AO4ELT, Paris 2009

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A Split LGS/NGS Atmospheric Tomography for MCAO and MOAO on ELTs

Luc Gilles and Brent Ellerbroek

Thirty Meter Telescope Observatory Corp.

AO4ELT ConferenceParis, June 22-26, 2009

AO4ELT, Paris 2009

Presentation Outline

Standard (integrated) tomography architecture for LGS MCAO and MOAO– Formulation– Benefits and practical considerations

A split tomography architecture for LGS MCAO– Formulation – Benefits and practical considerations

A Strehl optimal split tomography architecture for LGS MCAO and MOAO – Formulation– Benefits and practical considerations

Comparative Monte Carlo simulation results for NFIRAOS

AO4ELT, Paris 2009

LGS MCAO and MOAO on ELTs

Under consideration/development for the E-ELT, GMT, and TMTDemonstrators: MAD, Canopus, CANARYTypical wavefront sensing requirements:– ~6-9 sodium LGSs for atmospheric tomography– ~3 low-order NGS WFSs

Sense tip/tilt and tilt anisoplanatism errorsSense focus errors due to sodium layer range variations

Standard approach to tomographic wavefront reconstruction:– Minimal variance estimation + least squares DM fitting– LGS and NGS measurements concatenated into a single vector– “Pseudo open-loop” measurements used

AO4ELT, Paris 2009

Standard (Integrated) Control Architecturefor LGS MCAO and MOAO

“Open-loop”LGS Gradients

MinimalVarianceAtmosTomo

DMfitting

Modal Projection

andServo

filtering

Tip/Tilt and Diff. Focus

removal

DM/TTcommands

“Open-loop”NGS Gradients

(~12)

Low PassFiltering

Concatenate

AO4ELT, Paris 2009

Benefits and Practical Considerations

Strehl optimal in the limit of accurate tomographic solutionApplicable to both MCAO and MOAONGS and LGS WFS measurements are very different:– NGSs are typically faint, and measurements require pre-filtering

to optimize servo compensationRequires efficient joint estimation of both low- and high-order atmospheric modes – Impacts tomography algorithm (choice of “solver,” number of

iterations, memory …)Tomography step mixes LGS and NGS WFS operators– Impacts practical implementation of ray-tracing

Split LGS/NGS architecture preferred

AO4ELT, Paris 2009

A Split Tomography Control Architecture for LGS MCAO

“Open-loop”LGS Gradients

MinimalVariance

LGSAtmosTomo

DMfitting

Modal Projection

andServo

filtering

Tip/Tilt and Diff. Focus

removalLGS DM

commands

Closed-loopNGS Gradients

(~12)

Least-SquaresRank-5 Modal

Reconstruction

Servo filtering

NGS DM commandsTT commands

NGS-controlled modes are invisible to tip/tilt-removed LGS WFSs– Consist of Tip/Tilt and 3 “cancelling” quadratic modes on 2 DMs

AO4ELT, Paris 2009

Benefits and Practical Considerations

Tomography step contains only LGS operators– Relaxes computational requirements

Separate NGS servo compensation in 5 modesNGS reconstruction and servo compensation easy to update for each new NGS asterismSimple NGS reconstruction/control model requires good LGS correction to minimize aliasing of LGS DM commands into NGS loop (may impact sky coverage)Applicable to MCAO, inappropriate for MOAO (oversimplified definition/control of NGS modes)

AO4ELT, Paris 2009

A Strehl Optimal Split Control Architecture for LGS MCAO and MOAO

Concept derived from standard (integrated) tomography by application of the Sherman-Morrison matrix inversion formula Analytically equivalent to integrated tomography in the limit of an exact tomography matrix system solutionNGS modes dependent upon NGS asterism (location and magnitudes) and seeing– Must be pre-computed accurately and updated at ~0.1 Hz– A practical approach has been defined

AO4ELT, Paris 2009

Benefits and Practical Considerations

Robust to LGS/NGS loop cross-couplingPractical, similar to previous split MCAO control architectureStrehl optimal in the limit of accurate LGS tomographic solution Applicable to both MCAO and MOAO Cost of NGS reconstruction dominated by background computation of NGS modes

AO4ELT, Paris 2009

Sample NGS Mode Distortion Patterns for NFIRAOS

Comparative Performance Evaluation for NFIRAOS in the high SNR Regime

4 NGS asterisms of 16th magnitude Common 800 Hz sampling of LGS and NGS loopsSimulated NGS WFSs: Z-Tilt NGS reconstruction: Z-Tilt WFS modelSimulated LGS WFSs: physical optics with short-exposure matched filtersTomography algorithms: CG30 and FD3 (split, new split, integ)

AO4ELT, Paris 2009

Sample Split Tomography Performance

Median seeingIncludes 116 nm RMS in quadrature of implementation errors15 arcsec FoV averaged WFE2400 frames averaged; single turbulence realization

AO4ELT, Paris 2009

Sample Comparative Performance

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Summary and Plans

A split wave-front control architecture has been introduced for LGS MCAO– NGS reconstruction and servo compensation easy to update for

each new NGS asterism– Requires good LGS correction to limit aliasing into NGS loop– Applicable to MCAO, unsuitable for MOAO

A Strehl optimal split control architecture has been developed for LGS MCAO and MOAO– Practical, similar to previous split LGS MCAO architecture– Applicable to both MCAO and MOAO– 35-60 nm RMS improvement for sample asterisms in the high

SNR regime for NFIRAOSDetailed sky coverage simulations planned in near-futureMOAO analysis planned at completion of MCAO analysis

AO4ELT, Paris 2009

Acknowledgements

The work is supported by the TMT project. The authors gratefully acknowledge the support of the TMT partner institutions. They are:– the Association of Canadian Universities for Research in Astronomy

(ACURA)– the California Institute of Technology, and – the University of California

This work was supported as well by– the Gordon and Betty Moore Foundation– the Canada Foundation for Innovation– the Ontario Ministry of Research and Innovation– the National Research Council of Canada– the Natural Sciences and Engineering Research Council of Canada– the British Columbia Knowledge Development Fund– the Association of Universities for Research in Astronomy (AURA)– and the U.S. National Science Foundation