It contains the twenty million stars of the Hubble GSC, version 1.1, recalibrated using the U.S.
By 2009 it was considered obsolete, superseded by such catalogues as GSC-2.2, B1.0, A2.0, and especially UCAC-3.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.