imwgsca uses the GSC-ACT Catalog, an update of Hubble Space Telescope Guide Star Catalog containing the same number (25,541,952) of entries. It will also work on about half of the 10 arcminute square images I've encountered. The USNO-A2.0 catalog has enough stars to fit almost any ground-based image, and is invoked using imwua2. The 2MASS Point Source Catalog (imwtmc) and the GSC II Catalog (imwgsc2) have fewer stars than the USNO-A2.0, but they are based on positions closer to the present, i.e. 1990's rather than 1950's.
A set of image stars may be hand-matched to sky positions and used to get a WCS or set initial values for a reference catalog-based WCS (See example).
|Using SExtractor to find stars in an image|