XMM-Newton Science Analysis System: User Guide

6.2 Description of the OM image data processing chain omichain

The processing of OM data with SAS is run by the pipeline at the SOC. Users can also process their data at their home institute using SAS. Three processing chains, omichain, omfchain and omgchain are used for image, fast mode data and grisms spectra processing, respectively. The chains are Perl scripts that concatenate the individual SAS tasks so as to go from the input raw data up to the final output results without further interaction. The tasks can also be run one by one having more control on all needed parameters.

In the following pages we describe this processing paying attention to the concepts and functions implemented in the different tasks to accomplish all corrections and calibrations necessary to exploit OM data. A detailed description of the tasks and the parameters controlling their functionality is provided in the SAS on-line documentation and processing threads.

It should be noted that the pipeline does not run the processing chains we describe, but an equivalent concatenation of the same SAS tasks.

The whole processing of image and fast mode data is depicted in figure 50 and figure 51. Figure 52 gives an overview of the reduction of spectral data.

In order to process the OM image data successfully, an input dataset is required. Input datasets are not changed during the OM processing with omichain. They are copied to intermediate datasets and keywords. Extensions are added to the FITS data files as needed. Intermediate task products, which are passed from one task to another, are generally not described.

The processing of OM image data can be divided in three parts (see figure 50),

• Data preparation. Before real processing, if present, the four files composing a full frame low resolution exposure are combined into a single one using omcomb. Also a flat-field for later usage is obtained with omflatgen.

• Image processing. All corrections, source detection, astrometry and photometry are applied to each image file, exposure by exposure. omichain will process, automatically, all image data consecutively. However, the user can process single image files by applying the different tasks one by one in a semi-interactive way.

  This core of the image processing can be divided as well,

  • Preparation of tracking corrections
  • Corrections: bad pixels, fixed pattern, flat-field
  • Source detection, astrometry and photometry

• Combined results. The source lists corresponding to different exposures, using different filters, are combined with omsrclistcomb. Colour indices are also calculated. The images corresponding to different windows observed with the same filter are mosaiced (or stacked) in a single image file using ommosaic. Mosaiced images may be source-searched for fainter sources and the results merged into the combined source list.

As can be seen in the flow chart of the OM processing chain (figure 50), the OM tracking information for each exposure is treated before, and independently from, the image data processing. In the following, for each task description, a reference is given to the analysis step in the processing example.

Figure 50: SAS processing chain for data acquired in the OM imaging mode.