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Дата изменения: Wed Jul 2 10:27:05 2008
Дата индексирования: Fri Sep 5 17:24:54 2008
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XMM-Newton Science Analysis System


omatt (omatt-2.46.2) [xmmsas_20080701_1801-8.0.0]

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Astrometric correction

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:

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

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.