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Molten Mirrors

Liquid mirrors could enable more-powerful space telescopes.

Canadian researchers have developed a liquid mirror that could operate in a future telescope located on the moon, allowing researchers to peer back into the origins of the universe with extraordinary clarity. Telescopes relying on liquid mirrors can be hundreds of times more powerful than those with glass mirrors–for the same cost–and they should be easier to assemble in space.

Molten mirror: A liquid mirror like this two-inch silver-coated pool of salt could enable a powerful, moon-based telescope.

A liquid-mirror telescope could reveal much fainter objects than the Hubble Telescope can, says Ermanno F. Borra, a physics professor at the Université Laval, in Quebec, who is leading the development of the new mirror. The power of a telescope is proportional to the surface area of its mirror. The James Webb telescope, which is scheduled to launch in 2013 and is far more powerful than the Hubble, has a diameter of about six meters. (See “Giant Mirror for a New Space Telescope.”) A lunar liquid-mirror telescope could be as large as 20 to 100 meters, says Borra.

The liquid mirror, which was funded by NASA, consists of a pool of an ionic liquid coated with a film of silver. Such ionic liquids are carbon-containing salts that freeze only at very low temperatures and have very high viscosity. The salt used in the Laval mirror is liquid down to -150 ºC and does not evaporate below room temperature, even in a vacuum–suggesting that it could withstand the harsh environment of the moon.

There are two limitations on cosmologists’ observations of the early universe: “The objects you want to observe are incredibly distant and incredibly faint,” says Borra. Telescopes in orbit like the Hubble, whose views are unobstructed by Earth’s atmosphere, are limited in size and power; telescopes on Earth can be larger and more powerful but produce fuzzier images because of the atmosphere. Liquid mirrors couldn’t go into orbit, but they could operate on the moon, which has no atmosphere.

Large, powerful liquid-mirror telescopes should be less complicated to take into space than their glass counterparts. “To put a glass mirror into a rocket, you have to break it into segments and then reassemble them,” says Borra. “You can carry a liquid mirror in a jug.” But none developed so far have been space worthy. The University of British Columbia’s Large Zenith Telescope uses a liquid mirror made of mercury to observe the early universe. Mercury solidifies at -40 ºC–much warmer than the temperature on the moon.

Borra searched for a better liquid to make telescope mirrors and found that ionic liquids seemed promising. Unlike mercury, however, these molten salts are not reflective, and they require a metal coating to function as a mirror. “Depositing a layer of silver on liquid is like painting on air,” says Borra. Laval graduate student Omar Seddiki adapted the technique used to coat glass mirrors: in a vacuum chamber, Borra and Seddiki run an electrical current between pieces of silver, which vaporize and form a thin coating over the liquid salt. The Laval researchers have so far made a small mirror, about two inches in diameter, to demonstrate the technology.

Making a large, perfectly smooth, concave optical surface out of glass is an involved and expensive process. Very tiny flaws in the glass can make a mirror unusable. The containers that hold liquid mirrors, says Borra, don’t need precisely smooth surfaces and would be much cheaper to manufacture. Telescopes that rely on liquid mirrors would cost about 100 times less than glass-mirror telescopes of comparable size, says Borra.

“The forces of nature conspire to give the right shape,” Borra says of liquid mirrors, which need only be rotated to form a flawless reflective surface. As the mirror spins, centrifugal force and gravity pull the liquid into a smooth parabola. Unlike with a glass mirror, if the liquid is perturbed, it can move right back into shape.

Borra expects that a liquid-mirror telescope would be assembled on the moon by robotics. “A container holding the liquid will be sent to the moon and opened up like an umbrella,” he says of an imagined future system. A liquid-mirror telescope could not be put into orbit because gravity is necessary to form the optical surface–and because it would spill.

“There’s a tremendous amount of research to be done” to fine-tune the mirror, cautions Robin D. Rogers, a chemistry professor at the University of Alabama. He points out that there are hundreds of other ionic liquids that might have a better set of properties than those used in the Laval mirror.

“It may take 20 years before it’s built,” Borra says of his telescope. If it does come about, however, such a system could help cosmologists observe faint signals from when the universe was only a billion years old, “at that time when matter first assembled into stars, stars into galaxies,” says Borra.

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