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We present the measurements of sky surface brightness on Mount GrahamInternational Observatory obtained during the first binocular-mode science runsat the Large Binocular Telescope (LBT). A total of 860 images obtained on 23moonless nights in the period Feb 2008-Jun 2008 were analyzed with our dataquality assessment procedure. These data, taken at the solar minimum, show thatMt.Graham, in photometric conditions, still has one of the darkest skies,competing with the other first-class observatories. The zenith-corrected valuesare 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 ~0.1mag/arcsec^2 higher than the median when observing toward Tucson and Phoenixbut it may be up to ~0.5 mag/arcsec^2 higher in non-photometric conditions. Thesky at Mt.Graham is ~0.32 mag/arcsec^2 brighter at airmass ~1.4 than at zenithbut no significant trend was found with the time of the night. We demonstratedthe dependence of the sky background at Mt.Graham on the solar activity for thefirst time. In fact in 2008, at B and V bands, the sky was ~0.3 mag /arcsec^2darker than in 1999-2002. With these results we conclude that Mt.Graham isstill a first-class observing site, comparable to the darkest sites in Hawaii,Chile and Canary Islands.

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