SIMBAD references

A method is presented for estimating the background at a given location on a sky map by interpolating the estimated background from a set of concentric annuli that surround this location. If the background is nonuniform but smoothly varying, this method provides a more accurate (although less precise) estimate than can be obtained with a single annulus. Several applications of multiannulus background estimation are discussed, including direct testing for point sources in the presence of a nonuniform background, the generation of surrogate maps for characterizing false alarm rates, and precise testing of the null hypothesis that the background is uniform.