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: http://xmm.vilspa.esa.es/sas/8.0.0/doc/omdetect/node7.html
Дата изменения: Wed Oct 22 17:31:25 2008 Дата индексирования: Tue Aug 18 12:12:02 2009 Кодировка: Поисковые слова: enceladus |
The parameter nsigma (default value=2 controls the minimum detection threshold,
ie all pixels with a value greater than or equal to background value + nsigma 
background noise
will be considered to be part of a possible source. For values greater than 2, the part of the
algorithm that searches for very faintsources is bypassed, and when you are only
interested in the relativley bright sources then it is a good idea to set this parameter
to 2.01, which will significantly decrease the execution time (this will cause the algorithms
that specifically look for very faint sources to be bypassed).
The parameter minsignificance (default value=2) controls what sources are kept in the source-list- all detected sources with a computed significance less than or equal to minsignificance are discarded. If this parameter is set to 0 then all the detected sources will be kept in the source-list. Of course, the lower the value of minsignificance the more spurious sources may be in the source-list. If it is thought that there are too many spurious sources, then it is a good idea to re-run omdetect with a minsignificance value of 3, or maybe even 4.
The parameter detectextended determines if the algorithm is to search for ``large'' extended sources (true), or not (false). If you are not interested iin such sources, a considerable amount of computing time can be saved if this parameter is set to false.
Simulations, and checks on odfs, have shown that this latest version of omdetect produces significantly fewer spurious sources compared to earlier version and also a significant improvement on faint-source detection, due to a couple of minor program tweaks.