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: http://xmm.vilspa.esa.es/sas/6.5.0/doc/omdetect/node3.html
Дата изменения: Tue Aug 16 06:31:57 2005 Дата индексирования: Sat Dec 22 08:58:46 2007 Кодировка: Поисковые слова: seawifs |
This task determines point and extended sources in 2 separate operations. In order to detect sources the program computed the mode of the background, and then any pixel with a value greater than or equal to nsigma * backgound mode is considered to be likely associated with a source. In detecting point sources, each such pixel in the image is analaysed to see if it may be centred on a point-source. If a likely point-source is found, the pixels assigned to the source are set equal to the source number and its parameters calculated and stored. All other pixels above the detection threshold are then assumed to be associated with ``extended'' sources.
Having obtained a source list, comprising their positions, sizes, counts and background counts, aperture-photometry is then performed on each source to produce a count value for each source that is consistent with the CAL calibration procedure.
The output file is a source list comprising source positions and error (in pixels), parameters measuring the FWHM of the major and minor axes of an ellipse characterising the source shape, source count-rate and error, psf1 correction (point-spread function correction applied to counts to extrapolate from source extraction area to coincidence-loss area), aperture radius, no of counts associated with an extended source, coincidence-loss correction, significance of the source and flags reflecting whether the source is affected by quality, confusion or extension problems. A count-rate limit is written to the source list file header (keyword CRLIMIT) reflecting the mean nsigma above background detection threshold for the image, where nsigma is a parameter to the task.