Ratio between the background map (source-free) and the source map (detected sources + background). Pixels, where the ratio is smaller than 0.8, are displayed. This map is used to assign the positional flag and indicate that the upper limit calculation will be affected by detected sources. If the area used for the aperture photometry intersects or includes such a region, the “close neighbor” flag is set to 1 for this pixel, otherwise 0. The red square highlights the sky region displayed in Fig. 2.