IRAS Explanatory Supplement
XI. Known Processing Anomalies
A. Processing of Extended ("Cirrus") Sources as Point
Sources
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As discussed in Section V.C, the point source recognizer can be triggered by sources that are of unlimited extent in the cross-scan direction long as they are less than about 1' in the in-scan direction. While this characteristic was recognized early in the design of the software, it was felt that this situation would arise much less frequently than the case where several true point sources triggered many cross-scan detectors simultaneously. The presence of "infrared cirrus" and the extended sources in the Galactic plane means that at 100 µm in particular the sky is dominated by extended structures rather than by clusters of point sources. The detection and confirmation software thus created strings of pseudo-point sources spread along the cross-scan direction.
In retrospect, it would have been better to adopt a different approach. for example, whenever four or more cross-scan detectors were triggered, all such detections should have been discarded from the point source catalog or placed in a special data base to compare different scans to verify that these detections were in fact due to several true point sources. The consequences of the present approach are clear. At 100 µm the point source catalog is unavoidably contaminated by the presence of point-like condensations within the extended emission and by the effects of cirrus on discrete point sources that include missing fluxes at shorter wavelengths caused by band-merging diffculties.
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