Source Collation
At this stage of the processing we have a number of individual source tables, one for every image obtained during a pointing. The tables are combined into a single structure, merging astrometry, photometry and quality data of unique sources where necessary.
Astrometry
Detections recorded in multiple tables are considered to be the same source when they meet a positional criterion. If the distance between two point sources r < max{1.7”,3σr} (or r < max{4”,3σr} for bright sources) then the detections are adopted as the same source, where σris the rms uncertainty in the distance. The criteria for extended sources is slightly different, where r < max{3”,3σr}. Source positions and filter-dependent morphologies are refined by adopting mean values, weighted by uncertainty.
Photometry
Separate detections through one filter that have been identified as the same source have their photometric information combined. Count rates and detection significances are averaged without weights, with their errors propagated.
In addition, if several exposures are taken during a single pointing they are combined by summing up the images and re-running the detection algorithm. Sources that were not detected significantly in the separate exposures but are detected in the summed images are added to the catalogue.
Detections of the same source through different filters are appended to one table entry rather than to separate rows in the catalogue.
Quality
Quality flags are propagated and combined for averaged sources detected through the same filter, but not for detections of the same source through a separate filter.
Only sources observed multiple times over a single pointing are combined in the fashion described above. Sources which are observed multiple times during different pointings are not combined, therefore such sources will appear multiple times, once per pointing within the catalogue.
SAS tasks: omsrclistcomb