and absolute position errors have been reduced to between 0.3 and 0.4 arc seconds. Catalog format When quoting a catalog entry, the format is GSC FFFFF-NNNNN, or alternately, GSCfffff0nnnnn (fffff0nnnnn). The F-sequence references a Hubble sky region code. Guide Star Catalog II A significant expansion of the catalog, Guide Star Catalog II, was published in 2008. The Guide Star Catalog II (GSC-II) was compiled by the Catalog and Surveys branch of the Space Telescope Science Institute and the astrometry team of the Astronomical Observatory of Torino (Italy). It has entries for 945,592,683 stars, and has positions, classifications, and magnitudes for 455,851,237 stars. The latest revision of this version (2.3.4) is in active use for accurately positioning the Hubble Space Telescope. See also Hubble Guide Star Catalog - Astrographic Catalog/Tycho (GSC-ACT) Hubble Space Telescope Space Telescope Science Institute Star catalog Fine Guidance Sensor (HST) References External links GSC I and GSC II home GSC II Hubble Space Telescope Astronomical catalogues of stars