Finally, when all the data for each filter has been processed, omsrclistcomb runs to combine the source lists from separate exposures into a single master list, to compute source fluxes, standard and AB magnitudes, and to set source flags. As for omatt, if a USNO catalogue fits file is available, and the parameter omsrclistcombusecat is set to TRUE then it will also correct the astrometry (using the combined source-list) for any offset between the OM and USNO RA and dec axes, adding the columns RA_CORR and DEC_CORR to the combined source-list. For more details, please see the documentation on omsrclistcomb.
The omatt computed astrometric offsets are ignored by omsrclistcomb. This is because omatt cannot always do an astrometric correction (particularly in the UV) and the OM pointing is generally very stable during an observation. Omsrclistcomb can generally do a more accurate astrometric correction, but those done by omatt are useful as a check on the pointing stability.
The parameter omsrclistcombalignaxes is used to set the omsrclistcomb's parameter alignaxes and defaults to true. In the default case omsrclistcomb attempts to align the RA and DEC axes of the input source-list files before constructing the combined source-list containing the unique sources from the multiple detections of a unique source. This can improve the astrometry and reduce the chances of erroneous source matches. Once the combined source-list has been formed an astrometric correction will be computed using the sources in the list. This parameter should only be set to false if the user has a very good reason that it has not worked very well.
XMM-Newton SOC -- 2021-11-30