The best observing sites on Earth
ALMA is located at a truly unique and unusual place: the Chilean Atacama desert. While the astronomers will operate the telescope from the Operations Support Facilities (OSF) Technical Building, at 2,900 metres above sea level, the array of antennas will be located at the Altiplano de Chajnantor, a plateau at an altitude of 5,000 metres.
Satellite image of the Chajnantor area, 8 June 1999
Of course, this location was selected because of many well justified scientific reasons, particularly dryness and altitude. The ALMA site with the average annual rainfall below 100 mm is the perfect place for a new telescope capable of detecting radio waves just millimeters in wavelength. Indeed, radio waves penetrate a lot of the gas and dust in space, and can pass through the Earth’s atmosphere with little distortion. However, if the atmosphere above ALMA contained water, the radio signals would be heavily absorbed – the tiny droplets of water scatter the radio waves in all directions before they reach the telescope, and would degrade the quality of the observations.
Furthermore, the flat and wide land at the ALMA site is suitable for the construction of a large-scale array.
Considering these aspects, the ALMA Observatory will not only be unique because of its ambitious scientific goals, and the unprecedented technical requirements, it will also be unique because of the very specific, harsh environment and living conditions in which the most challenging radio telescope array will operate with high efficiency and accuracy.
Besides, the host country, Chile, is very cooperative in accepting ALMA project, and has offered the use of their land for many telescopes so far.
Last but not least, another reason why Chile was chosen for the site is that there are a lot of important and unique objects in the southern sky, including the center of the galaxy and Large and Small Magellanic Clouds.
