Figure 8: The kurtosis of the ADU statistics versus fractional JD of sky flats taken at the Mont Sec observatory during dusk twilight, five days apart. The different symbols refer to the different Johnson-Cousins filters B, V, R, and I. Within each night, the kurtosis stays remarkably constant, allowing for immediate identifications of flats affected by cosmics or clouds. The kurtosis offset in the blue filters visible between the two nights is probably caused by different efficiency in the light-straying process in Earths atmosphere.