In a similar fashion to the task omatt, 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), omsrlistcomb will attempt to correct the source RAs and Decs in the combined-souce list. If successful, the columns RA_CORR and DEC_CORR will contain the corrected RAs and DECs, respectively, the key-word POSCOROK will be set to t and the key-words RAOFFSET, RAOFFERR, DEOFFSET, DEOFFERR, ROT_CORR, ROT_ERR, NMATCHES, RMSRESID will contain the computed RA offset, RA offset error, DEC offset, DEC offset error, rotation, rotation error, the number of sources used in the astrometric fit and the root-mean-square residual of the fit, respectively. If the fit was unsuccessful then POSCORROK will be set to f and the columns RA_CORR and DEC_CORR will be set equal to the uncorrected values.
The user can use the task parameters maxradecerr and maxrmsres to change the settings that decide if an astrometric fit is okay POSCORROK=t or not POSCORROK=f.