UBVRI night sky brightness during sunspot maximum at ESO-Paranal

In this paper we present and discuss for the first time a large data set of UBVRI night sky brightness measurements collected at ESO-Paranal from April 2000 to September 2001. A total of about 3900 images obtained on 174 different nights with FORS I were analysed using an automatic algorithm specifically designed for this purpose. This led to the construction of an unprecedented database that allowed us to study in detail a number of effects such as differential zodiacal light contamination, airmass dependency, daily solar activity and moonlight contribution. Particular care was devoted to the investigation of short time scale variations and micro-auroral events. The typical dark time night sky brightness values found for Paranal are similar to those reported for other astronomical dark sites at a similar solar cycle phase. The zenith-corrected values averaged over the whole period are 22.3, 22.6, 21.6 20.9 and 19.7 mag arcsec(-2) in U, B, V R and I respectively. In particular, there is no evidence of light pollution either in the broadband photometry or in the high-airmass spectra we have analysed. Finally, possible applications for the exposure time calculators are discussed.