Relative error in NSB calculation for one image of subset 1 derived from Rome/PSPT observation taken on 21/08/2000 shown in Fig. 7 S1/1. The NSB was derived with our method and running window median filter width of a) R/ 2, b) R/ 4, c) R/ 13, and d) R/ 20 pixels. Also shown are the values of the rms, mean, mean absolute, and maximum relative differences by comparing image regions within 0.98 R. The colour bars apply to the images below them.