Sky background and related parameters for the winters 2008 to 2011 and for all the winters combined, from left to right. From top to bottom: sky background as a function of time, sky background distribution, sky background as a function of Sun altitude, epoch RMS σ_e (see text) as a function of sky background, and σ_e as a function of Sun altitude. The sky background histograms were calculated in flux units and are displayed in magnitude units. For the last three rows, we computed an outlier resistant mean using 50 bins across the abscissa range (red lines).