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MCAO at CfAO meeting

MCAO at CfAO meeting. M. Le Louarn CfAO - UC Santa Cruz Nov. 22 2000. Goal of MCAO modeling. Can one use 1 NGS measuring 5 modes instead of 3 NGS measuring TT ? Limiting magnitude with NGS ? Sky coverage ? Astrophysical implications of MCAO

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MCAO at CfAO meeting

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  1. MCAO at CfAO meeting M. Le Louarn CfAO - UC Santa Cruz Nov. 22 2000

  2. Goal of MCAO modeling • Can one use 1 NGS measuring 5 modes instead of 3 NGS measuring TT ? • Limiting magnitude with NGS ? • Sky coverage ? • Astrophysical implications of MCAO • ELTs: Is it possible to use N LGS and only 1DM (closed loop problem ?) for correction in the visible ? • Effect of spot elongation on MCAO • […] • AO engineering modelling (engineering tolerences) • Lasers ? • NGS - MCAO ? Probably not because of low SC, but can use a lot of faint stars (?) • Effect of the NGS star distribution (no perfect triangles) • MCAO: smaller SC than with single NGS AO. • JPV: MCAO SC on HST targets -> MCAO CoDR

  3. MCAO Modeling • Full numerical Monte Carlo Simulation • 3 DMs, 5 LGS, 7 atmospheric layers (~ Gemini) • 13x13 SH for each LGS (with SH spot speckles) • 3x3 SH for 1 NGS (instead of 3 TT NGS @ Gemini) • Least-squares inversion of IM (SVD) • Closed loop, obtain PSF anywhere in the FOV • Analytic covariance code (Tokovinin et al.) • Covariances of Zernikes computed for measurements & reconstructions • Knowledge of Cn2 profile • Optimal estimation of the correction • Open loop • Obtain residual WF error anywhere in the FOV

  4. Analytic vs. Numerical Black: Analytic & Numerical isoplanatism, Red: Numerical MCAO, Blue: Analytical MCAO

  5. H Band

  6. J Band

  7. What could go wrong in simulations ? • Atmosphere is not as simulated: • There are many turbulent layers, almost continuous profile, outer scale L0 -> effect on high altitude DM density ? • Strong turbulence profile variations in closed loop • Propagation effects • Non-Kolmogorov turbulence, Non-Taylor turbulence • Na layer / LGS properties • Height variations: difficult to put in but can get an estimate • Spot elongation (  ELTs): can be simulated for 8m tel. • Lasers for MCAO • Hardware issues: AO simulation validity ??? • High order aberrations in mirrors, DM mis-registration... • MCAO waffle modes on real DMs • DM hysteresis effects • edge effects • laser launch telescope jitter -> total least squares

  8. Cn2 profiles

  9. Cn2 profiles

  10. Lab/sky tests • Atmosphere: • Continuous turbulence (not repeatable ?) + thin layers (?) • Turbulence generator • Phase screens • Propagation effects • Phase screens • Non-Kolmogorov screens • Na Layer: • Studies on LGS AO systems ? • Site monitoring (Na profile measurements) • Lab MCAO system: • control algorithm testing, system tolerances • DM hysteresis • AO Systems: DO NGS AO system work as modelled ? (Lick AO, Keck, Pueo…)

  11. Why lab / sky tests ? • Lab test = acceptance test of the MCAO system: saves money • Have you thought of everything ? • 1DM LGS AO system: • Na Layer saturation checked • Engineering for Keck, VLT,... • Sky testing / site monitoring -> Do we have the right parameters ? • Ragazzoni test on the sky did it bring anything good ?

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