XMM-Newton Science Analysis System


omichain (omichain-1.74.10) [xmmsas_20230412_1735-21.0.0]

Processing of the mosaiced sky-images

This functionality was introduced into SAS 9 to allow source-detection on the mosaiced sky-images. By default this is set, via the optional parameter processmosaicedimages, to true.

The way in which the mosaiced sky-images are processed is shown in Figure 4 and it works in the following way:

1. Loop through each OM filter present and:
   • Run omdetect on the mosaiced sky-image file to produce an output FITS source-list file. The SRCLIST FITS table in this file will contain RA and Dec coordinates, computed using the WCS keywords in the FITS header. The photometry will have been done using the mosaiced EXPOSURE image in the sky-image file, and no corrections will have been done for coincidence-losses.

   • Run ommag on the source-list file to add instrumental magnitudes to it.

   • Run omqualitymap to set source-quality flags using the QUALITY image in the mosaiced sky-image file.

2. Run omsrclistcomb, using these new source-list files (one per filter) as the input files, to produce a second observation source-list file.

3. Run ommergelists to create a third observation source-list file from the merging of the first two.

As it has been already noted in section 4, running omichain on a large mosaic field (more than around 2 degrees x 2 degrees), the source detection process may encounter memory limits when processing the mosaic images, and fail to complete. Large OM mosaics often have low filling factors and little overlap of exposures, and therefore add little in terms of source detection beyond that achieved from the individual exposure processing. In the case of large mosaics, therefore, it may be advisable to avoid source detection by setting processmosaicedimages=n. Where source detection on mosaics is performed, new detections will be added to the final observation source list.