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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.
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.
I'd assumed they'd make multiple, smaller lenses, just like they do now with glass.
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.
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.
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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baroque1980
4 Comments
doubtful
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??
Reply
cyberpageman
53 Comments
Re: doubtful
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.
Reply
cjameshuff
1 Comment
Re: doubtful
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.
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