Sky level in e − s-1 px-1 (with 1 px being about 0.34′′ on the side) as function of time for the 6 half-nights of the GTO observing run. The data were taken in either NB118 or J band; the J-band sky level values have been scaled down by a factor of 20 to fit on the plot, as indicated on the panels. For each exposure, i.e. for each point in time, the sky level in the 16 detectors is plotted; a symbol legend is provided in panel f). In VIRCAM each detector has its own filter, and filter-to-filter variations within the set of the 16 NB118 filters are likely the reason for the large detector-to-detector variations seen for the NB118 sky levels. Dot-dashed lines mark the start of twilight. In panels a) and b), solid grey regions mark episodes where the OBs had to be restarted due to minor technical (software) problems, and dotted lines mark obtained J-band exposures for which CASU did not provide reduced images (see Sect. 3.2). The UT date for the observations in panel (a) is 2010 − 01 − 18, and each subsequent panel is one day later.
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