Panel (a): chip #79 reduced OmegaCAM image of M22. Panel (b): q_FIT vs. u_SDSS of sources measured in chip#79, with the orange lines separating the stellar (black) from the excluded (gray). Panel (c): chip #81 and #89 footprints, colored in gray and teal, respectively. Panel (d): zoomed-in view of their coordinates (gray dots and teal crosses) in the overlapping area. Panel (e): chip #81 and #89 magnitude difference against u_SDSS of chip #81. Gray and red lines indicate the zero-difference and the observed offset, respectively.