Known Issues
Source Detection
Within the version 1.0 release of the catalogue, fields containing >5,000 sources are not included. Source detection and photometry are biased by poor background statistics and source confusion in these dense fields. The absence of these fields is particularly noticeable in the optical band samples which, relative to the UV, are dominated by high Galactic latitude sources rather than those in the direction of the Galactic plane or Magellanic clouds. It is intended that more-crowded fields will be included within future releases of the catalogue, once suitable detection and photometry algorithms have been devised.
Astrometry
This issue is not directly relevant to the contents of the OM source catalogue. But catalogue users should note that while source positions within the table have been astrometrically corrected by ground processsing using correlations with USNO-B1.0 sources, the images produced by the XMM pipeline are not corrected. Be prepared therefore to find small (~ 1 arcsec) positional offsets between sources contained in the images and catalogue. Astrometric corrections to OM sky images will be added to the pipeline at a future date.
While stable over an observation, the absolute pointing knowledge of the XMM spacecraft is good to a few arcsecs. A fine correction to the pointing and source positions is made during the data reduction pipeline. Statistically speaking the corrections in RA and Dec over the duration of the mission are not completely random, with a bias of 0.3" in a particular direction. The same offset is observed when comparing USNO-B to Swift-UVOT fields and USNO-B to SDSS fields (Scott Koch; private communication). Users should be aware that the XMM-OM sources are corrected RELATIVE to the USNO-B catalogue.
Photometry
- Sources are not extracted from the deepest possible images
While the positions of all sources within the catalogue have been corrected using the USNO-B1.0 survey as reference, the images are not subsequently corrected using the RA and Dec offsets determined during the correction. Therefore historically the XMM-OM images have not been mosaiced in order produce the deepest possible image data. As of SAS8.0 sky images are aspect corrected and because of this improvement in data quality, the next version of the catalogue will extend the faint source limit significantly deeper.- Lower limits for non-detections are not provided
Currently, sources with non-detections in certain filters are given a detection significance of -999σ. While this does provide a useful selection diagnostic, it would be more useful to provide the specific background limit for each non-detection. The functionality to perform aperture photometry over the positions of these non-detected sources is not currently used within the pipeline, but is a desirable feature for future versions.- Accuracy of coincidence loss in extended sources is unquantified
Count rates are under-estimated in bright sources because of photon coincidence loss. These are corrected for during the data reduction pipeline. Using photometric standards the effect has been well-calibrated in point sources, but the calibration has not been tested upon bright extended sources. Events from an extended source will have a different positional distribution compared to point source. When combined with the modulo-8 fixed pattern, the coincidence correction of extended sources will come with an unquantified systematic error.- Magnitudes per unit area of extended sources are not reported in the catalogue
Reported magnitudes are integrated over extraction apertures and the aperture sizes are not recorded in the catalogue; for point sources these are either 17.5" radius circles (UVW2, UVM2, UVW1) or 6.0" radius circles (U, B, V). For extended sources it is not possible to accurately convert magnitudes to magnitudes per unit sky area. This can be done in an approximate way however using the major and minor axis Full-Width Half-Maximums recorded in the FILTER_MAJOR_AXIS and FILTER_MINOR_AXIS catalogue columns.- Bias of unknown origin in size of extended sources
The size and morphology of a source on the sky is represented by a 2-dimensional Guassian, calculated from moments. The FWHM of the minor axis, FWHM of the major axis and orientation of the axes are recorded in the catalogue. From the distribution of the position angle of the source