Example of the reflection halo of OmegaCAM in an r′-band mosaic image. The position of the reflection halo (marked with the blue dashed line) depends on the location of its parent star (red dashed line) on the instrument’s focal plane. Objects residing within the red and blue dashed circles are excluded from the sample due to the halo contamination. The surface brightness of the area highlighted with the blue circle is ~15 mag fainter than the peak surface brightness of the source star (Capaccioli et al. 2015). In typical seeing conditions, the stars brighter than m_r′ = 8.3 mag have central surface brightness of μ_0,r′ = 9 mag arcsec^-2, which will cause a halo with the surface brightness of 24 mag arcsec^-2. In the studied 4 deg² area, there were ~40 stars bright enough to cause halos which had to be excluded. The colorbar corresponds to the surface brightness in the image in units of mag arcsec^-2.