Histogram of the number of analyzed nights. The four different fields F1, F2, F3, F4 are colored blue, red, yellow and green, respectively. Periods marked in gray show the 61 days (from April 1 to May 31) during which M31 can hardly be observed (see also Table 1). Our most complete seasons (2000/2001 and 2001/2002) are marked with the labels “00–01” and “01–02.”