Relative sensitivity curves normalized at 6000 Å from simulated (noise free) standard star observations with PPak at a seeing of 1.2′′. Single pointing observations were simulated for a grid of pointing positions in α and δ with respect to the standard star. Simulations were performed for airmasses of 1.5 (gray), 1.3 (green), 1.1 (blue), and 1.0 (red) to take into account the change in the differential atmospheric refraction. The blue-to-red photometric calibration accuracy achievable is not sufficient even at low airmasses simply because of wavelength dependent aperture losses.