All data is extracted from the observing logs.
The same plot with only photometric or nearly-photometric nights is available here . The gap around days 140-180 is due to the July monsoons and August shutdown. Note the strong seasonal effect, with the Ks levels dropping in the fall and H background levels climbing. The decrease in Ks background can be attributed to the decrease in thermal background, as can been in the plot of telescope temperature vs. background . The slight anti-correlation between J and H background is presumably to the seasonable correlations of date vs. temperature and date vs. airglow.