D.J.Mink
SAO
LIBWCS is a library of utility subroutines, written in portable C, for setting and using the world coordinate system (WCS) of a FITS or IRAF image. The WCS is the relationship between sky coordinates, such as right ascension and declination, and image pixels, and can be described using standard keywords in an image header. A single subroutine library is currently used by the image display and browsing programs SAOimage, SAOtng, and Skycat to translate between image pixels and sky coordinates. Some subroutines have been improved and new ones have been added to read and write all legal FITS image data types and to deal with IRAF .imh (OIF) files as easily as FITS files. A program, IMWCS, based on earlier work presented by Elwood Downey and Robert Mutel of the University of Iowa at the 1995 ADASS Conference, has been written to automatically find stars in an image, match them to a reference catalog, and fit a WCS to the image. Each of the individual functions is available separately. IMSTAR lists the stars in an image, with sky coordinates if a WCS exists in the image header. IMGSC lists stars from the HST Guide Star Catalog CDROMs which are found in the area defined by an image WCS. IMUJC lists stars from the USNO J Catalog found in such an image-defined area. XY2SKY and its inverse, SKY2XY, convert pixel coordinates to sky coordinates and sky coordinates to pixel coordinates for an image with a WCS defined in its header. These programs can be used in combination. If a CCD image has too few Guide Stars in it for a good WCS fit, a Digitized Sky Survey image can be extracted and IMSTAR can be used to list stars in that image and their positions. This list can then be used by IMWCS as a reference catalog to set the world coordinate system of the image. Source code and online documentation for this library are available at http://tdc-www.harvard.edu/software/libwcs.html.