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: http://xmm.vilspa.esa.es/sas/7.1.0/doc/omsource/node107.html
Дата изменения: Mon Jul 9 08:09:26 2007 Дата индексирования: Sat Dec 22 12:03:32 2007 Кодировка: Поисковые слова: martian surface |
Extended-source photometry cannot be done in the same was as is done by omdetect, because information about the sources that omdetect used is not available to omsource. Basically, omdetect uses the pixels assigned to a source to compute source counts from the source, and the value obtained obviously depends on the ``thresh-hold level'' (please see the omdetect documentation for more information).
Omsource uses each pixel within the aperture to obtain the source counts. For each pixel within the aperture a coincidence-loss correction factor is computed by assuming this pixel is centred on a point-source and then computing the correction-factor as for a point-source. The pixel value is then corrected using the correction value. The background is computed from the pixels within the background annulus, which can be elliptical. The apertures can be elliptical, and the user can fix the ellipse parameters (the semi-minor axis, ellipticity and position-angle of the major-axis). Please note that the accuracy of this method has not yet been evaluated.
The background can be determined by one of three ways-
The source photometry parameter values are displayed at the top of the window. In the middle of the window spin-boxes allow the user to change the photometry aperture radius and the inner and outer background annuli. The user can also select the background-determination method