OM data reduction with SAS: image data processing chain

A detailed description of each task can be found in the SAS documentation, both in HTML and Postscript format. A step by step description of the image mode chain and examples of the processing by individual execution of all tasks is given in the SAS User's Guide and also at this location.

OM image data are fully processed by the XMM-SAS Pipeline: for each exposure of a given observation all necessary corrections are applied to the data files. Then a source detection algorithm is used to identify the sources present in the image. Standard aperture photometry is applied to obtain the count rates for all detected sources. These rates are corrected for coincidence losses and dead time of the detector and finally OM instrumental magnitudes and standard colour corrections are computed. A final source list is obtained from all exposures and filters. Astrometric corrections are applied to each source's position and also the whole image is converted to sky co-ordinates space and rotated so as to have the North on the top. Default image windows are also combined to obtain a mosaic (per filter) of the FOV.

In principle there is no need for further data reduction. The observation source list should contain the calibrated data with their errors. However, some checking is convenient to verify the consistency of the data output.

The whole data processing can be repeated easily by any Guest Observer or Archives User, should any calibration file be updated, and what is more important, in case of doubtful results: the pipeline applies default options in all SAS tasks which eventually can be changed by the GO in order to improve the quality of the results. In particular, the source detection is very sensitive to the artifacts which are very common in OM.

When comparing the data files obtained from the standard SSC pipeline with those obtained by running omichain, the user will notice two differences, the PPS files are compressed while the products from omichain are not, and in addition some intermediate files, with name starting with I, are preserved.

We outline here the checking that any user should perform on OM processed data (by the standard Pipeline or by running SAS)  and the use of one of the tasks, omdetect, where the user can modify parameters affecting the source detection and therefore the overall results of the data analysis.

• 1. Checking omichain output products:

• If the user has PPS products from the processing of his/her observation, or from the XSA, then the first thing to do is to overlay the image source list onto the sky image. The task implot will do it:

 

 
 
 
 
 

implot  set=P0125320701OMS002SIMAGE2000.FTZ withsrclisttab=yes srclisttab=P0125320701OMS002SWSRLI2000.FTZ  device='/XW'

This will overplot the detected sources on the corresponding image, and it will allow us to check that all sources are real and that the one(s) we are interested in have been properly identified.
If the background is strongly affected by straylight features, this check is very important
 

                 High resolution default window  (sky coordinates)                                       Low resolution default window (sky coordinates)
 
 
 
• If the user has run omichain then a similar check can be done by displaying with ds9 the image mod-8 corrected, e.g.
P0125320701OMS002IMAGE_0000.FIT

and then loading the corresponding region file I0125320701OMS002REGION0000.ASC  (this file is not a PPS product).




 
 
 
• Inspection of the combined source list (e.g. using fv ) will allow us to check that the sources of interest have been picked up in all the filters where they are visible and that the combined list contains colors and standard magnitudes for them.



 
 
 
 
 

• Check the tracking corrections: although the pointing stability of XMM is very good, one can verify it by examining the corresponding tracking history PDF file



 

• 2. Improving the source detection:

 
• If the source of interest is close to straylight features or to other sources it may not be detected with the default settings of the omdetect task. We have to change some parameters in omdetect: as nsigma , (sigma above background for detection), or minsignificance (minimum significance of source for its inclusion in the source list) (see SAS documentation, or run omdetect -h for details).

 
 
Invoking omdetect with regionfile=your_region.ascwill allow you a fast checking by overlaying the 7.0.0ly detected sources positions on the image with SAOImage using the created region file.

• Alternatively, the interactive task omsource can be used to perform aperture photometry on the desired image.