Examples of potential analysis fields. Upper panel: high significance source (PCCS2 857 G172.20+32.04). The histogram (shown in the inset; Y-scale is log) is a mixture of a Gaussian component from the background pixels, plus a strong upper tail generated by the source in the centre. Lower panel: field with no detected sources in it (PCCS2 857 G172.20+32.04; Y-scale is linear). This time only the Gaussian component is present. The tails of the distribution are compatible with “just background”. Each field is 25 × 25 pixels (1 pixel ). The pixel intensities (Eq. (A.13)) are unitless.