XMM-Newton Science Analysis System


omichain (omichain-1.74.10) [xmmsas_20230412_1735-21.0.0]

Processing of the data for each filter

The omichain cycles through the filters in the filter list and for each one processes each exposure in the exposure list for that filter. The following summarizes the steps taken by the omichain in this stage.

1. First it checks to see if the exposure is a full-frame low-resolution mode image (Engineering 2), and if so it runs omcomb to combine the four separate image segments into a single full-frame image, which is then processed instead of the individual image segments.

2. The tracking-history pipeline (please see Figure 2) is first run to produce tracking history information using omprep, omdrifthist and omthconv.

3. The imaging pipeline (Figure 3) is then run to process the image for this exposure, and the following programs are executed in turn:
   1. omprep - converts the image from integer to real and adds some key-words to the header.
   2. omcosflag - adds the quality array to the image.
   3. omflatfield - flatfields the image.
   4. ommodmap - attempts to correct for mod-8 patterning.
   5. omdetect - detects sources, computes source positions, source moments (semi-major, minor and position angle), raw and corrected count rates, and sets some flags for each source.
   6. omqualitymap - transfers source quality-flag information from the source-list table to the QUALITY image of the image file.
   7. ommag - computes instrumental magnitudes for each source.
   8. omatt. - computes celestial coordinates (Right-Ascension and declination) from the X and Y pixel coordinates and produces a “sky-image”. If a USNO catalogue fits file is available, and the parameter usecat is set to true then it will also correct the astrometry for any offset between the OM and USNO RA and dec axes, adding the columns RA_CORR and DEC_CORR to the source-list. Details about the catalogue fits file are given in the documentation on omatt and omsrclistcomb. It should be noted that in the case when the USNO subset catalogue is not available but the parameter usecat is set to true, then the task omatt will attempt to generate its own subset of the USNO catalagoe by using the scat tool from the package WCSTOOLS, which in this case should be installed on the user's computer. If the package WCSTOOLS is not installed, the task omatt will skip the aspect-correcting stage of processing and the output image and source lists will remain uncorrected.