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Friday, June 22, 2007

Molten Mirrors

Liquid mirrors could enable more-powerful space telescopes.

By Katherine Bourzac
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Molten mirror: A liquid mirror like this two-inch silver-coated pool of salt could enable a powerful, moon-based telescope.
Credit: Omar Seddiki (Univ. Laval)

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.

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.
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Comments

  • doubtful
    baroque1980 on 06/22/2007 at 12:28 AM
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    scientists say they can take the liquid mirror to the moon in a jug. but article points out a thin layer silver should be deposited on the ionic liquid. so how to put the liquid mirror with silver layer into a jug??
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    • Re: doubtful
      cyberpageman on 06/22/2007 at 10:42 AM
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      I imagine the scientists will find a reflective material that can be carried in a jug.  It doesn't sound too difficult--a detergent with a gold atom at one end would collect on the surface and form a mirror.

      I tried making a molten mirror back in 1944 out of a  a phonograph turntable and a pie dish with water in it and aluminum powder sprinkled on the  water surface.  It worked a bit, but the vibrations from the turntable made the focus jitter around.
      Rate this comment: 12345
    • Re: doubtful
      cjameshuff on 06/22/2007 at 11:01 AM
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      Simple: deposit the silver layer after setting the telescope up on the lunar surface. All you need is a way to vaporize small amounts of silver and direct the vapor onto the liquid surface.
      Rate this comment: 12345
  • how smooth?
    Bruceahz on 06/22/2007 at 10:32 AM
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    liquid mirrors are theoretically great but practically troublesome, particularly as they get larger. To achieve the proper, smooth optical surface you have to rotate the mirror at a constant angular velocity and keep it isolated from all other forces.

    Liquid mirrors are not new. This report addresses only a new material for liquid mirrors - not the issues of building a "20 to 100 meter" mirror on the moon. It's almost laughable for someone to talk this way when their demo so far is a 2 inch lab model.
    Rate this comment: 12345
    • Re: how smooth?
      Monsterboy on 06/24/2007 at 7:17 PM
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      I'd assumed they'd make multiple, smaller lenses, just like they do now with glass.
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      • Re: how smooth?
        Bruceahz on 06/25/2007 at 11:54 AM
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        Liquid mirrors cannot be "segmented" the way glass mirrors can. The parabolic mirror surface is generated by the sum of gravitational and centripetal forces, the latter being applied by the rim of the cup holding the liquid. The depth of the liquid in the center of the mirror (on the axis of rotation) is significantly less than the depth at the edge.

        If you segment the mirror with individual "cups" and rotate each cup individually, you get a fly's eye array of lenses; if you rotate all the cups on a big turntable you little parabolic segments that don't match up at their edges.

        Even if you could adjust the volume of liquid in each cup to make the edges match (effectively what is done with glass segments) you still have the problems I commented on - vibrations in your turntable, non-uniform angular velocity, etc.
        Rate this comment: 12345
  • dust a problem
    revluc on 06/25/2007 at 9:36 PM
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    One of the big problems with working on the moon is the dust.  Moon dirt - regolith - is highly abrasive, has particluate sizes around 70 microns (like silt), and can become electrostatically charged.  Any type of disturbance of the lunar surface sends up clouds of dust that clings to non-grounded surfaces, and there is some evidence to suggest that incoming charged particles from space actually cause charging of the dust resulting in levitating dust clouds up to meters above the moon's surface. Once a large mirror, whether solid or liquid, is covered with dust all the clarity gained from being on the moon is quickly negated.  Commonly proposed techniques for removing dust build up on moon based structures and mechanisms involve mechanical agitation and "wiping" and/or electic discharging, in combination with gravity to remove the dust. A tricky task for a solid mirror, let alone a liquid one. It would appear that the dust problem would be a big issue for a horizontal liquid mirror because once dust is on the liquid mirror how could it be removed without replacing the whole liquid volume.  Commonly prosed approaches won't work and simply recoating the silver over the dust won't provide an optical quality surface and the dust is abrasive and on a scale at which it would easily penetrate a typical <10 micron thick silver mirror layer.  There are other approaches - like gossamer mirrors - made from very thin, collapsible polymers - think shiny Mylar ballons -that have been demonstrated on earth too, that may end up having more practical utility - either as moon based telescopes or as space arrayed imagers.
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    • Re: dust a problem
      permit on 06/27/2007 at 1:48 AM
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      Perhaps the first stage of deployment on the moon would include some effort to reduce the dust-up?  Think mists of water and a large tarp.
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  • [no subject]
    quseio on 06/11/2008 at 2:27 AM
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    ouldnt theymake a mirror out of ice ?  heh nice lil backup water supply assumeing u can use  h20
    Rate this comment: 12345
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