MEASUREMENTS OF SKY BRIGHTNESS AT THE CALAR-ALTO-OBSERVATORY

We report measurements of the sky brightness at the Calar Alto Observa tory during 18 moonless nights in the years 1989, 1990, 1991, and 1993 . Most of the measurements were performed in eight intermediate band f ilters between 350 nm and 820 nm. A set of measurements was obtained i n the broad band UBV RI filters during three nights in 1990. Typically 10 to 20 readings were obtained each night for one or two fixed posit ions (fixed in or, delta), selected to be free of stars down to a limi ting magnitude of about B = 20. The sky brightnesses were calibrated u sing standard stars. We discuss the sky brightness variations both dur ing individual nights and from year to year and compare the observed s ky brightnesses to measurements from other observatory sites. 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.