Observing Conditions at Mt.Graham: VATT UBVR Sky Surface Brightness and Seeing Measurements from 1999 through 2003

Abstract

We present measurements of sky surface brightness and seeing on Mt.Graham obtained at the Vatican Advanced Technology Telescope (VATT) during 16 observing runs between April 1999 and December 2003. We show that the sky surface brightness is significantly darker during photometric conditions, and can be highly variable over the course of a single observing run as well as from one run to the next, regardless of photometricity. In our photometric observations we find an average low-airmass (sec z < 1.2) sky surface brightness 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 sky surface 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 in part to the strict dark-sky ordinances in place in Tucson and Safford. We expect the sky over Mt. Graham to be even darker than our 1999--2003 results during solar minimum (2006--2007). We find a significant improvement of about 0.45 arcsec in our measured stellar FWHM after improvements to the telescope were 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 0.97 arcsec to 2.15 arcsec. Significantly sub-arcsecond seeing was occasionally achieved with values as low as 0.65 arcsec FWHM in R. There may possibly still be a significant telescope contribution to the seeing at the VATT, but nearby trees as high as the dome are currently the dominant factor.