Distributions of the S/N in a sky image after deconvolution of a cleaned shadowgram for different radii used to exclude the sources. Left column: distribution of the S/N in the 20 s sky image of Fig. 13 (centre). Right column: distribution in 20 min sky image (same Fig., right). The red dashed lines correspond to a Gaussian function fitted to the distribution. With these distributions, the threshold to be applied for source detection would be 1.15 × 6.5 = 7.5 for the 20 s case, and 4.86 × 6.5 = 31.6 for the 20 min case, as indicated by the red vertical lines.