Geometry of the Hubble Space Telescope Wide Field/Planetary Camera field

We present a solution for the relative positions and orientations of the four CCD chips on the Hubble Space Telescope (HST) Planetary Camera (PC). An accurate solution is required when matching HST images with ground-based images or with one another. The solution is accurate to about 1/4 PC pixel or about $0{.hskip-2pt ''} 01$, a 30-fold improvement over the best previous solution. The CCDs are rotated relative to one another by up to $1^circ$. The solution is based on images taken between December 1990 and June 1992 and is stable over that entire period. We find that the pixel size is $0{.hskip-2pt ''}04373pm 0{.hskip-2pt ''}00004$ based on comparison with HST Guide Stars, in good agreement with previous Guide-Star based calibrations but $sim 1%$ smaller than a globular-cluster based calibration.