Sky brightness in AB mag/arcsec² versus time in days for all VISTA observations in the CASU QC tables. Panel a) shows the median NB118 sky brightness, with the median taken over the 16 detectors, i.e. over the 16 individual NB118 filters. Green diamonds: SV, blue triangles: UltraVISTA, red circles: GTO, grey skeletal symbols: standard stars taken at the start of the night (either in evening twilight or in the first 90 min of the night), black crosses: standard stars taken the rest of the time. Each point represents the stack made by CASU of the data coming from one OB and one pawprint position. There are therefore 18 GTO points from the 6 GTO OBs (one per night) each observing 3 pawprint positions. Panel b) shows the (NB118 − J) sky brightness difference for standard star observations taken in these two filters almost simultaneously. Again the median values over the 16 detectors are used, and symbols are as in panel a). Panel c) shows the NB118 sky brightness in the 16 individual detectors/filters minus the median NB118 sky brightness (with the latter being what is plotted in panel a). The median value in bins of one calendar month is plotted. The shown errorbar represents the standard deviation (if it could be computed). A legend is provided on the panel. The sense of the y-axis is such that e.g. detector 13 has a brighter sky and detector 1 has a darker sky. Panel d) is as panel c) but for J. These two panels have an identical scale on the y-axis in terms of mag/arcsec² per cm on the page. The CASU magnitudes were transformed to AB using NB118(AB) = NB118(Vega) + 0.853 and J(AB) = J(Vega) + 0.937.