We present the characterization of additional properties of the night-sky at the Calar Alto observatory , following the study started in Sánchez et al .
( 2007 ) , hereafter Paper I .
We focus here on the night sky-brightness at the near-infrared , the telescope seeing , and the fraction of useful time at the observatory .
For this study we have collected a large dataset comprising 7311 near-infrared images taken regularly along the last four years for the ALHAMBRA survey ( J , H and Ks -bands ) , together with a more reduced dataset of additional near-infrared images taken for the current study .
In addition we collected the information derived by the meteorological station at the observatory during the last 10 years , together with the results from the cloud sensor for the last \sim 2 years .
We analyze the dependency of the near-infrared night sky-brightness with the airmass and the seasons , studying its origins and proposing a zenithal correction .
A strong correlation is found between the night sky-brightness in the Ks -band and the air temperature , with a gradient of \sim - 0.08 mag per 1 ^ { \circ } C. The typical ( darkest ) night sky-brightness in the J , H and Ks -band are 15.95 mag ( 16.95 mag ) , 13.99 mag ( 14.98 mag ) and 12.39 mag ( 13.55 mag ) , respectively .
These values have been derived for the first time for this observatory , showing that Calar Alto is as dark in the near-infrared as most of the other astronomical astronomical sites in the world that we could compare with .
Only Mauna Kea is clearly darker in the Ks -band , but not only compared to Calar Alto but to any other observatory in the world .
The typical telescope seeing and its distribution was derived on the basis of the FWHM of the stars detected in the considered near-infrared images .
This value , \sim 1.0 \arcsec when converted to the V-band , is only slightly larger than the atmospheric seeing measured at the same time by the seeing monitor , \sim 0.9 \arcsec .
Therefore , the effects different than the atmosphere produce a reduced degradation on the telescope seeing , of the order of \sim 10 % .
Finally we estimate the fraction of useful time based on the relative humidity , gust wind speed and presence of clouds .
This fraction , \sim 72 % , is very similar to the one derived in Paper I , based on the fraction of time when the extinction monitor is working .