From Period 65 (April-September 2000) to its decommissioning on March 31, 2009, the value of the sky brightness of Paranal is constantly monitored using FORS1. Since P79 (April-September 2007) the sky brightness is also determined for FORS2. The FORS1 measurements are reported and discussed in Patat (2009, A&A 481, 575). 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 of the day, and corrected for the atmospheric extinction and for the instrument color-terms using the closest in time values logged in the QC1 database. Corrections for the contributions of the zodiacal light and of the moon are also applied.

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.

History

The available data do not cover P67 and P68 due to an operational break.

Since 2005-11-15 we use instead the zeropoints obtained for photometric nights within the last 3 months. Also the corrections for zodiacal light and moon light are provided, but not applied.

Publications about the skybrightness:

• The dancing sky: 6 years of night-sky observations at Cerro Paranal
  F.Patat, 2008, A&A 481, 575
• UBVRI night sky brightness during sunspot maximum at ESO-Paranal
  F.Patat, 2003, A&A 400, 1183-1198 (pdf | ps)
• A robust algorithm for sky background computation in CCD images
  F.Patat, 2003, A&A 401, 797 (pdf | ps)
• Night sky brightness during sunspot maximum at Paranal
  F.Patat, 2004, The Messenger 115, 18 (pdf)