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: http://xmm.vilspa.esa.es/sas/8.0.0/doc/omdetect/node41.html
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Omdetect classifies a source as being extended if its computed semi-major axis is greater than that of the point-spread function's semi-major axis (computed by using moments), by at least twice the error computed for the former. Each extended source has a group of connected pixels associated with it, each one having a value greater than a minimum height above the corresponding background image pixel. These pixels can be seen by looking at the level image map (parameter levelimage) and are used to compute the source count-rate through the following steps:
Loop through each source pixel and
End of looping through each source pixel.
The raw count-rate is computed by subtracting the total raw background counts from the total raw pixel counts and dividing by the exposure counts. The quantity corrected background counts is computed by applying a coincidence-loss correction to the total raw background counts, using the mean background-counts per pixel and a 12 pixel (unbinned) aperture. This correction is generally negligible. The corrected count-rate is computed by subtracting the corrected background counts from the total corrected pixel counts and divied by the exposure.