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Deformable Liquid Mirrors For Adaptive Optics 196

Posted by timothy
from the mirror-mirror-on-the-magnet dept.
eldavojohn writes "Want to make a great concave mirror for your telescope? Put a drop of mercury in a bowl and spin the bowl. The mercury will spread out to a concave reflective surface smoother than anything we can make with plain old glass right now. The key problem in this situation is that the bowl will always have to point straight up. MIT's Technology Review is analyzing a team's success in combating problems with bringing liquid mirrors into the practical applications of astronomy. To fight the gravity requirement, the team used a ferromagnetic liquid coated with a metal-like film and very strong magnetic fields to distort the surface of that liquid as they needed. But this introduces new non-linear problems of control when trying to sync up several of these mirrors similar to how traditional glass telescopes use multiple hexagonal mirrors mounted on actuators. The team has fought past so many of these problems plaguing liquid mirrors that they produced a proof of concept liquid mirror just five centimeters across with 91 actuators cycling at one kilohertz and the ability to linearize the response of the liquid. And with that, liquid mirrors take a giant leap closer to practicality."
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Deformable Liquid Mirrors For Adaptive Optics

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  • Re:dumb question... (Score:5, Informative)

    by Bigjeff5 (1143585) on Thursday June 17, 2010 @04:24PM (#32606990)

    Mercury changes color as it solidifies, from silvery white (highly reflective) to tin white (not nearly as reflective).

    In other words, if you freeze it it's not a good mirror any more, and then what's the point?

    That's not even considering the serious difficulty in getting a spinning liquid to freeze uniformly.

  • by mea37 (1201159) on Thursday June 17, 2010 @04:34PM (#32607100)

    Ignoring the mischaracterization of glass that you're trying to start a debate about, the answer to your question is: No, becuase mirrors are not made of glass.

    Bathroom mirrors have a protective layer of glass, but the reflective layer is silver. At best that would be "partially liquid" if we pretend that glass is a liquid. Many mirrors do not have such a protective layer, though; the mirror I use for backpacking is simply a thin metal sheet. Mirrors for lab optics typically don't include a glass layer because it would serve no purpose and would interfere with the mirror's intended use.

    The defining element of a mirror is the reflective part, which is made of metal and is usually solid.

  • by kg8484 (1755554) on Thursday June 17, 2010 @04:35PM (#32607118)

    You are wrong in 4 ways:

    • You misspelled "since"
    • The proper term is "cue," as in, "Cue the lights," a direction for stage-hands.
    • The idea that glass is a liquid is an urban legend. See here [unl.edu].
    • Not all mirrors are made from glass.
  • by questionsaddict (1277150) on Thursday June 17, 2010 @05:07PM (#32607442)
    um... it's nothing like the thing on the article.. what you're talking about is a lens with adjustable focus. what they're talking about is a mirror with ultra smooth adjustable shape. though they may seem similar, there's a big difference. also, you can afford a lot more focal dispersion (i just made that word up) in eyeglasses than telescopes**, hence the bitchiness about it being ultra smooth.

    **just take think about it this way. if you have a small imperfection in the eyeglasses, you may get a really tiny solid angle of the image wrong. astronomers like to look at objects that cover very very very tiny solid angles from where they look :P

  • Re:dumb question... (Score:5, Informative)

    by clone53421 (1310749) on Thursday June 17, 2010 @05:12PM (#32607506) Journal

    Usually frozen stuff floats, in comparison to its liquid state. (Apples, churches, very small rocks.)

    False. Water is one of the rare exceptions. Usually frozen stuff sinks in comparison to its liquid state.

  • Oh please. You need a good dose, and constant exposure. Otherwise your body will purge it. Don't drink it, but don't drink motor oil either.

    Just be careful. You average hobbyist has no problems with it.

  • by siddesu (698447) on Thursday June 17, 2010 @05:46PM (#32607796)

    Actually, most telescope mirrors are made from glass (some are made of special glasses, that have low thermal expansion and so on, but nevertheless glass), glass being the important "ingredient" of the mirror. The reason is that glass has no crystal structure and can be polished to very high degree of accuracy and achieve the required figure (a paraboloid) with very high precision. Glass is also a very stable medium if prepared (annealed) properly.

    Since the purpose of an astronomical mirror is to collect light in a precise way, the figure of the mirror is of most importance. The role of the metal layer on the surface is only to increase the reflectivity of the glass. There were (and, for some specialized uses probably are) some metal astronomical mirrors (made of speculum metal, mostly before glass got into wide use) but they allow a figure that is no better than the glass ones, and are difficult to polish and maintain.

    In fact, metal coating isn't even necessary to use a glass mirror. When you make a telescope mirror, before you send it off for coating you'd perform what is known as "star tests". You'd set up your telescope, put in the uncoated mirror in it, and look at stars to see if the mirror shape is good. I could easily see a lot of planetary detail with my last (40") mirror while I was testing it without coating. Looking at the Moon was blinding.

  • by burris (122191) on Thursday June 17, 2010 @06:06PM (#32607996)

    In a front surface mirror such as in a telescope, the surface of the glass is in fact the mirror because the important part is the shape and smoothness of the surface. You do not need to coat it with a reflective material because the glass itself is somewhat reflective. A large noncoated mirror is good for viewing the moon, which has a lot of detail but is very bright.

    After countless hours grinding and polishing or thousands of dollars spent on an optician with a good reputation or even tens of thousands spent on ion milling of the glass, you might want to have your mirror tested with an interferometer. That is done before the mirror is coated. That's because the glass is the mirror. The coating simply makes it more efficient.

    It is true that some mirrors are made with non-glass materials such as quartz or zero expansion ceramic. ...or Mercury.

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