The night sky at the Calar Alto observatory

We present characterization of the main properties of the night sky at the Calar Alto observatory for the time period between 2004 and 2007. We use optical spectrophotometric data, photometric-calibrated images taken in moonless observing periods, together with the observing conditions regularly monitored at the observatory, such as atmospheric extinction and seeing. We derive, for the first time, the typical moonless night-sky optical spectrum for the observatory. The spectrum shows a strong contamination by different pollution lines, in particular from mercury lines, whose contribution to the sky brightness in the different bands is of the order of similar to 0.09, similar to 0.16, and similar to 0.10 mag in B, V, and R, respectively. Regarding the strength of the sodium pollution line in comparison with the airglow emission, the observatory does not fulfill the IAU recommendations for a dark site. The zenith-corrected values of the moonless night-sky surface brightness are 22.39, 22.86, 22.01, 21.36, and 19.25 mag arcsec(-2) in U, B, V, R and I, which indicates that Calar Alto is a particularly dark site for optical observations up to the I band. The fraction of astronomical useful nights at the observatory is similar to 70%, with similar to 30% of photometric nights. The typical extinction at the observatory is mag in the winter season, with little dispersion. In summer the extinction has a wider k(v) similar to 0.15 V range of values, although it does not reach the extreme peaks observed at other sites. The analysis of the winter and summer extinction curves indicates that the Rayleigh scattering is almost constant throughout the year. The rise of the extinction in the summer season is due to an enhancement of the aerosol extinction, most probably associated with an increase of dust in the atmosphere. The median seeing for the last two years (2005-2006) was similar to 0.90 '', being smaller in the summer (similar to 0.87 '') than in the winter (similar to 0.96 ''). We conclude in general that after 26 years of operations, Calar Alto is still a good astronomical site. Its main properties are similar in many aspects to those of other major observatories where 10 m-like telescopes are under operation or construction, thus being a natural candidate for future large aperture optical telescopes.