ACS/WFC Geometric Distortion and CTE V. Kozhurina-Platais & ACS team

ACS/WFC geometric distortion is changing linearly with time Anderson J., ISR-ACS/WFC, August 2006 a) Standard astrometric reference field (47Tuc) ~ 53,000 stars b) User friendly software - XYM2MAT Our plan was to: a) Check the time-dependency on FLT images; b) Implement the time dependency in MultiDrizzle c) Verify the implementation

The observations of 47Tuc F606W filter (PIP ) different orientations different postargs different exposures: s The reductions: effective PSF fitting on FLT - X,Y mag; standard frame: 2002 March, 690s, F606W,  =-163 

Ouv - standard orthogonal system MXY - measured system Kiselev A.A, 1989, pub.,”Nauka”, St.Petersburg, Russia X Y XoXo YoYo O M  90+   u v Y X uouo vovo

720 s,  =-163  s,  =- -23  s,  = 51  s,  = -117  s,  = -67  s,  = -122 

The skew term as

Many variables: PID : The same pointing Exposures - 400s, 30s Two orientations : ,  exposure  pointing  orientation  epoch

LAMP=OFF/OFF 400sec--> 400sec, > sec--> 400sec, > sec--> 400sec, > sec--> 400sec, >-29.4 LAMP=OFF/OFF LAMP=MED/OFF LAMP=HIGH/OFF LAMP=LOW/OFF

Schematic illustration of the WFC1 and WFC2 plane: - Red arrows show the readout direction - Green color shows the PSF profile affected by CTE

Schematic illustration of the WFC1 and WFC2 plane: - red arrows show the readout directions - Green color show the PSF profile effected by CTE  Y > 0 - the difference in Y coordinate between CTE induced cenroid shift and cenroid without CTE  mag < 0 - the difference in magnitudes between CTE- induced photometric loss in the bottom of WFC1  Y < 0 - the difference in Y coordinate between CTE induced cenroid shift and cenroid without CTE  mag < 0 - the difference in magnitudes between CTE- induced photometric loss in the top of WFC2

Conclusions  The CTE-induced centroid shift depends on: magnitude of a star position on a CCD chip sky background  The amplitude of CTE-induced centroid shift is ~ 0.1 pix  CTE effect leads to a discontinuity in the reference frame  Accounting for CTE effect is critical to reach a 1 mas accuracy requirement for JWST standard astrometric field