Top panels: distribution of the Mahalanobis distances of all test sources as a function of G-band magnitude. The grey horizontal line indicates the mean of the chi distribution. Bottom panels: histograms of the Mahalanobis distances for sources with G <  10 mag (grey) and G >  16 mag (green). The red line is the corresponding chi distribution. The left-hand side plots are for the first five coefficients, with indices 0–4, and the right-hand side plots are for the five coefficients of highest order, with indices 50–54.