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Of ELTs and OWLs

European astronomers get serious about a stadium-size telescope.
Robert Adler
OWL views
The European Southern Observatory's Overwhelmingly Large Telescope (OWL) would consist of a 100-meter (330 foot) segmented mirror using adaptive optics. The telescope's protective enclosure has been retracted in this simulated view.
ESO
April 7, 2005
A ground-based telescope up to 100 meters — 330 feet — in diameter could see first light by 2020, a new European study finds. With 100-times the light-gathering power of the Keck, and 40 times the resolving power of the Hubble, a telescope of this class would give observers an unprecedented view of the universe.

Links to ELT projects
OWL

Euro50

Giant Segmented Mirror Telescope (GSMT)

Thirty Meter Telescope (TMT)

Very Large Optical Telescope (VLOT)
Big does not even begin to describe the telescope an international working group of astronomers wants to build. They've set their sights on what they are modestly calling the European Extremely Large Telescope (ELT), a 100m diffraction-limited monster utilizing several thousand modular primary mirrors constantly fine-tuned by fully integrated adaptive optics. Researchers are convinced it can be done.

"Initial studies are very positive, suggesting that a 50- to 100m segmented telescope could be built within 10 to 15 years for a cost of around 1 billion Euros [$1.3 billion]," says Isobel Hook of Oxford University, project scientist for the ELT. Will astronomers be using a giant scope like this within 20 years? "I think it's a certainty," she says.
Eli Atad, who heads the applied optics group at the Astronomy Technology Centre in Edinburgh, Scotland, says that the final design has not yet been chosen. However, earlier work on a 100m telescope called the OWL, for Overwhelmingly Large Telescope, suggests that light will be folded between 4 or 6 mirrors to reduce the telescope's height and minimize the size of the mirrors that will constantly be reshaped to compensate for atmospheric blurring.

Despite the folded light-path, the telescope will still stand 125m (410 feet) tall and weigh between 15 and 20 tons. The structure will glide on an alt-azimuth mount, and will also be actively controlled to maintain alignment and tracking accuracy.

"The largest telescopes we have now already make me feel very small when I stand next to them," says Hook. "It's hard to imagine what it would be like standing next to a telescope 5or even 10 times bigger!"

According to Atad, the telescope could mostly be built with today's technology. However, he says, engineers face "a huge technological adaptive optics challenge" with the 4m active mirror requiring millions of actuators, which in turn will require "extremely complex control and system engineering," plus a dedicated super-computer.
OWL at sunset
Light form the setting Sun glints through OWL's massive superstructure in this computer-generated view.
ESO
While the engineers tackle these challenges, European astronomers are making the scientific case for the ELT. At this week's meeting of the Royal Astronomical Society, Hook summarized a new report prepared by more than 100 of her colleagues.

The possibilities appear remarkable, including images of solar-system objects that would require a space probe today, views of Earth-sized exoplanets, and spectroscopic studies of the first stars and galaxies. However, Hook is more excited by the discoveries she can't predict.

"Personally, it's the unexpected results that I'm looking forward to," she says. "Every time we build a new class of telescope … it has turned up something that we hadn't predicted."
Robert Adler is a freelance science writer living in Santa Rosa, California. He is the author of Science Firsts: From the Creation of Science to the Science of Creation (Wiley & Sons, 2002).
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