  We 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 in the period Feb 2008-Jun 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 non-photometric conditions. The
sky at Mt.Graham is ~0.32 mag/arcsec^2 brighter at airmass ~1.4 than at zenith
but no significant trend was found with the time of the 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 Canary Islands.
