The small, colored points represent individual scans. Figures 2 and 3 show that the integrated residual distributions have tails towards larger values. The vertical error bars on these points represent the RMS difference from the mean of the trimmed sample in each bin. The two figures above show a weak dependence of zero point measurement uncertainty on airmass. Linear fits of the trimmed average sigma versus airmass, weighted by the RMS in each bin yields the following relationships for each band.
Sigma is the mean zero point uncertainty, in magnitudes, X is the airmass.
The first two numbers in parentheses are the uncertainties in each term in the fit, and the third is the reduced chi-squared. These linear fits for each hemisphere are included in Figures 5 and 6. The weak dependence of zero point uncertainties on airmass does not appear to be sufficient to de-emphasize the high airmass calibration scan measurements. The nightly zero point offsets in each band are derived from either the mean or linear fit versus time of the individual scan offsets. Individual scan zero point offsets can obviously differ from the nightly values.