UBVRI twilight sky brightness at ESO-Paranal

Context. Twilight studies have proved to be important tools for analyzing the atmospheric structure with interesting consequences on the characterization of astronomical sites. Active discussions of this topic have started again recently in connection with the evaluation of Dome C, Antarctica, as a potential astronomical site and several site-testing experiments, including twilight brightness measurements, are being prepared. Aims. The present work provides the first absolute photometric measurements of twilight sky brightness for ESO-Paranal (Chile). They are meant both as a contribution to the site monitoring and as reference values in the analysis of other sites, including Dome C. Methods. The UBVRI twilight sky brightness was estimated on more than 2000 FORS1 archival images, which include both flats and standard star observations taken in twilight and covering a Sun zenith distance range 94 degrees-112 degrees. Results. The comparison with a low-altitude site shows that Paranal V twilight sky brightness is about 30% lower, implying that some fraction of multiple scattering has to take place at an altitude of a few km above sea level.