Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 1 Adaptive Optics Requirements, Concepts, and Performance Predictions for Extremely Large Telescopes Brent Ellerbroek and Francois Rigaut Gemini Observatory SPIE Opto-Southwest / Astronomical Optics September 17, 2001

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 2 Presentation Outline Requirements and AO operating modes Implementation concepts and issues Towards performance estimates Summary

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 3 AO Performance Objectives for GSMT Narrow field AO MCAOLow order AO Field-of-view10”2’ Wavelength range,  m Resolution, arc secDL Strehl, %80 (90)50- Emissivity, %20 10 Stability over 3600 s, %152 Astrometric accuracy, arc sec1e-41e-31e-2 Photometric accuracy, %152 (DL: Diffraction limited)

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 4 Representative Parameters for 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 # Actuators Servo BW, Hz80125 FoV diameter, arc sec NGS magnitude (Hartmann WFS) Implementation errors, nm3423

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 5 Gemini-South design scalable to D = 30 m … … with increased laser power –Increase between O(D) and O(D 2 ) Better correction over wider FoV implies more DM’s, WFS’s –Scaling not yet fully quantified Representative Parameters for MCAO,  m Target Strehl Target FoV, arc sec 12 # of DM’s 35-9? Actuator pitch, m # of sodium LGS or NGS 59? Power/LGS, W 30?120? Servo Bandwidth, Hz 40 Implementation errors, nm 9050

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 6 Representative Parameters for Low 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: –Limiting magnitude: –Will achieve near-diffraction limited performance for > 5  m

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 7 AO Imple- mentation concept for GSMT MCAO Inst. LLT LLT? Inst. Adaptive M2 (LOAO) LOAO MOMFOS Elevation Axis M1 Support Prime Focus Inst. NFAO

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 8 Ideas 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

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 9 Guide Star Options for MCAO ProsCons 1) NaturalNo lasers!Questionable sky coverage (at desired Strehls w/o a revolutionary WFS) 2) Rayleigh1.Mature laser tech. 2.Safe for satellites 3.Fixed LGS range 1.Performance questions 2.Many guide stars required 3.Optical design questions 3) Sodium1.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

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 10 Coping 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 ~3 2 for a Hartmann WFS with 2 2 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 –Multiple lasers and launch telescopes per LGS?

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 11 MCAO 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)

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 12 MCAO Opto-Mechanical Layout OAP’s Collimated space for ADC’s, beamsplitters DM’s & TTM

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 13 Issues 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

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 14 AO Modeling Issues and Methods First-order system sizing and performance analysis –Number/geometry of guidestars, DM’s, WFS –Performance vs WFS noise, control loop bandwidth  Classical 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 Science Instrument LGS + NGS WFS’s Turbulence - Filtered white noise - Taylor hypothesis Science Fields LGS’s NGS’s LGS Pointing Tip/Tilt Offload DM’s TTM Recon- structor Common- and Noncommon Path Errors Strehl Histories Mean PSF’s Wave Optics Simulation for Evaluation of Higher- Order Effects Shack- Hartmann Geometric or Wave Optics Gain/bias calibration 3-D LGS Photon + Read Noise Misregistration Zonal 2 nd order Dynamics Misregistration Minimal Variance

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 16 Sample Modeling Results for MCAO on Gemini-South Strehl Ratio Center FoV Edge FoV Corner FoV J band H band K band D = 8 m, median Cerro Pachon Seeing,  = 0 0, 1’ square FoV, no WFS noise Linear Systems Analysis Wave Optics Simulation Solid: Extended laser beacons and wave optics propagation Dashed: WFS noise, DM misregistration

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 17 Computation Requirements vs. Aperture Diameter Scaling with diameter D for a fixed density of DM actuators and WFS subapertures Optical propagationO(1) (assumes fixed phase screen size of at least twice outer scale) Compute WFS measurementsO(D 2 ) Compute DM actuator commandsO(D 4 ) Compute DM surface figures from commandsO(D 2 )  Compute estimation matrix before simulationO(D 6 ) Conventional matrix computation Factor of 4096 in scaling from D = 8m to D = 32m !

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 18 Sparse Matrix Methods Reduce Computation Requirements First studied in for NGS AO with classical least squares estimators –Scaling with aperture diameter ~O(D 3 ), not O(D 6 ) 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

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 19 Conventional NGS AO Results Using Sparse Matrix Methods Aperture Diameter, m Order Actuators RMS WFE, (Optimal Estimator) RMS WFE, (Sparse Estimator) Time to compute sparse Estimator, s RAM to store sparse Estimator, MB NA19.3 Appears scalable to very high order NFAO

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 20 MCAO Results (and Predictions) Using Sparse Matrix Methods NGS MCAOLGS MCAO Aperture, m Actuators (9700) (9700) Subapertures (17120) (17124) Optimal RMS WFE,  m Time, hours0.51(32)(2103)0.51(32)(2103) RAM, MB13.7(176)(2827)13.8(177)(2848) Sparse RMS WFE,  m Time, hours (19.61) (13.44) RAM, MB (4128) (3189) Equivalent Performance Performance a weak function of D Computation times reasonable Memory requirements at least feasible

Gemini AO Program SPIE Opto-Southwest September 17, 2001 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s 21 Summary 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