XMM-Newton Science Analysis System
omichain (omichain-1.74.9) [xmmsas_20211130_0941-20.0.0]
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:
- 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.
- Run omsrclistcomb, using these new source-list files
(one per filter) as the input files, to produce a second observation
- 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.
XMM-Newton SOC -- 2021-11-30