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Adaptive Optics Requirements, Concepts, and Performance Predictions for Extremely Large Telescopes

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. Presentation Outline. Requirements and AO operating modes

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Adaptive Optics Requirements, Concepts, and Performance Predictions for Extremely Large Telescopes

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

  2. Presentation Outline • Requirements and AO operating modes • Implementation concepts and issues • Towards performance estimates • Summary Ellerbroek/Rigaut [SW01-114] AO … for ELT’s

  3. AO Performance Objectives for GSMT (DL: Diffraction limited) 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 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s

  5. Representative Parameters for MCAO • 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 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: 500-1000 • Limiting magnitude: 15-17 • Will achieve near-diffraction limited performance for l > 5 mm Ellerbroek/Rigaut [SW01-114] AO … for ELT’s

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

  9. Guide Star Options for MCAO 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 ~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 msec pulses, duty cycle of 0.05-0.10 • Multiple lasers and launch telescopes per LGS? 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) Ellerbroek/Rigaut [SW01-114] AO … for ELT’s

  12. Collimated space for ADC’s, beamsplitters DM’s & TTM OAP’s MCAO Opto-Mechanical Layout 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 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?? Ellerbroek/Rigaut [SW01-114] AO … for ELT’s

  15. Wave Optics Simulation for Evaluation of Higher-Order Effects • Zonal • 2nd order Dynamics • Misregistration Minimal Variance • Shack-Hartmann • Geometric or Wave Optics • Gain/bias calibration • 3-D LGS • Photon + Read Noise • Misregistration Science Fields NGS’s LGS’s • Turbulence • Filtered white noise • Taylor hypothesis LGS Pointing Tip/Tilt Offload Recon- structor DM’s TTM Common- and Noncommon Path Errors LGS + NGS WFS’s Science Instrument Strehl Histories Mean PSF’s

  16. Sample Modeling Results for MCAO on Gemini-South Wave Optics Simulation Linear Systems Analysis D = 8 m, median Cerro Pachon Seeing, y = 00, 1’ square FoV, no WFS noise Solid: Extended laser beacons and wave optics propagation Dashed: WFS noise, DM misregistration 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 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 ! Ellerbroek/Rigaut [SW01-114] AO … for ELT’s

  18. Sparse Matrix Methods Reduce Computation 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 Ellerbroek/Rigaut [SW01-114] AO … for ELT’s

  19. Equivalent Performance Computation times reasonable Memory requirements at least feasible Performance a weak function of D MCAO Results (and Predictions) Using Sparse Matrix Methods Ellerbroek/Rigaut [SW01-114] AO … for ELT’s

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

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