An Improved Distortion Solution for the Hubble Space Telescope's WFPC2

Having an accurate distortion correction is critical to doing accurate astrometry with the Wide Field Planetary Camera 2. Currently available solutions have serious systematic errors (~0.2 pixel) of a skewing nature in their linear terms. We use the inner calibration field of ω Centauri to find an improved cubic solution that is accurate to +/-0.01 pixel in the WF chips and to +/-0.02 pixel in the PC chip. Improvements beyond this will be difficult, as there are many factors that perturb the solution at the ~0.01 pixel level: breathing, variations with filter, and the physical movement of the chips. There are probably not enough data available to constrain the solution to better than ~0.01 pixel. Nevertheless, our solution is a major improvement and will allow many astrometric projects to obtain near-optimal results. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.