Omatt produces the sky-images and computes the celestial coordinates of the sources. Additionally, in order to correct for any small error in the OM pointing direction, leading to small RA and Dec offsets, it can also try to correct the source coordinates for these offsets by using the USNO-B catalogue. If this astrometry correction is successful, omatt will set a FITS key-word POSCOROK to TRUE in the headers of the source-list and sky-image files and RAOFFSET and DEOFFSET to the computed RA and Dec offsets. The values of CRVAL1 and CRVAL2 will also be corrected. If it fails, POSCOROK will be set to FALSE and the other key-words to zero.
For each input sky-image ommosaic will check the POSCOROK keyword and if it equal to FALSE, if possible it will correct the reference pixel coordinates (CRVAL1 and CRVAL2) by interpolation or extrapolation using the successful correction values. It will add the FITS keyword POSINTRP to the sky-image header, and it will be set to TRUE if an interpolation/extrapolation has been done, or FALSE otherwise. The keywords RA_OFF and DEC_OFF will also be added giving the interpolated/extrapolated values (0.0 if no interpolation or extrapolation).
When ommosaic is run with its parameter mincorr set to 0.0 (default value- no image-alignment attempted) the astrometric accuracy of the computed stacked image will be determined by the accuracies of the individual reference-pixel coordinates (CRVAL1 and CRVAL2), which in turn will depend on the accuracies of the computed astrometric correction values. A single bad computed RA or Dec offset can have an adverse effect on the stacked image and astrometry. Therefore, in order to try to warn the user of any possible problems, ommosaic will analyse the computed RA and Dec offsets (using a MAD test described later on) and warn the user of any possible discrepant values.
If none of the input sky images had a successful astrometric correction (POSCOROK=false), then the keyword POSCOROK in the FITS header of the output image will be set to FALSE. In this case the astrometry contains the uncertainty due to the unknown error in the OM pointing direction.
If at least one of input sky images had a successful astrometric correction (POSCOROK=true), then the keyword POSCOROK in the FITS header of the output image will be set to TRUE and the keywords RAOFFSET and DEOFFSET to the mean RAOFFSET and DEOFFSET values from all the input images, respectively (some of which may have been computed by interpolation or extrapolation.)
However, a successful astrometric correction may have been done by omsrclistcomb.- this is because there may be exposure source-lists containing too few sources for an astrometric correction to be performed by omatt (particularly in the UV) , but in the observation source-list produced by omsrclistcomb there may be sufficient sources for a successful correction.
XMM-Newton SOC -- 2021-11-30