XMM-Newton Science Analysis System


ombackground (ombackground-0.7.5) [xmmsas_20230412_1735-21.0.0]

Description

The main purpose of the program is to compare the default OM background-determination algorithm that omdetect uses when performing aperture-photometry with the mean, median and mode estimators.

The idea is that if you have an image with no sources and you compute the background-subtracted counts within square apertures placed on the image, the mean of these values should be zero for a good background algorithm. The program produces various plots that are stored in the pdf file specified by the parameter plotfile.

1. Graph 1 A histogram of the distribution of pixel values on the image specified by the parameter imageset. Various estimates of the mean background-counts per pixel (using all the good pixels) are computed.

2. Graph 2 Histograms of the distribution of computed background-counts per pixel for the various background algorithms, using square apertures that try to avoid source regions.

3. Graph 3 Histograms of the distribution of computed background-subtracted counts per pixel for the various background algorithms, using square apertures that try to avoid source regions.

4. Graph 4 Histograms of the distribution of computed background-counts per pixel for the various background algorithms, using square apertures that try to avoid source regions.

5. Graph 5 Histograms of the distribution of computed background-subtracted counts per pixel for the various background algorithms, using square apertures that do not try to avoid source regions.

From SAS 10 onwards it is possible to change the default background-determination method that omdetect use through its parameter backgroundmethod. It is not advised to change it without very good reasons.

If required, the program will compute a background image and store it in a FITS file- the algorithm used is nearly identical to that used by omdetect.