Instrument Science Report 2003-02:

Toward a Multi-Wavelength Geometric Distortion Solution for WFPC2

V. Kozhurina-Platais, J. Anderson, A. M. Koekemoer

July 1, 2003

Abstract:

The goal of an astrometric calibration of HST WFPC2 is to obtain a coordinate system free of distortion at the level of precision of 1 mas. So far such calibration has been done only for the wide bandpass F555W filter. The inner calibration field of w Cen exposed through filters F300W, F555W and F814W has been used to examine the geometric distortion of WFPC2 as a function of wavelength. The improved PSF-fitting technique by Anderson and King has been used to obtain the working sets of x,y coordinates for stars in assorted HST WFPC2 frames with the globular cluster w Cen in these three band-passes. We used a bicubic polynomial model to derive geometric distortions in the F300W and F814W filters relative to the distortion-free coordinates in the F555W filter. For each filter, four sets of distortion corrections, corresponding to each of the four WFPC2 chips, have been optimized independently. The main conclusion of this study is that the difference in the amount of distortion in the F300W filter is about 3% higher than in the F555W but in the F814W filter it is about 1% smaller than in the F555W. Tables with distortion coefficients are also presented.

The Complete Paper (PDF 8.8 MB) is available.