MOONS: The New Multi-Object Spectrograph for the
VLT

Michele Cirasuolo, Alasdair Fairley, Phil Rees, Oscar A. Gonzalez, William Taylor, Roberto Maiolino, Jose Afonso, Chris Evans, Hector Flores, Simon Lilly, et al.

<figure><img src="image_1.png" /></figure>

To cite this version:

Michele Cirasuolo, Alasdair Fairley, Phil Rees, Oscar A. Gonzalez, William Taylor, et al. MOONS: The New Multi-Object Spectrograph for the VLT. Eso Messenger, 2020, 180, pp.10-17. 10.18727/0722-6691/5195 . hal-02955869

HAL Id: hal-02955869

https://hal.science/hal-02955869v1

Submitted on 19 Sep 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
MOONS: The New Multi-Object Spectrograph for the VLT

Michele Cirasuolo¹, a

Alasdair Fairley²

Phil Rees²

Oscar A. Gonzalez²

William Taylor²

Roberto Maiolino³

Jose Afonso⁴

Chris Evans²

Hector Flores⁶

Simon Lilly⁵

Ernesto Oliva⁷

Stephane Paltani⁸

Leonardo Vanzi⁹

Manuel Abreu⁴

Matteo Accardo¹

Nathan Adams¹⁵

Domingo Álvarez Méndez¹

Jean-Philippe Amans⁶

Stergios Amarantidis⁴

Hakim Atek⁶

David Atkinson²

Manda Banerji¹²

Joe Barrett²

Felipe Barrientos⁹

Franz Bauer⁹

Steven Beard²

Clementine Béchet⁹

Andrea Belfiore¹¹

Michele Bellazzini¹³

Christophe Benoist³²

Philip Best¹⁰

Katia Biazzo¹⁴

Martin Black²

David Boettger⁹

Piercarlo Bonifacio⁶

Rebecca Bowler¹⁵

Angela Bragaglia¹³

Saskia Brierley²

Jarle Brinchmann¹⁶

Martin Brinkmann¹

Veronique Buat¹⁷

Fernando Buitrago⁴

Denis Burgarella¹⁷

Ben Burningham¹⁸

David Buscher³

Alexandre Cabral⁴

Elisabetta Caffau⁶

Leandro Cardoso¹⁶

Adam Carnall¹⁰

Marcella Carollo⁵

Roberto Castillo¹

Gianluca Castignani¹⁹

Marco Catelan⁹

Claudia Cicone²⁰

Andrea Cimatti²¹

Maria-Rosa L. Cioni²²

Gisella Clementini¹³

William Cochrane²

João Coelho⁴

Miriam Colling²³

Thierry Contini²⁴

Rodrigo Contreras⁹

Ralf Conzelmann¹

Giovanni Cresci⁷

Mark Cropper²⁵

Olga Cucciati¹³

Fergus Cullen¹⁰

Claudio Cumani¹

Mirko Curti³

Antonio Da Silva⁴

Emanuele Daddi²⁶

Emanuele Dalessandro¹³

Francesco Dalessio¹⁴

Louise Dauvin⁹

George Davidson²

Patrick de Laverny³²

Françoise Delplancke-Ströbele¹

Gabriella De Lucia²⁷

Ciro Del Vecchio⁷

Miroslava Dessauges-Zavadsky⁸

Paola Di Matteo⁶

Herve Dole²⁸

Holger Drass⁹

Jim Dunlop¹⁰

Rolando Dünner⁹

Steve Eales²⁹

Richard Ellis³⁰

Bruno Enriques⁵

Giles Fasola⁶

Annette Ferguson¹⁰

Debora Ferruzzi⁷

Martin Fisher³

Mauricio Flores⁹

Adriano Fontana¹⁴

Vincenzo Forchi¹

Patrick Francois⁶

Paolo Franzetti¹¹

Adriana Gargiulo¹¹

Bianca Garilli¹¹

Julien Gaudemard⁶

Mark Gieles³¹

Gerry Gilmore³

Michele Ginolfi⁸

Jean Michel Gomes¹⁶

Isabelle Guinouard⁶

Pablo Gutierrez¹

Régis Haigron⁶

François Hammer⁶

Peter Hammersley¹

Chris Haniff³

Chris Harrison¹

Misha Haywood⁶

Vanessa Hill³²

Norbert Hubin¹

Andrew Humphrey¹⁶

Rodrigo Ibata³³

Leopoldo Infante⁹

Derek Ives¹

Rob Ivison¹

Olaf Iwert¹

Pascale Jablonka¹⁹

Gerd Jakob¹

Matt Jarvis¹⁵

David King³

Jean-Paul Kneib¹⁹

Philippe Laporte⁶

Andy Lawrence¹⁰

David Lee²

Gianluca Li Causi³⁵

Silvio Lorenzoni⁴

Sara Lucatello³⁴

Yerco Luco⁹

Alastair Macleod²

Manuela Magliocchetti³⁵

Laura Magrini⁷

Vincenzo Mainieri¹

Charles Maire⁸

Filippo Mannucci⁷

Nicolas Martin³³

Israel Matute⁴

Sophie Maurogordato³²

Sean McGee⁴³

Derek Mcleod¹⁰

Ross McLure¹⁰

Richard McMahon³

Basile-Thierry Melse⁶

Hugo Messias⁴

Alessio Mucciarelli²¹

Brunella Nisini¹⁴

Johannes Nix²

Peder Norberg³⁶

Pascal Oesch⁸

António Oliveira⁴

Livia Origlia¹³

Nelson Padilla⁹

Ralf Palsa¹

Elena Pancino⁷

Polychronis Papaderos¹⁶

Ciro Pappalardo⁴

Ian Parry³

Luca Pasquini¹

John Peacock¹⁰

Fernando Pedichini¹⁴

Roser Pello¹⁷

Yingjie Peng⁴²

Laura Pentericci¹⁴

Oliver Pfuhl¹

Roberto Piazzesi¹⁴

Dan Popovic¹

Lucia Pozzetti¹³

Mathieu Puech⁶

Thomas Puzia⁹

Anand Raichoor¹⁹

Sofia Randich⁷

Alejandra Recio-Blanco³²

Sandra Reis⁴

Florent Reix⁶
Álvio Renzini³⁴

Myriam Rodrigues⁶

Felipe Rojas⁹

Álvaro Rojas-Arriagada⁹

Stefano Rota¹¹

Frédéric Royer⁶

Germano Sacco⁷

Ruben Sanchez-Janssen²

Nicoletta Sanna⁷

Pedro Santos⁴

Marc Sarzi⁴⁴

Daniel Schaerer⁸

Ricardo Schiavon³⁷

Robin Schnell⁸

Mathias Schultheis³²

Marco Scodeggio¹¹

Steve Serjeant³⁸

Tzu-Chiang Shen³⁹

Charlotte Simmonds¹⁹

Jonathan Smoker¹

David Sobral⁴⁵

Michael Sordet⁸

Damien Spérone¹⁹

Jonathan Strachan²

Xiaowei Sun³

Mark Swinbank³⁶

Graham Tait²

Ismael Tereno⁴

Rita Tojeiro⁴⁰

Miguel Torres⁹

Monica Tosi¹³

Andrea Tozzi⁷

Ezequiel Tresiter⁹

Elena Valenti¹

Álvaro Valenzuela Navarro⁹

Eros Vanzella¹³

Susanna Vergani⁶

Anne Verhamme¹⁹

Joël Vernet¹

Cristian Vignali¹³

Jakob Vinther¹

Lauren Von Dran⁴¹

Chris Waring²

Stephen Watson²

Vivienne Wild⁴⁰

Bart Willesme²

Brian Woodward²

Stijn Wuyts⁴⁶

Yanbin Yang⁶

Gianni Zamorani¹³

Manuela Zoccali⁹

Asa Bluck³

James Trussler³

¹ ESO

² STFC, UK Astronomy Technology Centre, Royal Observatory Edinburgh, UK

³ Cavendish Laboratory, University of Cambridge, UK

⁴ Instituto de Astrofísica e Ciências do Espaço and Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Portugal

⁵ Department of Physics, ETH Zurich, Switzerland

⁶ GEPI, Observatoire de Paris, PSL University, CNRS, France

⁷ INAF-Osservatorio Astrofisico di Arcetri, Florence, Italy

⁸ Department of Astronomy, University of Geneva, Versoix, Switzerland

⁹ Pontificia Universidad Católica de Chile, Santiago, Chile

¹⁰ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK

¹¹ INAF, IASF-MI, Milano, Italy

¹² Faculty of Engineering and Physical Sciences, University of Southampton, UK

¹³ INAF – Astrophysics and Space Science Observatory Bologna, Italy

¹⁴ INAF – Osservatorio Astronomico di Roma, Italy

¹⁵ Department of Physics, University of Oxford, UK

¹⁶ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Porto, Portugal

¹⁷ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France

¹⁸ School of Physics Astronomy and Mathematics, University of Hertfordshire, UK

¹⁹ EPFL, Observatoire de Sauverny, Versoix, Switzerland

²⁰ Institute of Theoretical Astrophysics, University of Oslo, Norway

²¹ University of Bologna, Department of Physics and Astronomy (DIFA), Italy

²² Leibniz-Institut für Astrophysik Potsdam (AIP), Germany

²³ STFC, Daresbury Laboratory, Sci-Tech Daresbury, UK

²⁴ Institut de Recherche en Astrophysique et Planétologie, Toulouse, France

²⁵ UCL Department of Space and Climate Physics, London, UK

²⁶ CEA, IRFU, DAp, AIM, Université Paris-Saclay, Université Paris Diderot, France

²⁷ INAF – Osservatorio Astronomico di Trieste, Italy

²⁸ Institut d'Astrophysique Spatiale, Orsay, Université Paris Sud, France

²⁹ School of Physics and Astronomy, University of Cardiff, UK

³⁰ Dept of Physics & Astronomy, University College London, UK

³¹ Astrophysics Research Group, Surrey University, UK

³² Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, France

³³ Observatoire Astronomique, Université de Strasbourg, France

³⁴ INAF – Osservatorio Astronomico di Padova, Italy

³⁵ INAF – Istituto di Astrofisica e Planetologia Spaziali, Rome, Italy

³⁶ Department of Physics, Durham University, UK

³⁷ Liverpool John Moores University, UK

³⁸ School of Physical Sciences, The Open University, Milton Keynes, UK

³⁹ BlueShadows Ltda., Santiago, Chile

⁴⁰ School of Physics and Astronomy, University of St Andrews, UK

⁴¹ Appleton Laboratory, STFC, Harwell Campus, UK

⁴² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China

⁴³ School of Physics and Astronomy, University of Birmingham, UK

⁴⁴ Armagh Observatory & Planetarium, Armagh, Northern Ireland

⁴⁵ Department of Physics, University of Lancaster, UK

⁴⁶ Department of Physics, University of Bath, UK

MOONS is the new Multi-Object Optical and Near-infrared Spectrograph currently under construction for the Very Large Telescope (VLT) at ESO. This remarkable instrument combines, for the first time, the collecting power of an 8-m telescope, 1000 fibres with individual robotic positioners, and both low- and high-resolution simultaneous spectral coverage across the 0.64–1.8 µm wavelength range. This facility will provide the astronomical community with a powerful, world-leading instrument able to serve a wide range of Galactic, extragalactic and cosmological studies. Construction is
now proceeding full steam ahead and this overview article presents some of the science goals and the technical description of the MOONS instrument. More detailed information on the MOONS surveys is provided in the other dedicated articles in this Messenger issue.

## Introduction

Over the last two decades several observational milestones have dramatically changed our knowledge of the Universe. Measurements of the Cosmic Microwave Background, high-redshift supernovae and large-scale structure have revealed that 96% of the density of the Universe consists of currently unexplained Dark Energy and Dark Matter, and less than 4% is in the form of baryons. Yet most of the information we have comes from luminous, baryonic matter. Understanding the nature of the dark components which dominate the global expansion and large-scale structure of the Universe along with the physical processes that affect baryons and shape the formation and evolution of stars and galaxies is amongst the most fundamental unsolved problems in science.

Answering these important questions requires an accurate reconstruction of the assembly history of stars and galaxies over virtually all of cosmic time in order to decode the building blocks of the Universe. The Milky Way offers a unique opportunity to reconstruct the assembly history of a prototypical spiral galaxy by looking at the individual ages, chemical abundances, and orbital motions of its stellar populations. Looking far beyond our Galaxy, it is also essential to trace the evolution of galaxy properties (star formation, chemical enrichment, mass assembly, etc.) over the whole cosmic epoch if we are to investigate the effects of age and environment. Ideally, these studies should be pushed to the highest redshifts — when the Universe was just a few hundred million years old — and young galaxies are key to understanding the physics of the early Universe and cosmic re-ionisation. Addressing these fundamental science goals requires accurate determinations of stellar and galactic physical properties, as well as

precise measurements of the spatial and chemical distribution of stars in the Milky Way and the 3D distribution of galaxies at different epochs.

In order to address this fundamental science, the MOONS instrument has been developed focusing on three essential parameters: sensitivity, multiplexing and wavelength coverage (the full list of key instrument parameters is given in Table 1)¹,². In respect of sensitivity, the VLT is currently one of the largest infrared and visible telescopes in the world in terms of collecting area, and every element of the MOONS instrument itself has been optimised for high transmission. The multiplex of 1000 is a factor of 20 larger than current spectrographs operating in the near-infrared; this is limited by the budget available, the capability to manufacture very large optics, and space

on the Nasmyth platform for the large cryostat (which is already more than 4 m high — see Figure 2). Finally, the broad wavelength coverage of MOONS, from 0.645 µm to 1.8 µm, extending into the near-infrared, is critical for observing heavily dust-obscured regions of our Milky Way as well as for opening a window onto the high-redshift Universe. To meet the aspirations of both the Galactic and extragalactic scientific communities, MOONS offers both low- and high-resolution spectroscopy. In the low-resolution mode ($R \sim 4000-7000$), the entire 0.645–1.8 µm range is observed simultaneously across the RI, YJ and H atmospheric windows. In the high-resolution mode the YJ channel remains unchanged at $R \sim 4000$, while the two high-resolution dispersers are inserted in the RI and H bands: one with $R > 9000$ around the Ca triplet region

<figure><img src="image_3.png" /><figcaption>**Figure 1.** The key science drivers that have shaped the requirements of the MOONS instrument:</figcaption></figure>

- The ability to obtain radial velocities and detailed stellar abundances for millions of stars, especially in the obscured regions of the Galaxy, to reconstruct the chemo-dynamical properties of our Milky Way.

- The capability to observe key spectral diagnostics for millions of distant galaxies up to the epoch of re-ionisation at $z > 7$, to determine the physical processes that shape their evolution and the impact of central supermassive black holes and the environments in which galaxies live, disentangling nature versus nurture effects.
<table><caption>Table 1. MOONS key instrument parameters.</caption><thead><tr><th>Parameter</th><th>Value</th></tr></thead><tbody><tr><td>Telescope</td><td>VLT, 8 m</td></tr><tr><td>Field of view</td><td>25 arcminutes in diameter</td></tr><tr><td>Multiplex</td><td>1001</td></tr><tr><td>On-sky aperture of each fibre</td><td>1.2 arcseconds</td></tr><tr><td>Field coverage</td><td>&gt; 3 fibres can reach any point in the focal plane</td></tr><tr><td># of fibres within a 2-arcminute diameter</td><td>7</td></tr><tr><td>Minimum fibre separation</td><td>10 arcseconds</td></tr><tr><td>Spectral channels</td><td>R<sub>I</sub>, Y<sub>J</sub> and H bands observed simultaneously</td></tr><tr><td>Resolution modes</td><td>Low and high resolution</td></tr><tr><td>Low-res simultaneous spectral coverage</td><td>0.64 – 1.8 µm</td></tr><tr><td>Low-res spectral resolution</td><td>R<sub>RI</sub> = 4100, R<sub>YJ</sub> = 4300, R<sub>H</sub> = 6600</td></tr><tr><td>High-res simultaneous spectral coverage</td><td>λ<sub>RI</sub> = 0.76 – 0.89 µm, λ<sub>YJ</sub> = 0.93 – 1.35 µm, λ<sub>H</sub> = 1.52 – 1.64 µm</td></tr><tr><td>High-res spectral resolution</td><td>R<sub>RI</sub> = 9200, R<sub>YJ</sub> = 4300, R<sub>H</sub> = 19700</td></tr><tr><td>Throughput</td><td>&gt; 30% in low resolution, &gt; 25% in high resolution</td></tr><tr><td>Sensitivity (point sources) in 1 hr integration</td><td>See Figure 3 for details</td></tr><tr><td>Continuum high res</td><td>S/N &gt; 60 at H<sub>AB</sub> ~ 17 and R<sub>AB</sub> ~ 17.5</td></tr><tr><td>Continuum low res</td><td>S/N &gt; 5 at mag(AB) ~ 23 rebinding to R = 1000 after sky subtraction</td></tr><tr><td>Emission lines</td><td>S/N &gt; 5 for a line flux of &gt; 2 × 10<sup>-17</sup> erg s<sup>-1</sup>cm<sup>-2</sup>, FWHM = 200 km s<sup>-1</sup></td></tr><tr><td>Calibration methods</td><td>Daytime flat fields, attached flats as part of observations, ThAr lamps for wavelengths</td></tr><tr><td>Observing overheads</td><td>Fibre positioning time &lt; 2 mins</td></tr><tr><td>Acquisition star limiting mag</td><td>Attached flats + 2 mins<br>V ~ 21 mag (in 30 sec exposure)</td></tr></tbody></table>

to measure stellar radial velocities, and another with R ~ 19 000 in the H band for detailed measurements of chemical abundances.

## The science drivers and the legacy value

The wealth of science that a highly multiplexed, near-infrared spectrograph like MOONS can generate is undeniably vast and it has been a common aspiration within the ESO community for a long time. MOONS will fill a crucial gap in discovery space which could never be addressed by only optical spectroscopy or low-multiplex near-infrared spectroscopy.

Within the 300 nights of Guaranteed Time Observations (GTO) obtained in return for building the instrument, the MOONS Consortium has developed a coherent set of surveys covering a large fraction of the history of the Universe, from cosmic dawn (13 billion years ago) to the present epoch, across many astrophysical fields (see Figure 1). About 100 GTO nights are devoted to Galactic surveys (see Gonzalez et al., p. 18). The aim is to investigate the nature of the heavily obscured regions of the Galactic bulge (unachievable with optical spectrographs), as well as providing new insights into the chemo-dynamical structure of the thin and thick Galactic discs, and for targeted studies of satellites and streams in the halo.

The other ~ 200 GTO nights will focus on galaxy evolution across cosmic time (see Maiolino et al., p. 24). The goal is to provide a complete picture of the integrated properties of the stellar populations and the ionised interstellar medium (ISM) of galaxies up to high redshift in a SDSS-like survey, including a large number of Lyα emitting galaxies up to z ~ 10, and use this to investigate in a systematic way the role that environment and black hole feedback have on the formation and evolution of galaxies with redshift.

The combination of MOONS GTO surveys and open-time surveys will provide an invaluable legacy. Even under the conservative assumption that MOONS is used only for 100 nights a year (i.e., sharing the

<caption>Figure 2. The central figure shows the computer-aided design (CAD) model of the MOONS instrument; the rotating front end and the spectrograph are highlighted. The other images show the real hardware being integrated. From top right and clockwise: the focal plate with four mounted fibre positioning units (FPUs) and one acquisition camera; the</caption>

structure of the rotating front end that will host the plate with the FPUs, the calibration unit and the metrology system; one of the slitlets with 16 mounted fibres; one of the 10 high efficiency VPH dispersers; one of the six Schmidt cameras; and the cryostat vessel.
telescope equally with the other 2 instru-
ments mounted on the UT), it will offer
the scientific community ~ 1000 000
fibre-hours every year. This figure will be
even more if in the future one of the UTs
is operated in survey mode. On a times-
cale of 10 years, which is the very mini-
mum lifetime of the instrument, Legacy
Surveys with MOONS will provide radial
velocities and detailed chemical abun-
dances for tens of millions of stars in our
Galaxy and beyond, as well as spectra
for millions of galaxies at 0 < z < 10, pro-
viding key spectral diagnostics and envi-
ronmental information. This will produce
a huge and unique dataset of high-quality
spectra and the essential deep spectro-
scopic follow-up of current and future
optical and near-infrared imaging surveys
or facilities (for example, Gaia, VISTA,
UKIDSS, VST, Pan-STARRS, Dark Energy
Survey, LSST, Euclid), as well as of
objects observed at other wavelengths
using, for example, ALMA, Herschel,
eRosita, LOFAR, WISE, ASKAP, Meer-
KAT, etc. Last but not least, MOONS will
offer a unique mine from which targets
will be selected for detailed follow-up
with ESO's Extremely Large Telescope
for years to come.

The MOONS instrument

MOONS is a fibre-fed spectrograph
designed to use the full 25-arcminute-
diameter field of view (FoV) of one of the
Unit Telescopes (UT) of the VLT. The
instrument consists of the three major
sub-systems shown in Figure 2: the part
that is mechanically attached to the
telescope and couples the light into the
optical fibres (called the rotating front
end); the two triple-arm spectrographs —
in which the light from the fibres is dis-
persed and recorded; and the instrument
control.

The first element in the optical path of
the instrument is the field corrector
made of two large lenses of almost 1 m in
diameter (and ~ 110 mm thick), which
provides a fully corrected field of 25 arc-
minutes in diameter; this is the largest
field possible at the VLT.

The fibres for science observations are
deployed on the focal plane created by
the field corrector using 1001 miniature

fibre positioning units (FPUs), allowing us
to configure an entire observation in less
than two minutes. Each fibre is con-
connected to its own pick-off unit, which has
a footprint of 25 mm fixed on the focal
plane and is equipped with two rotating
arms³. The combination of the two rota-
tions (like the combined motion of elbow
and shoulder) allows the fibre to patrol an
area with a diameter of 50 mm (~ 1.5 arc-
minutes on the sky), with an accuracy of
better than 20 µm (i.e., less than a third
of the diameter of a human hair), which
corresponds to 0.05 arcseconds on the
sky. The FPUs will be able to achieve this
positioning accuracy using state-of-the
art stepper motors, but in order to moni-
tor this and make any calibration adjust-
ments there is also an external metrology
system capable of precisely measuring
the position of each fibre. On the focal
plane, embedded within the FPUs, there
are 20 acquisition and secondary guiding
cameras used to acquire the science
field and do a fine alignment of the instru-
ment on the sky. The rotating front end
also hosts a novel concept of calibration
unit, which uses a projector (like those in
cinemas) to illuminate a screen coated
in a Lambertian diffuser to guarantee
high quality wavelength calibration and
flat fielding for all fibres. Indeed, to ensure
excellent sky subtraction it is critical that
the relative transmission of all the fibres is
known very accurately, to better than 1%,
and this highly homogeneous illumination
is achieved via the calibration unit.

Once the light from stars and galaxies
is collected at the front end it is then fed
through the fibres to two identical triple-
arm spectrographs enclosed in a single
cryostat vessel that keeps the optical
elements inside at a temperature of
-130 degrees C in order to reduce the
background in the near-infrared.

In each of the two spectrographs the
light from 512 fibres — arranged in
32 slitlets<sup>b</sup>, each containing 16 fibres (see
Figure 2) — is split by dichroic filters into
three wavelength ranges or channels (<i>RI</i>,
<i>YJ</i> and <i>H</i>). Each of the two MOONS
spectrographs has five highly efficient
volume phase holographic (VPH) dispers-
ers, three for the low-resolution mode
and two for the high-resolution mode.
The two triple-arm spectrographs are
mounted back-to-back on the optical

bench, which makes it possible to switch
between the high- and low-resolution
modes in the *RI* band (and similarly for
the *H* band) using a single common linear
mechanism that passes straight through
the optical bench. In each channel, the
light dispersed by the VPHs is refocused
by using the fastest large cameras ever
built for astronomy (to our knowledge).
Indeed, these Schmidt-like cameras have
a very fast *f*-number of *f/0.95*. Each
camera is also very compact and made
of just two lenses (glued one inside the
other) and one mirror to bring the image
into focus on a detector (see Figure 2),
and is therefore easy to align. Finally, the
light — which has travelled for billions of
years in some cases — will be recorded
on state-of-the-art detectors. The two
infra-red channels (*YJ* and *H*) will exploit
the new Hawaii 4RGs 15 µm-pixel detec-
tors and the optical channel (*RI*) will use
fully depleted Lawrence Berkeley National
Laboratory (LBNL) red-sensitive CCDs.

Since the very beginning of the project,
the focus (and the challenge to the engi-
neering team) has been to maximise the
quality and the throughput of the instru-
ment or, in other words, the mantra has
been "transmit as many photons as phys-
ically possible!". For this reason, all the
components described above have been
optimised in terms of design, material,
coating etc. to reach the high sensitivity
shown in Figure 3.

Observation preparation

For complex instruments and particularly
for multi-object spectrometers, the
usual ESO p2 software used to prepare
the observations is complemented by the
addition of instrument-specific detailed
configuration software. The observation
preparation software called MOONLIGHT
will perform automatic allocation of
fibres to science targets, including opti-
misation of fibre allocations and account-
ing for mechanical constraints of the
positioners. In order to have high alloca-
tion efficiency of the fibres on targets,
some overlap between neighbouring
patrol fields is needed, with one fibre
being able to patrol up to the centre of
the neighbouring cell. However, this
feature can increase the chances of colli-
sions during positioning. To avoid such
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

collisions, we have developed an algo-
rithm for the path analysis, which calcu-
lates in advance the best trajectory and
motion of each motor.

Observing strategies and sky subtraction

Accurate subtraction of the sky back-
ground is critical when observing faint
sources, particularly in the near-infrared,
where strong OH sky lines dominate the
background. To achieve this goal, we
have implemented multiple methods. First
of all, the spectral resolving power of
R > 4000 for the medium-resolution
mode (R > 6500 in the H band) ensures

that at least 60-70% of the observed
regions in the Y, J or H bands are com-
pletely free from OH airglow. Sky subtrac-
tion with fibres is challenging since their
efficiency might change (even slightly)
when they move. For this reason, particu-
lar attention has been devoted during
the manufacturing of the fibres and
their routing within the instrument to mini-
mise the variation of focal ratio degra-
dation (FRD), which has been measured
to be << 1%. In order to remove any
residuals, it is also possible to obtain a
fast attached flat after the fibres have
been reconfigured and are in their sci-
ence position. To further optimise the sky
subtraction, the fibre positioners have
been designed to have overlapping patrol

**Figure 3.** Top left: Spectral resolution *R* = *λ*/Δ*λ* as a function of wavelength for the low-resolution and high-resolution modes. In all the panels showing sensitivity, the thick solid lines show the typical value outside strong OH sky lines. At the resolution of MOONS more than 60–70% of the observed regions are completely free from OH sky lines. Top right: limiting flux for emission lines in low-resolution mode in the three simultaneous channels, for 1 hour on-source integration with S/N = 5 at the line peak. Bottom left: limiting magnitude in continuum for low-resolution mode in the three simultaneous channels, for 1 hour on-source integration with S/N = 5 per resolution element (~ 3 pixels) and dashed lines when rebinned to a resolution R = 1000 after sky subtraction. Bottom right: limiting magnitude in continuum for high-resolution mode in the three simultaneous channels, for 1 hour on-source integration with S/N = 60 per resolution element (~ 3 pixels).
<figure><img src="image_8.png" /><figcaption>**Figure 4.** The three possible MOONS observing strategies envisaged for sky subtraction.</figcaption></figure>

fields and are capable of being placed at a distance of 10 arcseconds from each other, so as to sample the sky very close to the science target.

These considerations are driving the three possible observing strategies envisaged for MOONS (see list below and Figure 4). During commissioning these strategies will be tested and the performance of sky subtraction evaluated in order to provide guidelines to users.

1. *Stare*: The vast majority of fibres will be on the targets, with dedicated sky fibres distributed across the focal plane. The number of sky fibres can be determined by the user.

2. *Stare+nod*: The majority of, if not all, fibres will be on the targets and the telescope is then nodded to a nearby sky position. This has the advantage that the sky flux will pass through the same fibre as the target, thus removing many instrumental effects. The quality of sky subtraction will depend on the frequency of sky nods.

3. *XSwitch*: This provides a pseudo-slit observation with the most accurate sky subtraction. Every science fibre will have an adjacent sky fibre at the same fixed distance ($10 < d < 30$ arcseconds) and same direction. The telescope is then nodded by the same distance and direction, so that object

and sky fibres are reversed. This observing pattern strategy allows both temporal and spatial sky variation to be removed, as well as accounting for instrumental effects (see Rodrigues et al., 2012 for more details and on-sky testing of this strategy).

<figure><img src="image_9.png" /></figure>

## The Consortium

Reflecting the wide range of science goals, the MOONS Consortium builds on the scientific and technical expertise of a range of institutes in Chile, France, Germany, Italy, Norway, Portugal, Switzerland, the United Kingdom, and ESO. It includes ~ 100 engineers and 150 scientists across ~ 50 institutes. Table 2 shows the main roles of each of the institutes involved in the construction of the instrument.

## Schedule

The MOONS project passed the Final Design Review (FDR) in 2017 and is now fully in the assembly integration and verification (AIV) phase. The vast majority of the components have been manufactured and are now being integrated in Edinburgh. The Provisional Acceptance in Europe (PAE) is foreseen for the end of 2021, followed by the installation and

commissioning at the VLT at the beginning of 2022 (see Figure 5).

## References

Rodrigues, M. et al. 2012, Proceedings of the SPIE, 8450E, 3HR

## Links

¹ The official MOONS website: www.vltmoons.org

² The MOONS website at ESO: https://www.eso.org/sci/facilities/develop/instruments/MOONS.html

³ See the fibre positioning units in action at https://vltmoons.org/resources/.

## Notes

<footnote>a For information contact Michele Cirasuolo at mciras@eso.org.</footnote>

<footnote>b A short anecdote: A very small fraction of the light coming from the fibres does not reach the detector immediately but bounces back and forth between the optical surfaces and when it reaches the detector it creates a "ghost" image! That image is adding noise and therefore degrading the science performance. When we discovered this problem, we had to think how to remove this effect. In order to do this each slitlet was equipped with a special component, tilted with respect to the optical axis, to deviate the "ghost" away from the detector. During a telecon someone said: "it works like an exorcist; the ghost has been evicted". Since then this special component is called "the Exorcist"!</footnote>
<table><caption>Table 2. Role of the consortium construction partners.</caption><thead><tr><th>Institute</th><th>Work package</th><th>Logos</th></tr></thead><tbody><tr><td>STFC UK Astronomy Technology Centre Edinburgh</td><td>Project office, fibre positioning units, calibration unit, cryostat, detector adjustment module, AIV, control software</td><td></td></tr><tr><td>Cambridge University</td><td>Camera opto-mechanics, assembly and testing</td><td></td></tr><tr><td> Eidgenössische Technische Hochschule (ETH) Zürich</td><td>Fibre positioning unit</td><td></td></tr><tr><td> INAF – Firenze</td><td>Optical design, exchange VPH mechanisms</td><td></td></tr><tr><td> INAF – Roma</td><td>Acquisition cameras end-to-end modelling</td><td></td></tr><tr><td> INAF – Milano</td><td>Observation preparation software and path analysis</td><td></td></tr><tr><td> GEPI – Paris</td><td>Fibre assembly, slit and shutters, data reduction software</td><td></td></tr><tr><td> University of Geneva</td><td>Instrument control electronics</td><td></td></tr><tr><td> Instituto de Astrofísica e Ciências do Espaço</td><td>Field corrector, rotating front end structure, cable wrap</td><td></td></tr><tr><td> Pontificia Universidad Católica de Chile</td><td>Metrology system, instrument control software</td><td></td></tr><tr><td> ESO</td><td>Detector arrays and CCDs</td><td></td></tr></tbody></table>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>Figure 5. Artist's impression of MOONS on the Nasmyth platform at the VLT.</figcaption></figure>