# Presentation: 853456

## AO concepts for extremely large telescopes

- Rich Dekany
- Jet Propulsion Laboratory
- Richard.Dekany@jpl.nasa.gov

## Today’s show...

- Extremely Large Telescopes
- Historical perspective
- CELT concept
- Adaptive optics observation modes
- Seeing limited
- Single conjugate
- Multiconjugate
- Extreme
- CELT configuration options

## Lessons of History

- Plot of largest optical/IR telescope size vs. time reveals exponential growth
- Remarkable given various social, economic, and technical factors
- Extrapolating from Keck 10 m:
- 10 m	1993
- 25 m	2034
- 50 m	2065
- 100 m	2097
- History does not explain how future gains will be made

**Log10 collecting area (meters****2****)**

- Courtesy J. Nelson

## Large telescope projects 1950-2020

- Hale
- Keck1
- Keck2
- MMT
- HET
- Gemini (x2)
- VLT (x4)
- Magellan
- ....others
- LBT (x2)
- GTC
- CELT

- HST
- SIRTF
- NGST

- 1949
- 1990
- 1995
- 2000
- 2005
- 2010
- 2015
- 2020

## California Extremely Large Telescope (CELT)

- Features
- 30 meter class optical / IR observatory
- Filled-aperture, fully pointable
- Advanced adaptive optics
- State-of-the-art instrument complement
- Cost ~ TBD
- Completion date ~ TBD

- Baseline design
- Two-mirror Ritchey-Chretien
- F/1.5 primary mirror
- ~1080 hexagonal segments
- 0.9 m (shortest edge) segment diameter
- Segments organized onto rafts for handling
- Non-interlocking edge sensors
- Final F/# ~ 15 (may shorten)
- Nasmyth foci only

## CELT Science

## NGST

- NASA mission currently in the planning/technology development stage
- 8-meter class aperture
- Diffraction-limited at 2 microns wavelength
- Optimized for 1-5 microns, but zodi limited to 10 microns in the currently passively cooled model
- Gold-coated optics + detectors will mean short wavelength cutoff of ~ 0.6 microns
- FOV of “wide-field” instruments 3-4 arcminutes
- 5-year lifetime in L2 orbit (no servicing possible)
- Launch in ~2008 time frame
- Total cost, including technology is ~$2B
- Instruments
- TBD, but includes 1-5 micron imager and spectrograph
- Resolutions to R~3000, but optimized for low-dispersion

## CELT modes of observation

- Seeing-limited (SL)
- Still requires wavefront information from ‘guider’ star
- Likely to be 10-20 Zernike modes or equivalent
- Single conjugate AO (SCAO)
- Sufficient for observations longward of  = 3.5 m
- Emphasizes low emissivity, possibly polarimetry (dust)
- Multiconjugate AO (MCAO)
- Wide FOV correction
- ~ 2 arcmin diameter optimized for  = 1-3.5 m
- Extreme AO (EAO)
- NGS-based system targeting highest-contrast science
- Nearby extrasolar companion searches and study

- _[Observing with CELT by band](file:///celt_obs_by_band.ppt)__ _

## Some CELT killer apps...

## High SNR, high imaging, and spectral resolution
Accretion environments
Study disks to centrifugal radius (Rc) ~ 10 AU at distances up to 1 kpc
R ~ 105 to obtain velocities to 3 km/s to estimate mass accretion rate
Measure infall rate as function of position and angular momentum
Cepheids out to redshift z ~ 0.1 
Measure H0 and omega matter
Age determination of giants (measure t0)
Measure age of stars via thorium decay in giants below RGB tip
Requires R ~ 3 x 105 and S/N ~ 1000.
Geometry of the Universe via SNe at z ~ 3 (measure q0)
Break degeneracy of omega matter and omega lamba
Spectroscopy of  z = 0.5 - 5 galaxies
Measure kinematics and chemistry
Planetary science
KBO searches
~ 20 km resolution imaging of Jupiter’s atmosphere
Comparable to Galileo images
Many others...

- Accretion environments
- Study disks to centrifugal radius (Rc) ~ 10 AU at distances up to 1 kpc
- R ~ 105 to obtain velocities to 3 km/s to estimate mass accretion rate
- Measure infall rate as function of position and angular momentum
- Cepheids out to redshift z ~ 0.1
- Measure H0 and omega matter
- Age determination of giants (measure t0)
- Measure age of stars via thorium decay in giants below RGB tip
- Requires R ~ 3 x 105 and S/N ~ 1000.
- Geometry of the Universe via SNe at z ~ 3 (measure q0)
- Break degeneracy of omega matter and omega lamba
- Spectroscopy of  z = 0.5 - 5 galaxies
- Measure kinematics and chemistry
- Planetary science
- KBO searches
- ~ 20 km resolution imaging of Jupiter’s atmosphere
- Comparable to Galileo images
- Many others...

## Courtesy M. Mountain

## IR exosolar planet studies with CELT

- CELT 5 m point-source sensitivity: 	0.6 x 10-6 Jy
- 5, 1 hour, no IR optimization
- Includes ‘reality factor’ of 25 seems with Keck LWS
- Predicted 5 m flux from 1 Gyr old Jupiter: 	3 x 10-6 Jy
- Planet self-luminous (around low-mass star), at 10 pc
- 200 stars with 10 pc, mostly K and M dwarfs
- 5 m flux from M2 star: 0.4 Jy
- PALAO demonstrating 1000x contrast at 40% Strehl at 1” (2.2 m)
- Extrapolation to CELT at 5 m = Star flux ~ 3 x 10-6 Jy
- Fluxes are very model-dependent
- Up to 100 times lower for 5 Gyr old planet
- Up to 106 times higher than blackbody, depending on wavelength
- Jovian frequency within 1 AU around G stars ~ 5% (Marcy & Butler, PASP, 112, 137) = 10-20 systems of interest

## CELT Telescope

## Scaling laws

- Cost
- Recent history:  Cost ~ D2.3
- Extrapolating Keck costs = 30 m cost ~$1.2B
- Need innovation to get break this ‘law’
- Define the scale size of interest as S
- Deflections due to self-weight,  ~ S2
- Deflection due to fixed load/m2, such as thin segments, ~ S1
- Angular change due to self-weight ~  /L ~ S1
- Angular change due to fixed load/m2 ~ S0
- Mass ~ S3 (extremely fast, so often ignored, use m ~ S3/2)
- Keck moving mass: 250 tons
- GBT (100m radio dish) mass: 8000 tons (law predicts 250,000 tons)
- Stiffness of structure, k ~ F/  ~ mG/  ~ S3/2/S2 ~ S-1/2
- Resonant frequency ~ -1/2 ~ S-1
- Wind loading
- Excitation frequency characteristic of eddy size ~ vwind/S ~ S-1
- Same scaling as the structure resonance frequency
- Deflections due to wind ~ F/k ~ S2/S-1/2 ~ S5/2

## Large telescopecost scaling laws

## Recent large telescope costs

- Item			Keck1($Y88)	Keck2($Y95)	HET
- 	Dome development		6.7		8.6		1.2
- 	Drive and control system	2.8		2.1		1.3
- 	Mirror support system	15.2		4.1		(optics)
- 	Observatory development	9.8		10.3		2.4
- 	Project managementand engineering		7.7		8.1		1.1
- 	SSC/support		1.7		0.4		0.2?
- 	Support Facilities		5.6		6.5		0.0?
- 	Telescope structure		9.2		10.8		1.2
- 	Operations Testing		0.0		5.5		2.0
- 	Total			93.7		77.7		16.5

## CELT Primary Mirror

## CELT primary mirror

**Fun Fact!  **
**Approximately the same as ocular collecting area of the population of the City of Los Angeles**
- Baseline
- Segment thickness ~ 45 mm
- Keck segment thickness is 75 mm
- Non-locking capacitive edge sensors
- Keck sensors interlock
- Segments grouped into rafts

- Equivalent collecting area to all major existing observatories combined

## CELT

- Keck

## Optimal Segment Size (Nelson, Chanan)

- Factors favoring many small segments:
- 	•  Lower mass, easier to support
-  	•  Lower material costs
- 	•  Easier to fabricate:
- Factors favoring fewer large segments:
- 	•  Smaller error propagation (sensors   actuators/wavefront)
- 		surface = 0.74 (Nseg)1/2  sensor
- 			4.4 for Nseg = 36
- 			15. for Nseg = 1098
- 	•  Easier to phase
- 		no. edges  Nseg =

-     84 	for Nseg = 36
- 3168 	for Nseg = 1098

- C22	= astigmatic coefficient
- a	= hexagon side length
- R	= off axis distance
- k	= primary radius of curvature

## Keck Stressed Mirror Polishing Set-up

## title

- Proposed CELT Stressed Mirror Polishing Set-up

## Keck Sensor Geometry

- Mirror Segment

- 7.5 cm

- Sensor Paddle

- Sensor Body

- Sensor Mount

- Conducting Surfaces

- 2 mm

- L

- R = 35 m

## title

- Proposed CELT Sensor Geometry

- Non-Interlocking Sensors

## CELT Adaptive Optics

## CELT modes of observation

- Seeing-limited (SL)
- Still requires wavefront information from ‘guider’ star
- Likely to be 10-20 Zernike modes or equivalent
- Single conjugate AO (SCAO)
- Sufficient for observations longward of  = 3.5 m
- Emphasizes low emissivity, possibly polarimetry (dust)
- Multiconjugate AO (MCAO)
- Wide FOV correction
- ~ 2 arcmin diameter optimized for  = 1-3.5 m
- Extreme AO (EAO)
- NGS-based system targeting highest-contrast science
- Nearby extrasolar companion searches and study

- _[Observing with CELT by band](file:///celt_obs_by_band.ppt)__ _

## Optical design issues for CELT AO

- NGS SL mode
- Minimize number of surfaces
- NGS SCAO
- Maintain large science field of view
- Minimize emissivity
- LGS MCAO
- Provide for multiple conjugation locations
- Get relatively large field through a complex relay
- LGS elongation
- LGS fratercide
- Requires multiple NGS, as well
- NGS EAO
- Minimize scattered light (high spatial frequency errors)

## Seeing-limited (SL) observing mode

- Even SL mode requires ‘aggressive’ active optics
- Primary focus mode
- Primary sensor error propagation
- Wind-induced vibrations in structure
- Regular telescope ‘guider’ replaced with low-order wavefront sensor

## Slide 29

## Single conjugate (SCAO) observing mode

- Natural guide star system
- Full sky coverage
- Based upon relatively loose wavefront error budget, which allows for large subaperture wavefront sensing and large isoplanatic angle
- Diffraction limited at ~ 3.5 m or longer
- Requires ~ 500-1000 actuators
- Single adaptive element – potentially an adaptive secondary
- Would prefer to minimize emmissivity, but resolution is the major science driver (NGST wins in sensitivity)

## Single conjugate (SCAO) observing mode

## SCAO issues

- Even for single-conjugate AO, many hard problems exist
- Segmented primary mirror
- Segment vibrations
- ACS error propagation of low order primary mirror modes
- Need more information than usual from ‘guider’ camera (now a wavefront sensor)
- Deformable mirrors
- Adaptive secondary mirrors may be desirable
- Minimizes emissivity for IR observations
- Requires technology development
- Number of actuators, size, stroke, linearity, hysteresis, coatings,...
- Control
- Incorporation of NGS data into control laws
- Computation cost
- Very large number of actuators require new algorithms
- Systematics
- Small diffraction width accentuates previously acceptable error

## Multiconjugate (MCAO) observing mode

- Natural guides stars do not provide sufficient sky coverage at near-IR wavelengths (1-2.5 m)
- Artificial beacons are necessary (ref. Ed Kibblewhite’s talk from Thursday)
- Focal anisoplanatism (‘cone error’)
- MS wavefront error ~ (D/d0)5/3
- 	where d0 is linear function of beacon height and scales ~ 6/5
- Mauna Kea model atmosphere
- d0, assuming 45 degree zenith angle,
- d0(10 km, MK, 45 zen) = 1.08m
- d0(92 km, MK, 45 zen) = 3.60m
- 	which leads to, for 1 mm observing wavelength, D = 30 m,
- 	FA(10km) = 2540 nm
- 	FA(92km) = 931 nm
- 	which are both obviously unacceptable wavefront errors.

## MCAO issues I

- Need multiple laser guide stars
- Due to focal anisoplanatism
- Required even for ‘narrow field’ AO (i.e. single isoplanatic angle)
- All guide stars would like to operate in ‘closed loop’ (i.e. enjoy AO correction)
- How much useful information, in any, is available from open-loop guide star information?
- Need multiple natural guide stars
- Tilt indeterminism of individual beacons leads to unsensed modes of turbulence
- Major contribution from focus and astigmatic atmospheric modes
- Ellerbroek: 3 NGS are sufficient for 5 LGS
- Research area: How does one exploit whatever wavefront information supplied by fortuitous NGS’s

## MCAO issues II

- Laser beacon elongation
- Sodium layer has finite thickness (typ. ~10 km)
- For Shack-Hartmann sensor, each subaperture sees elongated beacon
- Beam elongation ~ (beacon length * off-axis distance)/(height2)
- Possible solutions?
- Gating of wavefront sensor camera (natural for Rayleigh beacons)
- Membrane mirror for adaptive focus during pulse flight
- Elongated beacons still give good centroid estimates in one direction
- Is there really a problem?
- Laser beacon fratercide
- Multiple laser beacons tend to obscure each other with the Rayleigh scatter tail
- Worst effect for subapertures near beacon launch points
- Beacon launch from behind secondary always a problem

## MCAO issues III

- Many reflections
- Renewed emphasis on coatings
- Minimize emissivity and scattering
- Requires new calibration techniques
- Multiple wavefront sensors much each now be calibrated
- Above and beyond problem of lower acceptable rms wavefront error
- Complex software
- Multiple guide star acquisition

## Three MCAO geometries

- Offner relay
- Compact triad
- Crossed MCAO

## Offner Relay

- 1:1 relay
- Three concentric spherical surfaces
- Corrected field is determined by physical size of relay
- In other words, the corrected field (in mm) of a 2 meter Offner is twice the dimension of a 1 meter Offner
- Located beyond Nasmyth focus of an ELT, first two reflections of Offner occur at interesting conjugates
- Example:
- ELT secondary is first ‘conjugate’
- First two reflections of Offner conjugate to 2 km and 8 km altitude
- Requires large, powered adaptive elements
- Exploit adaptive secondary technologies?

## CELT AO - 2amin Offner relayConjugates at “0”km, 2km, 8km

## CELT - MCAO Offner relay (detail)

## CELT - MCAO Double Offner relay(Conjugates at “0”, 2, 5, 8, 12 km)

## Pupil magnification

- Internal CELT pupils are limited by FOV
- Practical angles at any (current) adaptive device about 20 degrees incidence
- For isoplanatic angle of 20 asec, limit of pupil magnification ~ 1/3600
- 8.33 mm pupil diameter
- Actuator spacing of 130 m for 30m telescope (for 4,000 Nact system)
- Selecting standard pupil diameter of 100 mm
- 20 asec FOV becomes < 2 degrees
- 1mm AO technology is initially usable (Nact ~ 8,000)
- 200 m MEMs development increases Nact to ~ 200,000

## Courtesy A. Meinel

## Courtesy A. Meinel

## Courtesy A. Meinel

## Extreme (EAO) observing mode

- Visible, high contrast observations
- Requires 10,000 – 100,000 phase actuators
- Will likely require amplitude correction
- Requires very high photon fluxes for sufficient SNR per subaperture
- Laser power required scales as -18/5
- Hard
- Return to NGS only system, at least initially
- Used for nearby stars only
- Limitations to exosolar planet detection may be imposed by primary mirror
- Diffraction from segment gaps
- Segment vibrations

## No-laser atmospheric tomography (Raggozoni, Gilmozzi)

- Large aperture telescopes sample an enormous volume of the Earth’s atmosphere
- Multiple NGS’s could be used to determine 3-D structure of turbulence
- Multiple (perhaps fewer) DMs could correct a significant field
- For example, for 30m CELT, beam shear of 15m (arbitrary), yields 5 arcmin radius
- Requires N guide stars
- However, corrected field grows as D2
- Finding 3 NGS’s of 13th mag within 5 arcmin  (30m CELT) reasonable (P ~ 50%)
- Finding 5 within 8.5 amin (50m CELT) likely (P ~ 90%)
- Exploit all useful NGS’s within technical field with multiple pyramids in Foucault-like wavefront sensor (ref. Gary Chanan’s talk from Monday)

## CELT Structure

## CELT – K structural schematic

- Courtesy S. Medwadowski

## CELT – MWD(millimetre dish) Concept

## CELT MWD – Servicing HiCass

## CELT MWD concept

- Concept by A. Meinel, D. Woody, and R. Dekany
- Model by A. Meinel

## CELT MWD - Inboard Nasymth

## CELT MWD - Outboard Nasmyth

## CELT MWD - Servicing Secondaries

## ELT AO Technology Roadmap