Note: The GSC-ACT (or GSC1.3) is a recalibration of the GSC1.1, the catalogue constructed to support the operational need of the Hubble Space Telescope, with the ACT, the combination of the Astrographic Catalogue and the Tycho catalogue derived from the Hipparcos mission; see http://www.projectpluto.com/gsc_act.htm for more details about this recalibration.