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: http://xmm.vilspa.esa.es/sas/8.0.0/doc/omdetect/node5.html
Дата изменения: Wed Oct 22 17:31:25 2008 Дата индексирования: Tue Aug 18 12:12:01 2009 Кодировка: Поисковые слова: cygnus loop |
The task first computes a background image in order to identify pixels that may be part of a source. The task uses two different algorithms for detecting ``point'' and ``extended'' sources. The former algorithm is run first, and for each pixel significantly higher than the local background value various checks are made to see if the pixel could be part of a source. If a likely source is found its position and shape parameters are computed by moments and the pixels assigned to the source are set equal to the source number. Since occasionally the detected source may be ``extended'', the source is classified as either ``point'' or ``extended'' based on a comparison of the width of the source to that of the PSF width. The algorithm makes many passes over the image, using different parameters, to pick up as many sources as possible. This algorithm largely overcomes the problem of locating ``point'' sources contaminated by scattered light features. The pixels are examined in the order of decreasing pixel brightness, and this overcomes other problems due to strong mod-8 patterns, etc.
The latter algorithm works by examining groups of pixels above a background threshold. Various checks are made to ensure that the group of pixels is not just part of a mod-8 pattern or part of a scattered-light feature. If a likely source is found, again its position and shape parameters are computed by moments and the pixels assigned to the source are set equal to the source number. The source is again classified as either ``point'' or ``extended'' from a comparison of its with to that of the PSF width. This is necessary because some faint ``point'' sources may have been missed by the point-source detection algorithm, but picked up by the extended-source detection algorithm. Again, the algorithm makes many passes over the image, using different parameters, to pick up as many sources as possible.
When the detection process is finished a check is made on extended sources to see if they can be split into two or more sources.
Having obtained a list of sources, comprising of various parameters such as positions, extension flags and shapes, aperture-photometry is then performed on each source to produce a count-rate for each source that is consistent with the OM calibration procedure. Omdetect does not compute instrumental magnitudes from these count-rates, and ommag needs to be run using the output source-list to accomplish this. Flags are also added to make the user aware of any potential problems with the photometry of a particular source.
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 5 sigma above background detection threshold for the image.
A more comprehensive discussion of how the program works is given in the section Algorithm.