Sky Surface Brightness at Mount Graham: UBVRI Science Observations with the Large Binocular Telescope

Abstract

ABSTRACTWe present the measurements of sky surface brightness on Mount Graham International Observatory obtained during the first binocular-mode science runs at the Large Binocular Telescope (LBT). A total of 860 images obtained on 23 moonless nights during the period February 2008–June 2008 were analyzed with our data quality assessment procedure. These data, taken at the solar minimum, show that Mt. Graham, in photometric conditions, still has one of the darkest skies, competing with the other first-class observatories. The zenith-corrected values are 21.98, 22.81, 21.81, 20.82, and 19.78 mag arcsec-2 in U, B, V, R, and I, respectively. In photometric conditions, the sky background is ∼0.1 mag arcsec-2 higher than the median when observing toward Tucson and Phoenix. but it may be up to ∼0.5 mag arcsec-2 higher in nonphotometric conditions. The sky at Mt. Graham is ∼0.32 mag arcsec-2 brighter at air mass ∼1.4 than at zenith, but no significant trend was found with the time of night. We demonstrated the dependence of the sky background at Mt. Graham on the solar activity for the first time. In fact, in 2008, at B and V bands, the sky was ∼0.3 mag arcsec-2 darker than in 1999–2002. With these results, we conclude that Mt. Graham is still a first-class observing site, comparable to the darkest sites in Hawaii, Chile, and the Canary Islands.