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POSSIBLY the clearest skies on Earth have been found - but to exploit them, astronomers will have to set up a telescope in one of the planet's harshest climates.

Michael Ashley of the University of New South Wales in Sydney, Australia, and his colleagues wanted to find the best sites for astronomy on the Antarctic plateau. Combining observations from satellites and ground stations with climate models, they evaluated different factors that affect telescope vision, such as the amount of water vapour, wind speeds and atmospheric turbulence.

The team found that the plateau offers world-beating atmospheric conditions - as long as telescopes are raised above its frozen surface. The ice makes the lowest layers of air on the plateau much colder than those above, forming an "inversion layer" that, together with the strong local winds, can lead to severe turbulence. This would blur a telescope's images.

The team's analysis showed the inversion layer is only about 20 metres thick, however. If a telescope was mounted above it, its view would be affected by far less turbulence than at other world-class observatory sites, says Ashley. "It's drier than Mauna Kea [in Hawaii] by a long way and drier than the Atacama desert [in Chile]," he says.

Such conditions would be good for studying star birth. Normally, water vapour in the atmosphere blocks telltale emissions from molecular clouds in star-forming regions of the Milky Way. But the air above the high area known as Dome A is so dry that a ground-based telescope there could observe stellar nurseries - something that's impossible anywhere else on Earth.

So far as Ashley's team know, Dome A seems the best site for astronomy (see map). China has already built a summer station there, with a small robotic observatory. Next best is Dome F, the site of a Japanese station.

But conditions may prove much better about 150 kilometres south-west of Dome A, at Ridge A (www.arxiv.org/abs/0905.4156). "We won't know until we make measurements there," says Ashley.

Life on the plateau isn't easy for telescopes, though. One problem is that ice can form on lenses and mirrors. Marc Sarazin of the European Southern Observatory's offices in Munich, Germany, says the harsh conditions demand the kind of approach used for space missions: "They will be single-purpose, short-lifetime instruments answering a precise scientific question," he says. General-purpose telescopes are best built in temperate latitudes like Chile's, he adds.

Ashley thinks the biggest difficulty lies elsewhere. "The main problems in Antarctica aren't so much the engineering," he says. "It's more convincing people that it is not as scary as it sounds."

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