mm

Mean mol. weight 2.3 H2O Hmag < 10.3 Hmag < 11.3

CH4 Hmag < 10.3 Hmag < 12.3

Mean mol. weight 28 H2O Hmag < 6.3 Hmag < 8.3

CH4 Hmag < 6.3 Hmag < 8.3
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contributes up to 10 km (a little more than 

the 8 km scale height) of extra absorption in 

the transmission spectroscopy of an Earth. 

The weaker methane features at shorter 

wavelengths are yet more heavily blended. 

In atmospheres with high cloud coverage, 

the strong IR molecular features absorb 

at altitudes above the cloud deck, which 

is a major advantage of long wavelength 

spectroscopy.

We have computed the transit spectrum 

of an Earth analog orbiting a late type star, 

exploiting the advantages conferred by M 

stars for transit measurements; in addition, 

the refraction effect mentioned above is 

much less of a concern for M stars than for 

solar type stars. Conservatively assuming 

that the occurrence of habitable zone rocky 

planets is 0.2 around late type stars, the 

nearest M-dwarf with a favorable transit will 

be about ~15 pc away.

In Figure 17.7 we show a simulated 

LUVOIR R=20 transit spectrum of an Earth 

transiting an M4V star (distance 15 pc, Lmag

= 7.7) in the 2–5 mm region; the underlying 

model simply uses an Earth transit spectrum 

and the instrument prescription described in 

this paper, for which the spectral resolution 

can be degraded from 200 to 20 with a 

√10 increase in S/N per resolution element. 

Figure 17.7. A 2–5 μm R~20 model transmission spectrum of a 1 REarth habitable zone planet 

with an Earth-like oxidizing atmosphere transiting an M4V star (Rstar = 0.32 RSun, Tstar = 3100 K) 

which is 15 pc from Earth. The 1-sigma errors per wavelength bin, obtained with the smaller 

LUVOIR design (D=9.2 m, T=273 K), are plotted along with the model. The total integration time 

is 120 hours in transit, corresponding to 60 transits of about 2 hours each. Features of H2O, CH4, 

and CO2 are detected. CH4 is blended with H2O at ~3.3 mm and determining its independent 

presence and total column would require full atmospheric retrieval; in the Earth’s atmosphere, 

CH4 and H2O contribute roughly equally to the 3.3 mm feature. A total throughput of 0.35 and an 

emissivity of 0.2 is assumed here. The y-axis is labelled in parts per million (ppm) relative to the 

total signal produced by the star. The overall depth of the transit for this case would be about 

800 ppm; the features shown in the spectrum would be manifest as changes in the transit depth 

with wavelength. The precision of a few ppm called for here would be a challenging target for IR 

detector technology.
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While the total time required for the spectrum 

shown in Figure 17.7 (~60 transits or 120 

hours) is large, it is not outrageously so 

(two exoplanets have had similar amounts 

of Hubble time awarded). The orbital period 

of a habitable zone planet around our M4 

star is 16 days, so about 2.5 years would 

be required to obtain spectra similar to that 

shown in Figure 17.7.

17.3.4 Summary: an infrared 

capability for LUVOIR

We have demonstrated that a ~10-m diameter 

telescope operating in space at room 

temperature would be able to make unique 

and important scientific measurements in 

the infrared out to at least 5 mm. By using 

the technique of transit spectroscopy, where 

for bright stars the noise is set by the stellar 

photons rather than the thermal background, 

such a telescope could outperform smaller 

but lower temperature telescopes for studies 

which could prove critical for the study of 

exoplanet atmospheres. It would also be 

capable of obtaining spectra of millions of 

infrared sources from the WISE catalog and 

provide new information about the formation 

of solar systems, which would link directly 

to the exoplanet observations. We feel 

that such a capability should be carefully 

considered for incorporation into LUVOIR, 

now or in the future, particularly if further 

studies of instrument concepts such as that 

presented below show that they can be 

easily incorporated into the payload.

17.3.5 A fiber-fed infrared 

spectrometer for LUVOIR

W16 describe a fiber-fed IR spectrometer 

for LUVOIR. The spectral resolving power of 

200 is chosen to permit disambiguation of 

molecular features in exoplanet atmospheres 

and also to permit study of the profiles of 

solid state absorption features in galactic 

protostellar and protoplanetary disks. We 

emphasize that other spectrograph designs 

and means of feeding the spectrograph could 

provide equivalent performance, and that 

coupling the starlight into and out of the fiber 

could stress the LUVOIR pointing system 

and perhaps other design parameters. An 

alternative approach using mirrors instead 

of fibers to feed the spectrograph ought 

certainly to be considered, for example.

This particular approach was chosen 

because, on the surface at least, it appears 

to minimize the system impact of the thermal 

infrared capability. Figure 17.8 is a concept 

sketch of the system. A pick-off mirror will 

be inserted into the LUVOIR telescope 

focal plane to relay the star signal into a 

prism-based spectrometer system (f/12.5), 

featuring a MWIR FPA (5 mm-cutoff H2RG 

from Teledyne). The reflective prism is made 

of calcium fluoride. A 10 m IR fiber (Fluoride 

Fiber ZBLAN SM from Le Verre Fluore) is 

used to carry the light from the telescope to 

the spectrometer, which is mounted on the 

Figure 17.8. Fiber-fed, prism-based IR 

spectrometer configuration.
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exterior structure of LUVOIR so that it can 

cool radiatively. To get optimal fiber coupling, 

2 relay mirrors with power (f/12.5 to f/2.5) 

are utilized. To minimize the spectrometer 

background noise, a cold shield box covers 

the full spectrometer system, and the second 

relay surface illuminates the spectrometer slit, 

which is mounted on the side of the otherwise 

sealed cold shield box. The performance of 

this system is detailed in W16 and used in 

the calculations presented above.

The design of the spectrometer described 

here was supported by a grant from the JPL 

Research and Technology Development 

fund. Portions of the work described in this 

paper were carried out at the Jet Propulsion 

Laboratory, California Institute Technology, 

under a contract with the National Aeronautics 

and Space Administration. 

References

Betremieux, Y. & Kaltenegger, L. (2014). 

ApJ, 791, 7

Greene, T. P., Beichman, C., Eisenstein, D., 

et al. (2007). SPIE, 6993, 69930G

Greene, T. P., Line, M. R., Montero, C., et al. 

(2016). ApJ, 817, 17

Misra, A., Meadows, V., & Crisp, C. (2014). 

ApJ, 792, 61

Sing, F., Fortney, J., Nikolov, N., et al. 

(2016). Nature, 529, 59

Werner, M., Swain, M., Vasisht, G., et al. 

(2016). JATIS, 2, 041205