Improvements to the Hubble ACS/WFC Geometric Distortion Correction based on GAIA DR2

The Hubble Space Telescope's Advanced Camera for Surveys (ACS) has a larger geometric distortion than the other HST instruments due to its off-axis configuration. Yearly observations of 47 Tuc have been taken over the lifetime of the instrument to characterize and correct this distortion to provide high quality images to the astronomical community. With the Gaia Data Release 2, we perform a new calculation of the geometric distortion correction for the ACS Wide Field Channel (WFC) that leverages the increased accuracy and precision of DR2 over the current HST standard catalog. We improve the fit of the scale, skew and time dependence by incorporating both the Gaia DR2 positions (with errors less than 5 mas) as well as revised WFC aperture location measurements in the Science Instrument Aperture File (SIAF). We present results from this effort showing the increased accuracy of the correction.