Recorded CCD counts c from the star HD 35708 as a function of air mass, converted to the stellar magnitudes m = m₀ − log_2.512c with an arbitrary reference point m₀. These counts were obtained with the green filter during the CCD imaging of the Crab Nebula region on nights with stable atmosphere transparency. Solid lines correspond to the fits of each night (see Eq. (B.1)) assuming α = const; their extrapolation to the zero air mass defines the reference calibration constant c₀ for the particular filter used. The RMS of these extrapolations sets the c₀ uncertainty and is ≈0.04 mag for this calibration sample.