The GSC-ACT Catalog
The ACT offers better positions and more stars than the reference catalogs originally used to reduce the Guide Star Catalog, so the astrometric fit can include more terms... and this is a very good thing, because the plates include distortions that the simple third-order GSC 1.1 fit cannot accommodate. In GSC-ACT, terms up to order 5 are included.
The GSC-ACT is available on two CDROMs from Project Pluto.
To enable rapid access of specific stars in the catalog, WCSTools software numbers each star using its RA region number (0000-2330) and a five-digit star number within each region, separated by a decimal point. sgsca lists GSC-ACT stars by number or sky region. imgsca lists the GSC-ACT stars within an IRAF or FITS image with a world coordinate system defined in its header.