Comparison for the CCD E2V-250 between the expected Poisson noise (red dashed line) and raw PTCs that are corrected by summing covariances up to 4 pixels distance. For a 100 ke⁻ flux level, these correlations add up to 18% of the variance. The corrected PTCs slopes coincide with Poisson law at ≈0.5%, indicating that more than 97% of the correlations are considered by the truncation at a 4 pixels distance of the integral of the correlation function.