The Hubble Guide Star Catalog – Astrographic Catalog/Tycho (GSC-ACT) is an astronomical catalog. It contains the twenty million stars of the Hubble GSC, version 1.1, recalibrated using the U.S. Naval Observatory's ACT (Astrographic Catalog/Tycho). By 2009 it was considered obsolete, superseded by such catalogues as GSC-2.2, B1.0, A2.0, and especially UCAC-3.[1]
GSC-ACT was a re-calibration of the Guide Star Catalog version 1.1 for Hubble.[2] The original GSC dated to 1989,[3] and one of the reasons for an update was improvements to star catalogs that took place after that time.[4] In this case the ACT catalog was used, which had for example more stars and whereas the GSC 1.1 had 3rd order fit for plate distortions, the ACT has a five-order available.[5] Another improvement of the GSC 1.1 was the GSC 1.2.[6]
GSC-ACT uses five classification codes:[7]