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 (similar to0.2 pixel) of a skewing nature in their linear terms. We use the inner calibration field of omega 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 similar to0.01 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 similar to0.01 pixel. Nevertheless, our solution is a major improvement and will allow many astrometric projects to obtain near-optimal results.