Precision astrometry is revolutionizing our understanding of the local universe, with Gaia and HST being key players. Yet the WFPC2 is one HST camera that has not seen much of its astrometric potential tapped, up to now. Importantly, the archival database for this imager can extend time baselines by 10-15 years compared to other HST cameras. To improve the astrometric calibration of WFPC2 we make use of all archived observations in filters F555W, F606W, and F814W.
These are calibrated both via internal comparisons and by reductions into three external, astrometric standard catalogs: Gaia DR2 and two HST catalogs constructed using other HST imagers in the fields of 47 Tuc and Omega Cen. We explore various centering techniques for the undersampled WFPC2 star images and attempt to model field-dependent systematics, of both high and low spatial frequency, and monitor these as a function of observation epoch.
We find no significant high-frequency systematics, besides the well-known 34-row correction, down to a resolution of 10 pixels. Low-frequency systematics, beyond the previously measured 3rd-order optical distortion, do exist with filter F606W markedly different from the other two filters. These systematics are seen to change gradually over time. Based on Gaia DR2, we provide distortion-correction maps at two epochs.