imwcs currently works only on images where the rotation about the center is known to within a few degrees It has been tested extensively only on images with near-zero rotation.
A plane-tangent projection (center right ascension and declination, plate scale, and rotation angle) is fit based on matches between stars in the image and stars in a reference catalog. The HST Guide Star Catalog, the USNO A, SA, and UJ Catalogs, or a user-supplied catalog may be used, as well as the Tycho-2, ACT, PPM, and SAO Catalogs for wider fields.
As of version 2.8.7, 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 6).
Portions of imwcs have been turned into stand-alone programs which are useful in their own right. imsize decodes header information to print the size of the image on the sky. imcat finds the reference stars on that portion of the sky. imstar finds star coordinates within an image. immatch matches stars in an image with stars in a reference catalog.
|1. Find image WCS using HST Guide Star Catalog Stars as references|