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 website: http://www.stsci.edu/hst/instrumentation/wfc3/data-analysis/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. See Section 6.2.2 of the Phase II Proposal Instructions, which gives detailed information on the relationship between detector coordinates, spacecraft coordinates, and ORIENT.

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; WFC3 ISR 2018-10).

The distortion in both UVIS and IR has been found to be stable over the years (see WFC3 ISR 2019-09 for analysis of WFC3/UVIS in the F606W filter, and WFC3 ISR 2018-09 for analysis of WFC3/IR in the F160W filter). 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). Astrometric accuracy of the WFC3/UVIS distortion solutions has been improved by incorporating correction for the lithographic mask pattern of the detector (WFC3 ISR 2013-14). Further work on the lithographic mask pattern and filter-dependent fine scale structure has reduced astrometric errors to the level of ~1 mas for many of the UVIS filters (WFC3 ISR 2014-12). The WFC3/UVIS distortion solution for F606W was found to be stable and accurate to +/-2 mas over 5 years using a standard astrometric catalog of the central region of Omega Centauri created from exposures made at different centerings and roll angles (WFC3 ISR 2015-02). With 10 years of data, the WFC3/UVIS distortion solution for F606W was confirmed as stable, based on F606W images corrected with the current geometric solutions to the Gaia DR2 source catalogue in the central region of Omega Centauri (WFC3 ISR 2019-09). Preliminary results based on 14 years of UVIS data indicate that the X and Y scales changed by ~0.2 and 0.1 UVIS pixels respectively at the detector edge (2048 pixels; 2024). Should this be confirmed and shown to significantly impact science data calibration, time-dependent distortion solutions can be incorporated into the IDCTAB reference files.

Delivered IDCTABs with distortion solutions for WFC3/IR filters and WFC3/UVIS filters are listed in Table B.1. Any IR filter or UVIS filter not listed in the delivered IDCTABs is still using the solution derived for F160W and F606W, respectively. All UVIS filters listed in the table have accompanying NPOLFILE (Non-polynomial Offset) reference files available. The NPOLFILE reference files are the 2-D look-up-tables of the filter-dependent fine scale distortion (WFC3 ISR 2014-12; WFC3 ISR 2018-10).

Table B.1: IDCTAB (Instrument Distortion Correction Table) deliveries for the IR and UVIS filters