Calar Alto Sky: still Dark, still Quiet
The quality of the night sky at Calar Alto Observatory has been extensively analysed using many different observational techniques, and with data gathered between 2004 and 2007. The conclusions indicate that Calar Alto is a dark site with a considerable fraction of useful nights for astronomical observation. The sharpness of the images (what astronomers call “the seeing”) and the overall properties of the night sky at Calar Alto place this observatory among the best sites for astronomical observations.
In the old days astronomical observatories were placed close or even inside towns and cities. However, the scientific drive to study fainter and fainter astronomical objects at the sharpest resolution possible led to the development of larger telescopes, outfitted with more sensitive instruments for faint light detection. This drive has placed modern observatories at high, isolated, and dark locations, carefully selected to guarantee the maximum scientific return from the economical, technical, and manpower efforts invested in their construction.
Following this tendency, Calar Alto Observatory was placed in South-East continental Spain after a careful study conducted from 1968 to 1973 by the institution Max-Planck-Gesellschaft (West Germany). Five main telescopes (0.80m, 1.23 m, 1.5 m, 2.2 m, 3.5 m) were installed at Calar Alto summit by West Germany and Spain from 1975 to 1984, and nowadays Calar Alto remains the most important astronomical facility in the European continent.
Good observing sites have to be dark, with clear air (that is, with low atmospheric extinction), with a good fraction of non-cloudy nights, and stable atmospheric conditions to provide sharp images (good "seeing"). A detailed study just made public analyses the night sky at Calar Alto from a complete set of data that allow to assess all these aspects. This research, to be published in the scientific journal Publications of the Astronomical Society of the Pacific, relies on spectroscopic, photometric, seeing and extinction measurements made at Calar Alto between 2004 and 2007.
The authors, Sebastián Sánchez and collaborators, conclude that "In the optical wavelength range, Calar Alto seems to be a particularly dark site, comparable to Mauna Kea" in Hawaii, where the giant Keck telescopes are installed. The darkness of moonless nights at Calar Alto can be compared even to those at Paranal (in Chile, housing ESO's Very Large Telescope), a result that the researchers consider "remarkable, considering the amount of light pollution present in Calar Alto spectra". They remark that "if the effects of light pollution could be reduced in the vicinity of Calar Alto, it would become a particularly dark site for optical observations."
The transparency of the sky ("extinction" in the astronomical jargon) at Calar Alto shows a seasonal tendency related to Saharian dust, but with a smaller effect than that perceived from the Canary Islands observatories. About 70% of the Calar Alto nights are useful for astronomical observation, and almost one third of these qualify as "photometric", the term used in astronomy to refer to the nights when the best measurements can be achieved. The sharpness of the stellar images (the "seeing") during the last two years has a median value of 0.9 arc-seconds, being slightly better in summer and slightly worse in winter.
In summary, the authors of this study infer that "Calar Alto remains a good astronomical site, similar in may aspects to places where 10 meter class telescopes are under operation or construction." This, linked to the lower construction and operation costs implied by the location in continental Europe, move the scientists to conclude that "this observatory is a good candidate for the location of future large aperture optical telescopes".