The Guide Star Catalog, version 1.2: An astrometric recalibration and other refinements

A collaboration between the Space Telescope Science Institute (STScI) and t he Astronomisches Rechen-Institut has produced an astrometric re-reduction of the Guide Star Catalog (GSC) published by Lasker et al. This version, GS C 1.2, has dramatically reduced the systematic errors present in the GSC 1. 1. As shown by Taff et al., the positions in the GSC 1.1 are affected by pl ate-based systematic distortions that are largest at the plate edges (1."0 in the north and 1."2 in the south). In addition, Morrison et al. have show n that these positions also suffer from systematic errors which are a funct ion of magnitude and radial distance from the plate center. This effect is small for radii under from the plate center, 2.degrees7 then rapidly increa ses, producing an average offset of the faint stars (15 mag) versus the ref erence stars (10 mag) of 0."9 at the plate edges. Since the magnitude term was similar for the two main plate collections comprising the GSC, which we re taken with different telescopes, exposure times, emulsions, and filters, we believe this effect is common to fast wide-field-of-view Schmidt plates . In this paper we report on the astrometric quality of the GSC 1.2 and pre sent the method we developed (for large plate collections) to quantify and reduce the positional errors which are dependent on location on the plate a nd magnitude. Instruction for obtaining the catalog may be obtained from th e STScI World Wide Web site, or the user can obtain the catalog directly fr om CDS (Centre de astronomiques de Donnees Strasbourg). This version of the catalog has not been installed in the HST Guide Star Selection system. The refore this version must not be used for HST observation planning.