The Night Sky at the Calar Alto Observatory II: The Sky at the Near-infrared

We present a 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, which contribution to the sky-brightness in the different bands is of the order of �0.09 mag, �0.16 mag and �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 recomendations of 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