Astrometry for NGAO Brian Cameron, Matthew Britton, Jessica Lu, Andrea Ghez, Rich Dekany, Claire Max, and Chris Neyman NGAO Meeting #6 April 25, 2007

2/14 The Limits… In Principle Photon Noise –AO helps with SNR and achieving  D Confusion –Important for densest fields

3/14 The Limits… In Practice Systematics… Physical optics –Geometric distortions An atmosphere –Refraction –Anisoplanatism All are ~10 mas, but can be reduced

4/14 Geometrical Distortion: Pin Hole Mask NIRC2 suffers from distortion. Characterize with pin hole slit mask –Polynomial fit –Residuals ~0.1 pixels (1-4 mas) Stable within the errors over the last ~6 months. Don’t dither! –Doesn’t help for heterogeneous data sets

5/14 State of the Art: HST Distortion from huge, well- dithered, rotated data set Understood at <0.01 pixel (<0.3 mas) –Both WFPC2 and ACS Key’s to success –PSF modeling –Stability On-sky experiment planned for summer 2007 Anderson & King 2003

6/14 Differential Tilt Stars separated by some angle sample different turbulence. Elongation of the PSF toward the TT reference.  DT ~ 20 mas (  /20”)(5m/D) 6/7 Averaged down by integration.

7/14 Differential Tilt Vector separation between 21” binary –100s of 1.4 sec exposures RMS ~20 mas –anisotropic Uses a PSF prediction technique –OTF measured from guide star –Propagated off-axis with ATF (calculated with help of DIMM/MASS) Britton 2006

8/14 Differential Atmospheric Refraction DAR contributes ~5 mas every 10” at transit. Can we detect? –Sort of…

9/14 Differential Atmospheric Refraction 12 zenith angle of 45 degrees, separation of 30” and  T ~ 5000 K Noise from correction: –Model Uncertainties –Uncertainties in 7 parameters. ParameterUncertaintyNoise (uas) Ground Temp. 3 K50 Pressure8 mb50 Zenith Angle 36”10 Separation30 mas10 Relative Humidity 10%-- Relative Stellar Temp K10 Gubler & Tytler 1998

10/14 Confusion Inner few mas of the Galaxy is limited by confusion –Goal < 0.1 mas Trade study (although noisy) shows ~140nm WFE required.

11/14 Scientific Applications Galactic Center –GR, young stars, high velocity stars ‘Isolated’ point sources –Galactic Plane targets Proper Motions

12/14 Distortion Corrected Compute Allan variance for diagnostics –variance after averaging over various timescales Recover ~ 1/sqrt(t) dependence until the next floor –~1 mas for isolated point source data.

13/14 GC Astrometric Precision

14/14 Recommendations Turbulence monitoring PSF monitoring camera Distortion solutions and monitoring Atmospheric dispersion corrector (?) System with < 170nm WFE to mitigate GC confusion

15/14 More DAR

16/14 Field Dependent Errors