... A common cause of errors in CCD differential photometry is an improper calibration of the array. ... Figure 1: Differential photometry as a function of the position on the CCD for three stars in a field observed with the University of Bochum 0.6m telescope at the ESO La Silla observatory. ... These variations are quite different from star to star. ... Differential data computed between widely separated stars can show errors of up to about 0.1 mag when a night-sky superflat correction is applied. ...