Astrometry and Photometry with HST WFC3. I. Geometric Distortion Corrections of F225W, F275W, F336W Bands of the UVIS Channel

An accurate geometric distortion solution for the Hubble Space Telescope UVIS channel of Wide Field Camera 3 is the first step toward its use for high-precision astrometry. In this work we present an average correction that enables a relative astrometric accuracy of similar to 1 mas (in each axis for well-exposed stars) in three broadband ultraviolet filters (F225W, F275W, and F336W). More data and a better understanding of the instrument are required to constrain the solution to a higher level of accuracy.