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: http://xmm.vilspa.esa.es/sas/6.5.0/doc/omdetect/node11.html
Дата изменения: Tue Aug 16 06:31:57 2005 Дата индексирования: Sat Dec 22 10:08:34 2007 Кодировка: |
When the source-detection algorithms has run omdetect will compute raw count-rates for each detected source. For point-sources (ie those sources having widths consistent with those of the point-spread function) the count-rates are obtained by aperture photometry. The background is subtracted using an annulus region lying just outside the circular aperture region.
These ``raw'' count-rates have to be corrected for coincidence-loss, dead-fraction and the time-dependent sensitivity degradation before instrumental magnitudes can be computed (using the task ommag). The default aperture is 12 unbinned pixels, but for some sources a smaller aperture may be used. For these cases an aperture correction is also applied using the point-spread function. For the filters UVW1, UVM2 and UVW2 a further point-spread function correction is made to extrapolate the count-rates to an unbinned aperture radius of 35 pixels.
For extended sources a different method is used to obtain the count-rates, since the above method is invalid for them. Each pixel of the source region is treated as a point-source and the counts obtained by summing the coincidence-loss corrected pixel counts. Please note that this method is only approximate- a more accurate method would require detailed modelling of the source.
XMM-Newton SOC/SSC -- 2005-08-16