Optical positions for a sample of ICRF sources

The aim of this paper is to present a large, coherent set of astrometric optical positions for ICRF sources in the Hipparcos reference frame. For this, we present positions for 315 ICRF sources measured in Digitized Sky Survey (DSS) images, with positions referred to the ACT, Hipparcos, and Tycho catalogs. The reductions were made relative to Hubble Space Telescope guide stars, which were previously corrected toward those same catalogs. The reductions errors are at 45 mas and even smaller for southern sources. The comparison between optical with ICRF radio positions shows that neither the residuals nor the internal errors exhibit any trend with magnitudes or with location on the DSS plates.