imwgsc2 uses the GSC II Catalog, an total update of the Hubble Space Telescope Guide Star Catalog containing 60 times as many stars, 998,402,801. The USNO-B1.0 catalog (imwub1) has few more stars (1,036,366,767), but its proper motions are not quite as good. The 2MASS Point Source Catalog (imwtmc) is another recent epoch catalog, and is based on IR images.
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|