Calar Alto Sky: excellent not only in the vertical direction (photo release)

Calar Alto nights enjoy excellent conditions and extremely clear skies. Especially in the winter season, clouds and fog very often stay below the observatory's height of 2168 m above sea level. Prominent topographic features like the mountains of Sierra Nevada, approximately 70 km to the west of Calar Alto, appear as clear and sharp as if they were next door, and on a good night one can see Africa towards the south. In this photo release we offer two recent photographs that illustrate these facts.

The first image was taken the 9th of November 2008 from the catwalk of the 2.2 m telescope, facing west, and shows the moonlit and snow-covered highest peaks of Sierra Nevada mountain range at night. The most outstanding summit is Mulhacén, the highest point in the Iberian Peninsula (3482 m). The total exposure time of this image was 5 minutes, causing stars to appear as trails due to the rotation of the Earth. These trails are visible almost down to the horizon, a proof of the excellent transparency of the air. A cloud layer can be seen at the bottom of the image, far below the mountains, filling the Nacimiento river valley, that separates Sierra Nevada from Filabres range, where Calar Alto Observatory is located.

The second image was taken in the early hours of November 14th 2008 in a cloudless night, also under moonshine, but towards the opposite direction (East-South-East). Here the horizon is defined by the sea at a distance of about 170 km. One can clearly see features at the local coast at a distance of 65 km, moving ships on the sea (appearing as trails due to the long exposure time) and, above all, a giant thunderstorm cloud. Though it may appear to be close, this thunderstorm was actually located at the coast of Algeria, close to the city of Mostaganem, at a distance of 270 km from Calar Alto. The location of the cloud was evaluated using Meteosat weather satellite image data, and the distance independently confirmed by assuming a typical maximum height of 10 km for the top of the thunderstorm clouds. This means that the flashes extend below the local horizon and may well hit not the sea but, actually, the African continent.  

The photographs were obtained by Felix Hormuth from the Max Planck Institute for Astronomy and currently a staff astronomer at Calar Alto with a Canon EOS 450D digital camera, using focal lengths of 60 mm for the thunderstorm and 100 mm for the image of Sierra Nevada, at a sensitivity of ISO 1600. Both images are stacks of 10 single exposures with 30 s exposure time each.

High resolution images:

Sierra Nevada photographed from Calar Alto Observatory.

Thunderstorm over the coast of Algeria photographed from Calar Alto Observatory.