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Liquid-mirror telescopes are a reality at last

Liquid-mirror telescopes are a reality at last

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Why waste good money on a fiendishly expensive glass mirror for a huge telescope, when you can have a cheaper, flawless mirror simply by spinning a pool of mercury?

The idea of a liquid mirror is an old one, and some experimental liquid-mirror telescopes have been built. But the potential of the concept for astronomy is only now reaching fruition. The world’s first dedicated astronomical observatory with a liquid-mirror telescope at a top-class observing site is expected to come online in 2009.

The International Liquid-Mirror Telescope will have a liquid mirror 4 metres wide. With funding from Belgium and Canada, the telescope will scan the skies from a mountaintop site in northern India.

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Another group is studying the possibility of constructing a telescope with a liquid-mirror twice as wide, and there are even proposals to put a liquid-mirror telescope on the Moon.

What is a liquid-mirror telescope and how does it work?

The concept is simple: spin a pool of liquid and its surface curves into a parabolic shape – by coincidence, exactly the shape needed for a telescope mirror to focus light. Using a reflective liquid like mercury, a telescope mirror can be made quickly and relatively inexpensively.

What are the advantages of a liquid-mirror telescope?

Liquid mirrors are lighter, simpler and much less time-consuming to construct. They can be built for just a tenth of the cost of a conventional glass telescope mirror.

So why haven’t liquid-mirror telescopes already replaced regular telescopes?

The major limitation of liquid-mirror telescopes is that they can only look straight up, so they can only observe what passes directly overhead.

What kind of observations are liquid-mirror telescopes suited for?

They are ideal for sky surveys. For example, an 8-metre telescope at the right location could observe 1 billion galaxies, just by staring at the strip of sky that passes overhead. It would also detect large numbers of supernovae, which would allow astronomers to better determine the properties of dark energy – the baffling effect that makes the expansion of the universe accelerate.

What liquid-mirror telescopes are being planned?

In addition to the ILMT, Arlin Crotts of Columbia University in New York, US, is leading design studies for a proposed Earth-based 8-metre telescope calledALPACA (Advanced Liquid-mirror Probe of Astrophysics, Cosmology and Asteroids). The ultimate liquid mirror telescope could one day be constructed on the Moon, with a diameter of 20 to 100 metres, which would offer insight into the formation of galaxies in the very early universe. A Canadian team is studying the possibility of a smaller 2-metre liquid-mirror telescope for the Moon as a step towards this.

What does the future hold for liquid-mirror telescopes?

Ermanno Borra of Laval University in Quebec City, Canada, and his colleagues are developing reflective liquids with high viscosity that could allow liquid mirrors to tilt up to 30 degrees from vertical, opening up much more of the sky to their gaze.

As astronomers push for larger and larger telescopes, their costs are increasing accordingly, making liquid mirrors more and more attractive, Borra says.

“If you go back to the beginning of the 20th century, it was mostly the refracting [lens-based] telescopes that were used and it took some time for glass mirrors to take over,” he told New Scientist. “My feeling is that we’re going to see the same thing with liquid mirrors.”

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