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Abstract: We present a characterization of the main properties of the night-sky at theCalar Alto observatory for the time period between 2004 and 2007. We useoptical spectrophotometric data, photometric calibrated images taken inmoonless observing periods, together with the observing conditions regularlymonitored at the observatory, such as atmospheric extinction and seeing. Wederive, for the first time, the typical moonless night-sky optical spectrum forthe observatory. The spectrum shows a strong contamination by differentpollution lines, in particular from Mercury lines, which contribution to thesky-brightness in the different bands is of the order of ~0.09 mag, ~0.16 magand ~0.10 mag in B, V and R respectively. The zenith-corrected values of themoonless night-sky surface brightness are 22.39, 22.86, 22.01, 21.36 and 19.25mag arcsec^-2 in U, B, V, R and I, which indicates that Calar Alto is aparticularly dark site for optical observations up to the I-band. The fractionof astronomical useful nights at the observatory is ~70%, with a ~30% ofphotometric nights. The typical extinction at the observatory is k V~0.15 magin the Winter season, with little dispersion. In summer the extinction has awider range of values, although it does not reach the extreme peaks observed atother sites. The median seeing for the last two years 2005-6 was ~0.90-,being smaller in the Summer ~0.87- than in the Winter ~0.96-. We concludein general that after 26 years of operations Calar Alto is still a goodastronomical site, being a natural candidate for future large aperture opticaltelescopes.

Autor: S.F. Sanchez, J. Aceituno, U. Thiele, D. Perez-Ramirez, J. Alves


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