Sky surface brightness at Mount Graham II. First JHKs science observations with the Large Binocular Telescope

•Sky brightness has stronger airmass dependency in the J, H bands than at the Ks-band.•A correlation was found between the J-band sky background and the fraction of the night.•At 10 deg from the moon, the J-band sky background can be 0.11 mag brigther.•Correlation found between the Ks-band sky-brightness and the air temperature.•At the Ks-band Mt. Graham is the darkest site in the world together with Mauna Kea.

We studied the near-infrared sky-brightness at J, H and Ks-bands as derived from the data taken during the first year and a half of routine science operations of the Large Binocular Telescope (LBT). This is the first comprehensive study of the near-infrared night sky-brightness ever conducted at the Mount Graham International Observatory (MGIO), based on a large dataset comprising 4699 near-infrared images taken in 52 nights. We analyzed the dependency of the near-infrared night sky-brightness with the airmass, the season and the moon phase and distance. The average night sky-brightnesses (dispersion) in the J, H and Ks bands scaled to the zenith is 15.82 mag/arcsec2 (0.21), 14.29 mag/arcsec2 (0.26) and 13.4213.42 mag/arcsec2 (0.32) respectively. Those values were derived for the first time at this observatory. At the J-band we found a tendency of the sky background to get darker by ∼0.35 mag at the end of the night with respect to the evening twilight. Also in the J-band we found that the sky background can be up to ∼0.11 mag brighter when observing at 10°10° distance from the full moon. A correlation was also found between the night sky-brightness in the Ks-band and the air temperature with a gradient of -0.06-0.06 mag per 1°C of temperature increase. If we compare the average sky brightness of the major observing sites we find that, at J-band, Mt. Graham is quite similar to the major sites but it quickly becomes the second darkest place at the H-band and definitely the darkest observing site at the Ks-band together with Mauna Kea.