To compute the S/N and FPF at a given position, we estimate the noise using n sets of reference positions (here: n = 3). The diameter of the circles is equivalent to 1 FWHM of the PSF template, which is not the size of the apertures used, but was chosen to ensure that the reference positions within each set are independent of each other, in the sense that they are further apart than the typical correlation length (which is given by the size of the PSF). We compute the S/N (and FPF) for each set of reference positions before averaging the results across all sets. □