Sky brightness at the ESO La Silla Observatory 1978 to 1988 ^*

¹ Observatory, P.O. Box 14, FIN-00014 University of Helsinki, Finland
² Astronomisches Institut, Ruhr-Universität Bochum, D-44780 Bochum, Germany

Send offprint request to: K. Mattila

Received: 4 November 1995
Accepted: 13 January 1996

Abstract

We have measured the sky brightness at the European Southern Observatory (ESO), La Silla, during 40 moonless nights in the years 1978 - 1988. Most of the measurements were performed in five intermediate band filters between 3500 and 5470 Å, including the Strömgren , and y bands. In addition we made measurements in the uvgr and BVRI photometric systems. Typically 20 to 40 readings were obtained each night at a fixed (α, δ) position, selected to be free of stars down to a limiting magnitude of 20. The sky brightnesses were calibrated using standard stars. We discuss the sky brightness variations both during individual nights and from year to year. We have found that the long-term variations are well correlated with the solar activity, for which the 10.7 cm radio flux is used as a measure. We compare the sky brightnesses as observed at La Silla with the published values for several other sites. Finally, based on the tight correlation of the airglow variations in two pass bands near 4000 Å  we propose a new method for the separation of airglow and zodiacal light contributions.