One fifth of the world population can no longer enjoy the sight of the night sky with the naked eye: close to urban and industrial areas, light pollution allows one to see only a few stars. It is a well known fact: anyone can perceive it, whenever looking up to the sky. If asked “which is the ideal site for astronomy?”, many would answer “far away from big cities.” Right, but not enough. Because escaping from light pollution is not the only problem to handle, in this case.
The key instrument in astronomy is the telescope. As the name itself says, its purpose is to look at objects which are located far away. In order to observe distant stars and galaxies, a telescope has to catch as much light as possible: this means that very large mirrors (currently with diameters of order 10 m, but up to 100 m for future generations) and very long exposure times are needed to achieve a good image. Unfortunately, before reaching the telescope, the light rays coming from stars and galaxies have to pass through the atmosphere of the Earth. Here, the cosmic light interacts with the turbulent atmosphere, thus losing track of much information about the objects (stars or galaxies) it comes from.
The atmosphere is not static nor homogeneous, but extremely turbulent. At every instant, a star or galaxy produces a slightly different image, according to the varying characteristics of the portion of atmosphere overlooking the telescope. A long exposure, needed to capture as much light as possible, is a superposition of the individual images obtained in several, consecutive instants. Because of atmospheric turbulence, the image resulting from such a superposition is blurred: this phenomenon is referred to as seeing.
It goes without saying that modern astronomy needs accurate and precise images, and the limiting effects due to the atmosphere have to be taken under control. Hence, in the last decades telescopes are being built in dry regions, preferably at high altitude, close to oceans or deserts, so that the surrounding air temperature is as constant as possible and the turbulence minimal. This is the reason why the ideal sites to build cutting-edge astronomical observatories are volcanic islands, such as Hawaii or the Canary Islands, and desertic plateaux, such as the Atacama desert in Chile.
An even more radical solution to remove the effect of seeing is a telescope orbiting around the Earth, outside of the atmosphere: though rather expensive, this formula has already been pioneered with the Hubble Space Telescope, and is the subject of several projects for the upcoming future.
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The photo, taken by Bobb Tubbs, shows the European telescopes in La Palma, on the Canary Islands: at an altitude of over 2,400 m, the observatory is higher than the clouds and the seeing is one of the best in the world.Translated from Il Denaro, 22.01.2009
