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

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

2. 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

3. The observations of 47Tuc • F606W filter • 2002-2006 (PIP-9018-10771) • different orientations • different postargs • different exposures: 22 - 1200s • The reductions: • effective PSF fitting on FLT - X,Y mag; • standard frame: 2002 March, 690s, F606W,=-163

4. Y v  Y vo Yo O u uo M  Xo X 90+ X Kiselev A.A, 1989, pub.,”Nauka”, St.Petersburg, Russia Ouv - standard orthogonal system MXY- measured system

6. 720 s ,  =-163.2 2002.4 22 s ,  = -122.9 2004.5 22 s ,  =- -23.9 2004.7 22 s ,  = -67.8 2005.0 22 s ,  = -117.9 2002.4 22 s,  = 51.9 2005.9

7. The skew term as

10. PID - 10368: • The same pointing • Exposures - 400s, 30s • Two orientations : +154.8, -29.4  exposure  pointing  orientation  epoch Many variables:

11. 400sec--> 400sec, 154.7-> 154.7 LAMP=OFF/OFF 400sec--> 400sec, 154.7-> -29.4 LAMP=OFF/OFF 30sec--> 400sec, 154.7-> 154.7 LAMP=OFF/OFF 30sec--> 400sec, 154.7-> 154.7 LAMP=MED/OFF 30sec--> 400sec, 154.7-> 154.7 LAMP=HIGH/OFF 30sec--> 400sec, -29.4->-29.4 LAMP=LOW/OFF

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

14. 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