How is the background map produced?
The image is made by excluding all point sources from the data (essentially, the point sources are cut out of the image, using a hack saw, perhaps?). A 2-D spline fitting is performed on what remains. This is the background map. The quality of the background map will depend upon the strength and variation of diffuse emission in the field, the count density (i.e., the exposure times surface brightness), and the density of removed sources (deep exposures of rich fields can have much of the area removed). Cross-like structures can be produced if there are relatively strong gradients in the diffuse emission, as the spline tries to fit the gradients.
The processing pipeline, since 3/92, has used a different algorithm to handle the background. This affects observations processed since that date, which, due to the backlog at the time in processing, actually affects data obtained sometime between 5/91 and 11/91 to the present. The new background maps no longer show rib structure (in contrast to the old approach). The procedure removes the point sources found with the local detect algorithm, then divides the image by the exposure map, and applies a spline fit to the result. The background image is re-constructed from the spline fit.
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