Since Period 79 (April-September 2007), the value of the sky brightness of Paranal is constantly monitored using FORS2 (first in addition to FORS1; since its decommissiong on March 31, 2009, solely with FORS2). The sky brightness is measured from the reduced SCIENCE images by running a code created by F. Patat (ESO) and described in A&A 401, 797 (2003). The code has been integrated in the DFO system to obtain automatically, for each reduced science image, the sky brightness measurements as part of the QC process. Images are scanned to select those best suited for a reliable sky brightness measurement. In this phase, the image sky background is evaluated automatically and those images affected by a very noisy backgrounds, by enhanced objects crowding, by saturation problems and by reflections or ghost images are filtered out. The measured sky brightness values (in instrumental magnitudes/arcsec sq.) were then calibrated on-the-fly using the photometric zeropoints obtained for photometric nights within the last 3 months, using the current corrections for the atmospheric extinction and for the instrument color-terms using the closest in time values logged in the QC1 database table fors_ext_color. Corrections for the contributions of the zodiacal light and of the moon are determined, but not applied.
Most of the sky brightness monitoring was done by processing science frames. With the rescoping of the QC group processes, science processing was terminated on Sep. 30, 2011, and so was the sky brightness trending. Still, the historical information could be useful.
The correction for atmosperic extinction is not correct, as the sky background is affected differently from stars by the earth's atmosphere - part of the sky background is actually created by the earth's atmosphere. We therefore strongly discourage the use of the columns sky_corr_z and sky_cal_z.
The final values are then automatically ingested in the sky brightness QC1 database.