Optical astrometric corrections in R.A. and decl. These relative offsets are computed as the positional difference between the HST WFPC2 and USNO coordinates for multiple stars in a given field. Both distributions are roughly Gaussian with mean values of ﹩( \Delta \alpha ,\Delta \delta ) =( -0.1^{\prime \prime },0.0^{\prime \prime }) ﹩, with standard deviations ﹩( \sigma ^{\mathrm{opt}\,}_{\alpha },\sigma ^{\mathrm{opt}\,}_{\delta }) =( 0.85^{\prime \prime },0.55^{\prime \prime }) ﹩. The ellipses represent the 1 σ, 2 σ, and 3 σ contours. These systematic offsets and uncertainties are used to determine the most likely optical identification for radio galaxies discussed in § 2.4.