Estimate of the effective influence of the noise correlations for the E noise images for test Ic and S/N ≃ 30. The $\overline{r}$ factor is plotted as a function of the side length M of the sub-regions of 2000 noise images and the error bars are the standard error of the mean. In this case, the value of $\overline{r}=1.32$ for large values of M gives the factor by which the S/N is overestimated when ignoring the noise correlations.