Illustration of the adaptive ring method for background estimation for a single observation (see Sect. 3.2.3). The HGPS significance image is shown in inverse grayscale and exclusion regions as blue contours. The analysis FoV for one observation is shown as a black circle with 2° radius and a blackcross at the observation pointing position. The red rings illustrate the regions in which the background is estimated for two positions in the FoV (illustrated as red squares). Only regions in the ring inside the FoV and outside exclusion regions are used for background estimation. For the position in the lower right, the ring was adaptively enlarged to ensure an adequate background estimate (see text).