Because the RA and Dec of the OM pointing direction 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), 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), omatt will attempt to correct the computed source coordinates. The correction is deemed to be a success if the following criteria are met:
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 true 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 false 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=true) or not (POSCORROK=false).
Please note that from SAS 7.2 the astrometric fit does not allow for any rotation between the OM and USN0 coordinate axes, since the computed rotations were never more than a few arc minutes- hence ROT_CORR will always be zero.
From SAS 13.0 the task has a functionality to generate its own USNO subset catalogue, provided the WCSTOOLS package is installed on the user's local Linux machine. This functionality is controlled by using the usecat and catfile parameters: if the parameter usecat is set to `yes' but the parameter catfile remains unspecified, then the task will attempt to generate a USNO catalogue subset with its default name usnocat.fit. If this attemt failes (e.g., due to the absence of the WSCTOLLS package on the local machine) then the USNO subset is not generated and the aspect-correction routine is skipped, the ODF image and source list remaining uncorrected.