A fit is performed at the peak position in each image, using the reference as an initial guess, and the tangent-plane X,Y offsets between the peak and reference positions are calculated. The offsets are applied to the WCS. Note that only a linear shift is performed; this recipe is not a full astrometry matching routine and does not deal with rotations between images.
The reference position is assumed to be RA/Dec coordinates if given in sexagesimal format, otherwise Galactic (in degrees).
Creates an output file for each input file with suffix _reg.
A reference position should always be given for non-calibrators.
The reference position should be that of a known source in each image, and that source must be present in all images.
Supported coordinate systems are GAPPT, J2000 (including FK5, ICRS) and Galactic.
The coordinate system of the reference position does not have to match that of the images to be registered.