François Rigaut, Gemini Observatory GSMT SWG Meeting, LAX, 2003/03/06 François Rigaut, Gemini Observatory GSMT SWG Meeting, LAX, 2003/03/06 GSMT AO Simulations

GSMT SWG Meeting, LAX, 2003/03/16 2 Ground Conjugated AO: GCAO/AO/MCAO relative merits Basic limitations and principles Performance (Monte-Carlo & Analytical) Astrometry with MCAO Presentation map

GSMT SWG Meeting, LAX, 2003/03/16 3 Ground Conjugated Adaptive Optics for the GSMT

GSMT SWG Meeting, LAX, 2003/03/16 4 Idea very similar to MCAO, but using a single ground conjugated deformable mirror instead. The ground layer is the strongest. The FWHM gains are modest, but the field of view much larger than AO/MCAO ➜ tailored to different applications. Performance independent of telescope D Borderline to use NGS, but LGS available Ground Conjugated AO GCAO

GSMT SWG Meeting, LAX, 2003/03/16 5 SNR gain ≈ Metrics (arbitrary?): 1→ SNR gain x FoV/FoV(AO) 2 → SNR gain x [FoV/FoV(AO)] / cost GCAO/AO/MCAO relative merit GCAO #photons obj uncomp. FWHM obj comp. #photons obj comp. FWHM obj uncomp. × ≈ Strehl comp x 0.7 x D/r 0

GSMT SWG Meeting, LAX, 2003/03/16 6 GCAO/AO/MCAO relative merit GCAO 8m,1μmAOMCAOGCAOSeeing SNR gain5421 FoV20’’80’’10’∞ Cost (AU)132- Metric ∞ Metric ∞

GSMT SWG Meeting, LAX, 2003/03/16 7 GCAO/AO/MCAO relative merit GCAO 30m,1μmAOMCAOGCAOSeeing SNR gain5421 FoV20’’80’’10’∞ Cost (AU)132- Metric ∞ Metric ∞

GSMT SWG Meeting, LAX, 2003/03/16 8 Geometry of the Problem GCAO

GSMT SWG Meeting, LAX, 2003/03/16 9 Geometry of the Problem GCAO Actuator grid Field of View h d The effective thickness of the compensated layer depends on: seeing wavelength (via d) Field of view → h eff ≃ 2 r 0 /θ for good seeing: h eff (K) = 1600m h eff (V) = 275m

GSMT SWG Meeting, LAX, 2003/03/16 10 Actuator Density (Analytical approach) Average FWHM at V band versus the diameter of the field of view for actuator densities of 25cm (lower solid) 50cm (dot) 100cm (dash) Pachon Cn 2 profile & r 0 GCAO Pachon Cn 2 Profile Natural V band seeing

GSMT SWG Meeting, LAX, 2003/03/16 11 Wavelength dependency (Analytical approach) Natural (straight upper lines) and Compensated (lower curves) FWHM averaged across field of view: Bands: V (solid), J (dot) K (dash) Cerro Pachon Cn 2 profile Actuator density = 1m. Pachon Cn 2 Profile GCAO

GSMT SWG Meeting, LAX, 2003/03/16 12 Developed a new AO simulation code (Yorick based) Handles any number of DMs (PZT/bimorph) at any altitude, any number of WFSs (SH/Curvature) at arbitrary location in the field. Cone effect included. Only simple least square implemented (upgrade). Include most of AO effects (fitting, aliasing, servo-lag, anisoplanatism, noise) Goal here is to validate the analytical results and investigate: dependence upon telescope diameter dependence upon number of guide stars dependence upon compensated field of view dependence upon the wavelength, Cn2 profile, etc... Uniformity and shape of the PSF across the field Monte-Carlo Simulations GCAO

GSMT SWG Meeting, LAX, 2003/03/16 13 Performance vs Field of View (Monte-Carlo Simulations) Field of View Diameter [arcmin] GCAO Conditions: λ = 1.25 μm Seeing(500nm)=0.5” Pachon profile ground level) 8-m telescope ( ⇔ 30m) 50cm pitch 10 WFSs GS placed on a circle No AO results: FWHM = (400± 14) mas Checked telescope diameter has little impact FWHM (mas)

GSMT SWG Meeting, LAX, 2003/03/16 14 Performance vs Field of View (Monte-Carlo Simulations) Number of Guide Stars GCAO Conditions: λ = 1.25 μm Seeing(500nm)=0.5” Pachon profile ground level) 8-m telescope ( ⇔ 30m) 50cm pitch GS placed on a circle No AO results: FWHM = (400± 14) mas FWHM (mas)

GSMT SWG Meeting, LAX, 2003/03/16 15 Astrometry with MCAO on GSMT

GSMT SWG Meeting, LAX, 2003/03/16 16 Bears the promise of 0.1mas astrometric accuracy (FWHM/30). But... Difficulty intimately linked to the ability of compensating high altitude turbulence Not a problem with GCAO or AO Difficult to analyze. Require a careful consideration of biases Established a preliminary error budget Key: design a system as little bias-prone as possible Astrometry with MCAO Astrometr y

GSMT SWG Meeting, LAX, 2003/03/16 17 relative astrometry 3+ tip-tilt GSs adequately located in FoV Astrometric references adequately located in the field of view (QSOs, background stars) used to establish astrometric referential from observation to observation TT/plate scale mode sensing done at the science detector Errors given at the Tip-Tilt GSs Error budget: Assumptions Astrometr y

GSMT SWG Meeting, LAX, 2003/03/16 18 Astrometry with MCAO Astrometr y SourceError (mas)Comment Distortion calibration (high order NCPA)0.2reduce by dithering Pixel QE inhomogeneities0.08 reduce by dithering (1.5 mas/pix) Residual local image motion (convergence) 0.1 long total exptime ≫ 1mn WFSs calibration/Zero point0.5?T.B. detailled Spatial Aliasing on TipTilt GS WFSs-astrometric reference in FoV Beam position error of LGSs< 0.1careful optical design Non-linear effects in WFS/DMs0.6place holder Total0.82needs more work Some of these items can be checked with current systems. I suggest a plan be prepared to get observing time on Altair or other systems.

GSMT SWG Meeting, LAX, 2003/03/16 19 Conclusions Ground-Conjugated: Provides factors of 2 improvement on 5- 20’ field of view Good PSF shape and homogeneity Monte-Carlo simulations confirm analytical derivations Astrometry: A preliminary error budget was developed. Need input from community (astrometry and AO) to investigate further. Possible tests on existing systems

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