Observing Conditions at Mount Graham: Vatican Advanced Technology TelescopeUBVRSky Surface Brightness and Seeing Measurements from 1999 through 2003

We present measurements of sky surface brightness and seeing on Mt.Grahamobtained at the Vatican Advanced Technology Telescope (VATT) during 16observing runs between April 1999 and December 2003. We show that the skysurface brightness is significantly darker during photometric conditions, andcan be highly variable over the course of a single observing run as well asfrom one run to the next, regardless of photometricity. In our photometricobservations we find an average low-airmass (sec z < 1.2) sky surfacebrightness of 22.00, 22.53, 21.49, and 20.88 mag arcsec^-2 in U, B, V, and R,respectively. The darkest run (02/00 in U and 02/01 in BVR) had an average skysurface brightness of 22.38, 22.86, 21.72, and 21.19 mag arcsec^-2 in U, B, V,and R, respectively. With these results we show that under the best conditions,Mt. Graham can compete with the darkest sites in Hawaii and Chile, thanks inpart to the strict dark-sky ordinances in place in Tucson and Safford. Weexpect the sky over Mt. Graham to be even darker than our 1999--2003 resultsduring solar minimum (2006--2007). We find a significant improvement of about0.45 arcsec in our measured stellar FWHM after improvements to the telescopewere made in Summer and Fall 2001. Stellar FWHM values are highly variable,with median R-band focus FWHM values in each observing run ranging from 0.97arcsec to 2.15 arcsec. Significantly sub-arcsecond seeing was occasionallyachieved with values as low as 0.65 arcsec FWHM in R. There may possibly stillbe a significant telescope contribution to the seeing at the VATT, but nearbytrees as high as the dome are currently the dominant factor.Comment: 22 pages, 11 figures, to appear in PAS