Examples of the calibration procedure on synthetic images from subsets 1–4 (from top to bottom) produced by Rome/PSPT image taken on 21/08/2000. From left to right: 1: density images, 2: imposed backgrounds, 3: calculated backgrounds, 4: calibrated contrast images, 5: pixel-by-pixel relative errors in NSB, and 6: relative errors in calibrated contrast images. Here we show the following cases: subset 2: – CC with slope of 0.1; subset 3: combination of the strongest under- and over- exposure (case 10 underexposure and 10 overexposure in Fig. A.5); subset 4: disc diameter of 1100 pixels. Also given (below the images in Cols. 5 and 6) are the values of the rms, mean, mean absolute and maximum relative differences within the disc up to 0.98 R.