B.1 Overview

WFC3 images exhibit significant geometric distortion, similar to that seen in ACS images. The required folding, with powered optics, of the light paths in both channels to fit within the instrument’s optical-bench envelope results in substantial tilts of the focal surfaces with respect to the chief rays. The WFC3 UVIS detector is tilted at ~21° about one of its diagonals, producing a rhomboidal elongation of ~7%. The IR detector has a ~24° tilt about its x-axis, creating a rectangular elongation of ~10%.

If these were the only distortions they would not present much difficulty: their impacts on photometry, mosaicking, or dithering could be computed simply. More problematic, however, is the variation of plate scale across each detector. For the WFC3 UVIS and IR channels, this variation in plate scale amounts to a change of 3.5% in x and y, and 2% in x and 6% in y, respectively, over the full field. Hence the area on the sky covered by a pixel varies, by about 7% for the UVIS channel and about 8% for the IR channel. Allowance for this change in plate scale must be made in photometric reductions of WFC3 data that have not been corrected for distortion. Further details are available in WFC3 ISR 2010-08 and at the pixel area map section of the WFC3 Web site:

http://www.stsci.edu/hst/wfc3/pam/pixel_area_maps

Dithering and mosaicking are complicated by the fact that an integer pixel shift near the center of the detector translates into a non-integer displacement for pixels in other locations. Even this is not a fundamental difficulty, but implies some computational complexity in registering and correcting images. All of these considerations make it necessary to obtain accurate measurements of the distortions. The orientations of the WFC3 detector edges for both detectors are at approximately 45° with respect to the V2 and V3 coordinate axes of the telescope. Figure 2.2 shows the WFC3 apertures in the telescope’s V2,V3 reference frame. For a telescope roll angle of zero this would correspond to an on-sky view with the V3 axis aligned with north and the V2 axis with east.

The first on-orbit measurements of the geometric distortion for the WFC3 detectors were made during SMOV (Servicing Mission Observatory Verification). Astrometric fields in 47 Tuc (NGC 104) and the LMC were observed with multiple offsets in programs 11444 (UVIS, filter F606W) and 11445 (IR, filter F160W). Geometric distortion solutions were derived from this data (WFC3 ISR 2009-33, WFC3 ISR 2009-34) and entered into IDCTAB files to support the use of MultiDrizzle to produce distortion-corrected images. (MultiDrizzle has since been replaced by DrizzlePac.) In the initial IDCTAB files, the solutions for filters F606W and F160W were applied to all UVIS and IR filters, respectively. Because there are small filter-dependent differences in distortion, exposures made with other filters during SMOV and in subsequent calibration programs observing Omega Centauri have been used to derive improved solutions for the more commonly used filters (WFC3 ISR 2012-07). The distortion has been found to be stable over two years (WFC3 ISR 2012-03). The relative displacement of stars in exposures made with different filters due to non-coplanarity of the filters is ~ 0.02 arcsec in most cases (WFC3 ISR 2010-12, WFC3 ISR 2012-01.)