XMM-Newton Science Analysis System


omsrclistcomb (omsrclistcomb-2.24) [xmmsas_20230412_1735-21.0.0]

Successful Astrometry Correction

The correction is deemed to be a success if the following criteria are met:

• A sufficient number of stars have been matched (if the number of input sources (n) is less than 30, max(5, n/4), otherwise 10).
• The computed offset errors are no greater than the value of the input parameter maxradecerr (arcsecs).
• The computed RMS residual is no greater than the value of the input parameter maxrmsres (arcsecs).

The user can use the task parameters maxradecerr and maxrmsres to change the settings that decide if an astrometric fit is okay.

The columns RA_CORR and DEC_CORR will contain the corrected RAs and DECs, respectively, the key-word POSCOROK will be set to t (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.

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 the FITS keyword ROT_CORR, in the SRCLIST table will always be zero.