Left: relative difference between the NSB and the imposed background (within 0.98 R) for all the synthetic data of subset 8. RMS difference (green), mean absolute difference (blue), mean difference (orange) and maximum difference (red). These differences are plotted vs. the date on which the original Rome/PSPT images (that were randomly distorted) were recorded. Note that the maximum difference values have been divided by ten to plot them on the same scale as the other quantities. The solid lines are 100 point averages and the shaded surfaces denote the asymmetric 1σ interval. Right: distribution of the relative difference values.