Improvements To Pan-Starrs1 Astrometry Using Gaia

We use the Gaia DR2 catalog to improve the astrometric accuracy of about 1.7 billion objects in Pan-STARRS1 Data Release 2 (PS1 DR2). We also obtain proper motions for these PS1 objects. The cross-match between Gaia and PS1 reveals residuals that are correlated on a scale of about 1 '. We apply a spatially adaptive correction algorithm for all PS1 objects having more than two detections to reduce these residuals and align the object positions to Gaia. For point-like PS1 objects that cross-match to Gaia, the algorithm reduces PS1/Gaia residuals by 33% in position (median value of 13.5 mas reduced to 9.0 mas) and by 24% in proper motion (median value of 6.3 mas yr(-1) reduced to 4.8 mas yr(-1)). The residuals for the corrected positions are smallest for objects with the most point-like morphologies and with intermediate magnitudes of about 17 mag. The residual errors in decl. are systematically larger than those in R.A.; the decl. errors increase with zenith angle in proportion to the air mass of the observations. The decl. positional residuals at a given decl. generally vary with color and are consistent with the effects of differential atmospheric refraction. In principle, these residuals could be reduced further by taking into account object color.