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Description of the Algorithms

For all detections produced by SASS, the preparation routine reads various SASS parameters and sets flags, depending on the values of single parameters or combinations of parameters. For example, the distance to the nearest (PSPC) rib is determined, and sources that are too close (and therefore strongly affected by the wobble of the satellite) are flagged both for suspect position and intensity. Also, the nearest neighbor to each source is found, and if it is so close that the flux determination is affected, the intensity flag is set (to both sources, in this case). This preparation step can also set sequence flags, e.g. in case the background is suspect.

Specifics of the tests and decisions can be found in the flagset documents, but we note here that some of the decision trees are quite complex. For the HRI, each field has 5 separate SASS detection outputs, each with a different size detect cell. Each detection, regardless of detect cell size, is assigned a source number. Obviously we wanted to deal with a given source, and not evaluate each separate detection of the same source. Hence we devised the 'u' flag, which is the answer to the question ``Is this source not unique (aka un-unique)?''. If the SASS processing made a tentative association of a given detection with a detection achieved with a smaller detect cell, then the smaller cell detection is considered to be the unique detection, and the larger cell detections receive a u=T flag. This procedure is quite satisfactory for most sources. However, occasionally, the smaller cell detection does not pass the S/N test, and is assigned n=T, but its larger cell detection exceeds the threshold, and thus becomes the detection of interest.
Wed Jun 3 11:20:57 EDT 1998