Distribution of uncertainty on the weighted mean G_BP value as a functionof the same G magnitude. The lines shown are equivalent to those of Fig. 9. In this case, the plot is restricted to all sources with 10 G_BP observations. The green line shows the expected uncertainties for sources with 10 G_BP transits and for a nominal mission with perfect calibrations. The red line shows the same uncertainty function, but with a calibration uncertainty of 5 mmag added in quadrature to the individual observations. The dashed black line has a slope of 0.4 and indicates that the faint end is sky dominated.