The New GSC-II and its Use for HST

The second generation HST Guide Star Catalog (GSC-II) provides a significant improvement over the original GSC-I. It provides ICRS-based positions (International Celestial Reference System) with improved accuracy for almost a bil- lion objects as faint as J=21. This will become the default reference frame for HST observations starting in cycle 15 and will reduce the absolute errors of the astrom- etry in the observation headers from 1-2" down to 0.3-0.5". In addition, the new astrometry is being incorporated into the Digitized Sky Survey (DSS) images and can also be used to improve the astrometric accuracy of archival HST data.