gemini ao program spie opto-southwest september 17, 2001 ellerbroek/rigaut [sw01-114] ao … for...

SPIE Opto-SouthwestSeptember 17, 2001

Ellerbroek/Rigaut [SW01-114]AO … for ELT’s

1

Gemini AO Program

Adaptive Optics Requirements, Adaptive Optics Requirements, Concepts, and Performance Predictions Concepts, and Performance Predictions

for Extremely Large Telescopesfor Extremely Large Telescopes

Brent Ellerbroek and Francois RigautBrent Ellerbroek and Francois RigautGemini ObservatoryGemini Observatory

SPIE Opto-Southwest / Astronomical OpticsSPIE Opto-Southwest / Astronomical OpticsSeptember 17, 2001September 17, 2001



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Gemini AO Program

Presentation OutlinePresentation Outline

• Requirements and AO operating modes• Implementation concepts and issues• Towards performance estimates• Summary



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Gemini AO Program

AO Performance Objectives for GSMTAO Performance Objectives for GSMT

Narrow field AO

MCAO Low order AO

Field-of-view 10” 2’ 2’

Wavelength range, m 1.0-2.5 1.0-2.5 1.0-20

Resolution, arc sec DL DL 0.1-0.2

Strehl, % 80 (90) 50 -

Emissivity, % 20 20 10

Stability over 3600 s, % 1 5 2

Astrometric accuracy, arc sec 1e-4 1e-3 1e-2

Photometric accuracy, % 1 5 2

(DL: Diffraction limited)



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Gemini AO Program

Representative Parameters for Representative Parameters for Narrow Field AO (NFAO)Narrow Field AO (NFAO)

• Standard scaling law analysis with nominal Cerro Pachon atmosphere

• Strehl budget divided equally into: fitting, servo lag, anisoplanatism, WFS noise, implementation errors

Target Strehl at m 0.8 0.9

# Actuators 59600 147000

Servo BW, Hz 80 125

FoV diameter, arc sec 1.75 1.1

NGS magnitude (Hartmann WFS) 9.3 8.0

Implementation errors, nm 34 23



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Gemini AO Program

• Gemini-South design scalable to D = 30 m …

• … with increased laser power– Increase between

O(D) and O(D2)

• Better correction over wider FoV implies more DM’s, WFS’s– Scaling not yet fully

quantified

Representative Parameters for Representative Parameters for MCAOMCAO

, m 1.65 1.0

Target Strehl 0.4 0.5

Target FoV, arc sec 1 2

# of DM’s 3 5-9?

Actuator pitch, m 0.5 0.25

# of sodium LGS or NGS 5 9?

Power/LGS, W 30? 120?

Servo Bandwidth, Hz 40 40

Implementation errors, nm 90 50



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Gemini AO Program

Representative Parameters for Low Representative Parameters for Low Order AO (LOAO)Order AO (LOAO)

• Standard Strehl scaling laws break down for low-order systems

• Initial parameter estimates can be derived from Hokupa’a performance on Gemini-North– Number of actuators: 500-1000

– Limiting magnitude: 15-17

– Will achieve near-diffraction limited performance for > 5 m



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Gemini AO Program

AO Imple-AO Imple-mentation mentation concept concept for GSMTfor GSMT

MCAO

Inst.

LLT

LLT? LLT?

Inst.

Adaptive M2 (LOAO)

LOAO

MOMFOS

ElevationAxis

M1

M1Support

PrimeFocus

Inst. NFAO



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Gemini AO Program

Ideas on NFAO ImplementationIdeas on NFAO Implementation

• NGS AO on bright guidestars

• Woofer/tweeter wavefront correction– Adaptive M2 (low order, large stroke)

– Fast tip/tilt mirror

– MEMS or other high-order DM (limited stoke)

• Shack-Hartmann wavefront sensor or point diffraction interferometer (PDI)– PDI would eliminate reconstructor and reduce detector count, but has

limited dynamic range

– Two-stage wavefront sensing possible

• Atmospheric dispersion compensation requirements and implementation TBD



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Gemini AO Program

Guide Star Options for MCAOGuide Star Options for MCAO

Pros Cons

1) Natural No lasers! Questionable sky coverage (at desired Strehls w/o a revolutionary WFS)

2) Rayleigh 1. Mature laser tech.

2. Safe for satellites

3. Fixed LGS range

1. Performance questions

2. Many guide stars required

3. Optical design questions

3) Sodium 1. Performance predictions

2. Few (5) guidestars required

1. LGS elongation

2. Immature laser tech.

3. Variable LGS range

4. Satellite predictive avoidance

4) Sodium + low-order Rayleigh

Enhanced sky coverage (tilt aniso. compensation with a single tip/tilt NGS)

2) 3 and 3) 1-4



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Gemini AO Program

Coping with LGS ElongationCoping with LGS Elongation

• LGS Elongation proportional to aperture radius– Increases WFS spot size by factor of ~3 for D = 30 m

• Increases laser power requirements– Factor of ~32 for a Hartmann WFS with 22 pixels per

subaperture– Factor of ~3 for a Shack-Hartmann WFS with many pixels

per subaperture• Increased sensitivity to sodium column fluctuations• Alternate approaches:

– Track short laser pulses through sodium layer?• Need ~ 1 sec pulses, duty cycle of 0.05-0.10

– Multiple lasers and launch telescopes per LGS?



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Gemini AO Program

MCAO Optical DesignMCAO Optical Design

• Derived from Gemini-South– Collimated space for DM’s, ADC’s, beamsplitters

between off-axis parabolas (OAP’s)

• Design constrained by magnification ratio tradeoffs– Low magnification permits near-normal angles of

incidence on DM’s, but …

• Requires large diameters for TTM, ADC

• Increases overall size, complicates packaging

– Current design uses M=60 (0.5 m beam diameters)



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Gemini AO Program

MCAO Opto-Mechanical LayoutMCAO Opto-Mechanical Layout

OAP’s

Collimated space forADC’s, beamsplitters

DM’s &TTM



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Gemini AO Program

Issues for Further WorkIssues for Further Work

• LGS WFS optical path– Noncommon path aberrations and pupil distortion with

variable LGS range

• Tip/tilt mirror requirements• Explore alternative approaches with reduced surface

count



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Gemini AO Program

AO Modeling Issues and MethodsAO Modeling Issues and Methods

• First-order system sizing and performance analysis– Number/geometry of guidestars, DM’s, WFS

– Performance vs WFS noise, control loop bandwidthClassical linear systems models

• Higher order effects and implementation errors– Wave optics propagation (atmosphere, optical train, WFS)

– Extended (three-dimensional) laser guide stars

– DM-to-WFS misregistration, noncommon path errors,…Wave optics propagation simulations

• Computation requirements for ELT’s??

Gemini AO Program

ScienceInstrument

LGS + NGSWFS’s

Turbulence- Filtered white noise- Taylor hypothesis

Science FieldsLGS’s

NGS’s

LGSPointing

Tip/TiltOffload

DM’sTTM

Recon-structor

Common- andNoncommonPath Errors

Strehl HistoriesMean PSF’s

Wave Wave Optics Optics Simulation Simulation for for Evaluation Evaluation of Higher-of Higher-Order Order EffectsEffects

• Shack-Hartmann• Geometric or Wave Optics• Gain/bias calibration• 3-D LGS• Photon + Read Noise• Misregistration

• Zonal• 2nd order Dynamics• Misregistration

Minimal Variance



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Gemini AO Program

Sample Modeling Results for MCAO Sample Modeling Results for MCAO on Gemini-Southon Gemini-South

Strehl Ratio

Center FoV

Edge FoV

Corner FoV

J band 0.590 0.478 0.475

H band 0.739 0.653 0.649

K band 0.843 0.787 0.783D = 8 m, median Cerro Pachon Seeing, = 00, 1’ square FoV, no WFS noise

Linear Systems Analysis Wave Optics Simulation

Solid: Extended laser beacons and wave optics propagationDashed: WFS noise, DM misregistration



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Gemini AO Program

Computation Requirements vs. Computation Requirements vs. Aperture DiameterAperture Diameter

• Scaling with diameter D for a fixed density of DM actuators and WFS subaperturesOptical propagation O(1)

(assumes fixed phase screen size of at least twice outer scale)

Compute WFS measurements O(D2)

Compute DM actuator commands O(D4)

Compute DM surface figures from commands O(D2)Compute estimation matrix before simulation O(D6)

Conventional matrix computation

Factor of 4096 in scaling from D = 8m to D = 32m !



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Gemini AO Program

Sparse Matrix Methods Reduce Sparse Matrix Methods Reduce Computation RequirementsComputation Requirements

• First studied in 1983-84 for NGS AO with classical least squares estimators– Scaling with aperture diameter ~O(D3), not O(D6)

• Features of more elaborate AO configurations and estimators require special treatment– Turbulence-related matrices appearing in optimal estimators

– LGS tip/tilt removal and NGS tip/tilt measurements

• Method to include these terms recently developed– Computing LOAO / NFAO estimators for GSMT almost trivial

– Computing MCAO estimators at least feasible



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Gemini AO Program

MCAO Results (MCAO Results (and Predictionsand Predictions) ) Using Sparse Matrix MethodsUsing Sparse Matrix Methods

NGS MCAO LGS MCAO

Aperture, m 8 16 32 8 16 32

Actuators 789 2417 (9700) 789 2417 (9700)

Subapertures 1020 4280 (17120) 1024 4284 (17124)

Op

timal

RMS WFE, m 0.138 - - 0.149 - -

Time, hours 0.51 (32) (2103) 0.51 (32) (2103)

RAM, MB 13.7 (176) (2827) 13.8 (177) (2848)

Sparse

RMS WFE, m 0.139 0.137 - 0.148 0.149 -

Time, hours 0.19 1.92 (19.61) 0.21 1.68 (13.44)

RAM, MB 111.9 679.7 (4128) 98.5 560.5 (3189)

Equivalent Performance

Performance a weak function of D

Computation times reasonableMemory requirements at least feasible



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Gemini AO Program

SummarySummary

• Three AO operating modes defined for GSMT– NFAO with a bright NGS (High Strehls in NIR, 1-10” field)

– MCAO using multiple sodium LGS (Good Strehls in NIR, 1-2’ field)

– LOAO using a dim NGS (Improved resolution in near- and thermal IR, 2’ field, low emissivity)

• Top-level implementation approach defined• Work on MCAO and NFAO designs in progress• Estimators can be efficiently computed/evaluated using

sparse matrix methods

gemini ao program spie opto-southwest september 17, 2001 ellerbroek/rigaut [sw01-114] ao … for...

Documents

ao performance objectives

nfao implementation

ellerbroekrigaut sw01

ao imple mentation concept

nfao slide

diffraction limited

nm3423 slide

geminisouth design scalable