UBVRI twilight sky brightness at ESO-Paranal ?

Context. Twilight studies have proved to be important tools to analyze the atmospheric structure with interesting consequences on the characterization of astronomical sites. Active discussio ns on this topic have been recently restarted in connection with the evaluation of Dome C, Antarctica as a potential astronomical site and several s ite-testing experiments, including twilight brightness m easurements, are being prepared. Aims. The present work provides for the first time absolute photome tric measurements of twilight sky brightness for ESO-Paranal (Chile), which are meant both as a contribution to the site monitoring and as reference values in the analysis of other sites, inclu ding Dome C. Methods. The U BVRI twilight sky brightness was estimated on more than 2000 FORS1 archival images, which include both flats and standard stars observations taken in twilight, covering a Sun zenith distance range 94 ◦ -112 ◦ . 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 fe w km above the sea level.

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