The RA and Dec coordinates are computed using the pointing direction of the OM camera as the origin, and because the RA and Dec coordinates of this point may not be exactly the same as the coordinates deduced from reference to a catalogue (differences of a few arcsecs in RA and Dec are typical), omsrclistcomb (in a similar fashion to omatt) allows the user to determine these shifts and correct the source positions.
If the task parameter usecat is set to true, and the parameter catfile is set to the name of the fits catalogue file (or it has the default name usnocat.fit), omsrclistcomb attempts to match the OM sources with those from the catalogue, determine the RA and Dec shifts and correct the coordinates.
