Azimuth (a) and zenith distance (b) random projection errors as a function of the calibration sample size (N_*). For zenith distance, the data are presented for z <  60° (blue) and z >  60° (red) separately to outline the plate scale degradation close to the horizon, whereas azimuth data (green) do not show any systematic dependence. The points represent the daily and monthly data, and the straight lines show the square root dependence.