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Liquid Mirror Telescopes Set For Magnetic Upgrade 64

Posted by timothy
from the calm-the-hubble-hubbub dept.
KentuckyFC writes "Liquid mirror telescopes start life as a puddle of mercury in a bowl. Set the bowl spinning and the mercury spreads out in a thin film giving the surface an almost perfect mirror finish. But these telescopes have two important limitations. First, they can only point straight up since tilting the mirror spills the mercury. And second, they cannot be made adaptive to correct for any blurring introduced by the Earth's atmosphere. But liquid mirror telescopes look set for an upgrade thanks to the work of a group of Canadian researchers. Their technique is to change the shape of the liquid mirror using powerful electromagnets. They use a ferromagnetic fluid of iron nanoparticles in oil instead of mercury which is too dense to be easily manipulated in this way. The work is just proof of principle at this stage but the idea is to use magnets to correct for the usual range of optical aberrations that telescopes have to deal with (abstract). And also to allow a liquid telescope to be tilted by using oil that is much more viscous than mercury and correcting any periodic deformation in the fluid that tilting might cause."
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Liquid Mirror Telescopes Set For Magnetic Upgrade

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  • by pwnies (1034518) * <j@jjcm.org> on Thursday July 17, 2008 @04:39PM (#24233545) Homepage Journal
    This is all a secret cover up for research on how to make a robot transform into liquid metal... Sarah Conner better keep on her toes.
  • I hope they have this thing on a serious UPS. Imagine the thing upright losing power, and all that mercury spilling onto the ground. (assuming that they get to the point that they can use real mercury)

    • Re: (Score:3, Informative)

      by FooAtWFU (699187)
      TFS says they're not using mercury in the tilting kind since it's too dense to work with effectively.
    • by RaguMS (149511)

      I'm sure it's not just an exposed spinning bowl of mercury outside with dirt underneath. Even so, they could just put a bucket under it to catch the mercury...

  • Alternately (Score:3, Insightful)

    by overshoot (39700) on Thursday July 17, 2008 @04:41PM (#24233573)
    they could still use mercury [1] and magnets while taking advantage of the fact that it's a moving conductor.

    [1] Despite facing the wrath of Jenny McCarthy

    • Re: (Score:3, Informative)

      by againjj (1132651)
      Mercury is too dense. From TFA:

      Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.

    • by S-100 (1295224)
      Since it's a single conducting mass, whether or not it's spinning won't matter. It won't be affected by a magnetic field to any significant effect. The OP claims they use a ferromagnetic liquid because mercury is too dense, mercury is also unaffected by magnetic fields, being nonferrous.
  • See Dune novels... (Score:3, Interesting)

    by zarozarozaro (756135) on Thursday July 17, 2008 @04:50PM (#24233731)
    for more info about oil based lenses.
  • by MBCook (132727) <foobarsoft@foobarsoft.com> on Thursday July 17, 2008 @04:53PM (#24233787) Homepage

    Would it really be possible enough to make the magnetic field smooth enough so that the mirror surface was smooth and not something like the surface of a 300 sided polyhedron?

    I would think it would be impractical to put enough small but powerful electromagnets behind the fluid so that you could make a smooth surface.

    Or could you use something to vastly increase the surface tension thus making it easier to create a smooth surface?

    • by hubie (108345) on Thursday July 17, 2008 @05:50PM (#24234467)
      From their abstract they say they corrected the residual wavefront error to 0.05-microns, which for the 659.9-nm light they tested works out to lamda/13, which is not bad for the imaging surface for a mirror. Then again, the mirror was only 37-mm in diameter, so it would be interesting to see how it scales (especially with actuator number).
    • by mikael (484) on Thursday July 17, 2008 @06:37PM (#24234941)

      The ferromagnetic liquid will always try to achieve an equilibrium point between gravity, surface tension and the surrounding magnetic field. Gravity and surface tension will make it try and remain flat. As a magnetic field is continuous, it should be possible to have a large number of small but powerful magnets to make the liquid adopt whatever position is desired.

      There are a good few videos on youtube: Magnetic sculpture [youtube.com]

    • Re: (Score:3, Informative)

      by kaiser423 (828989)
      Well, optical-grade corrections are extremely small. The magnets would be nowhere near to making something look like a polyhedron. Max deflection from these suckers would probably be millimeters if that....
    • by bcrowell (177657) on Thursday July 17, 2008 @10:04PM (#24236665) Homepage

      I would think it would be impractical to put enough small but powerful electromagnets behind the fluid so that you could make a smooth surface.

      Their paper says they surrounded the mirror with a Maxwell coil [wikipedia.org], which has an extremely uniform magnetic field on its interior.

      Would it really be possible enough to make the magnetic field smooth enough so that the mirror surface was smooth and not something like the surface of a 300 sided polyhedron?

      I think the idea is that they spin it so that it assumes a parabololoidal shape. A paraboloid is the mathematically perfect shape for bringing parallel rays to a focus. The magnetic fields are only used for small corrections to the shape. These small corrections might, for example, be used for adaptive optics, to correct for atmospheric turbulence on a real-time basis.

      The slashdot summary talks about pointing away from the vertical, whereas the paper doesn't talk about that explicitly. I may be wrong, but I think the idea is this. A liquid metal telescope can only view a certain circular strip of the sky, which depends on the latitude at which it's located. You want this strip to be as wide as possible. Theoretically, you can just move your CCD (or whatever instrument it is) off the axis, and it will get a field of view that's away from vertical. However, any optical device is subject to aberrations if you try to use it far off axis. Reading between the lines here, I think the idea is that you can correct for the aberrations using the magnets, so that it might be possible to get good-quality images very far off the vertical axis -- "very far" meaning, I dunno, maybe five or ten degrees or something.

  • Interesting design (Score:5, Interesting)

    by jd (1658) <imipak&yahoo,com> on Thursday July 17, 2008 @04:54PM (#24233797) Homepage Journal
    Liquid mirrors are ingenious and have many benefits over solid mirrors. It's hard to get a solid mirror into space without it sagging, whereas a mirror shaped by inertia or magnetic fields isn't going to care. Porting solid mirrors up the side of a volcano is also much harder than sending up a few tanker trucks. In principle, this means you can get far larger mirrors into key sites. It may also impact optical interferometry, as it would be easier to build large arrays - though you'd need to watch for magnetic fields from nearby telescopes interfering.
    • by hubie (108345) on Thursday July 17, 2008 @05:39PM (#24234303)

      When you talk about space, everything changes. If the ferrofluid has a volatile base liquid, it will all evaporate/boil away in the vacuum (and make a heck of a mess of the rest of the telescope). I couldn't tell from the ferrofluid manufacturer web site, but the material doesn't make any claims about vacuum compatibility (the stuff is used to make seals but those look to be hermetic and not vacuum seals).

      The other problem with space applications and these thin deformable mirrors is whether there is any savings in making a mirror out of them over glass. If the weight of all the actuators, actuator support structures, electronics to run the actuators and the control system, etc. weigh more than a proper piece of glass of comparable diameter, then you're better off going with a nice stable piece of glass.

      As an aside, I'm not so sure it makes it easier to build larger interferometric arrays. Everything behind the primary telescope mirrors stays the same and you are only talking about how much gain you get building these mirrors over glass. For interferometric arrays what is important is the "filled" area vs the area of the effective diameter, and unless you're talking about these mirrors being an order of magnitude larger (and much cheaper than the glass ones), I'm not so sure it impacts your "filled" vs "unfilled" area ratio.

      • by jd (1658)
        For a space telescope, I was thinking you'd have something sealing the mirror section to maintain a fixed pressure. That would avoid the vaccuum problem, though wouldn't give you as clear an image as one totally open to space, but the difference could probably be made marginal. The weight argument is complicated by the fact that glass distorts under high acceleration, which makes it very complicated to put large mirrors into space. The larger the mirror, the harder it is to protect it against the effects of
      • by vigour (846429) on Thursday July 17, 2008 @06:21PM (#24234785)
        Ferrofluids are generally dispersed in water. They consist of coated magnetite (Fe3O4) nanoparticles. It doesn't take a particularly strong magnetic field to distort them. If you place a small, good quality permanent magnet (NdFeB), which has a surface field ~ 150 mT, under a dish of a ferrofluid, the ferrofluid grows peaks (it look slike a hedgehog) to minimise its surface area.

        With small electromagnets, it is possible to generate fields of this magnitude, on this scale. The magnetic field inside a solenoid is
        B = mu_0 * (N/L) * I
        where mu_0 permeability 4*PI e-7
        N - Number of turns
        L - Length of solenoid
        I - Current in solenoid

        Typical Values of N = 5000, L = 1 cm, I = 0.5 A, B = 314 mT at the center (so ~ 150 - 200 mT at the edge).
      • by techno-vampire (666512) on Thursday July 17, 2008 @07:14PM (#24235279) Homepage
        The other problem with space applications and these thin deformable mirrors is whether there is any savings in making a mirror out of them over glass.

        There are other factors here. With a glass mirror, you're limited to the inside diameter of your launch vehicle. You also need extra mass for all the bracing and padding needed to protect the mirror during the launch. With a magnetic mirror, it can be sent disassembled, possibly in several shipments instead of all-at-once, making things much easier.

        • Re: (Score:1, Informative)

          by Anonymous Coward

          A big glass mirror can be put together from smaller hexagonal glass mirror pieces. Some of the larger surface bound telescopes are set up that way too. So the launch vehicle diameter is not a hard limit.

      • Why can't they just use some DSP algorithms for this? We've got quad core processors that can be had for $200, or Sony PS3s would be fitted for the job as well.

    • by boot_img (610085)

      Liquid mirrors are ingenious and have many benefits over solid mirrors.

      ... But one killer disadvantage: they only point straight up. Generally, astronomers want to look at what they want to look at, not just whatever is overhead at the moment. (There are some exceptions: for some cosmological surveys, any patch of sky will do).

      This is the main reason they are not widely used.

  • I love ferrofluid (Score:5, Interesting)

    by Xelios (822510) on Thursday July 17, 2008 @04:59PM (#24233887)
    So [youtube.com] many [youtube.com] interesting [youtube.com] things [youtube.com]
    • I love it too. I even have a little bottle of it. Unfortunately, I have yet to find a transparent container to put it inside that it won't stick to and discolor. Otherwise I'd keep it in a little box on my desk. I don't know why it stains so well, but you spill even a drop of it and it will never come off.
  • Laval University (Score:5, Interesting)

    by geogob (569250) on Thursday July 17, 2008 @05:01PM (#24233919)
    These works are done by a group from the Centre d'Optique et de Photonique Laser (COPL) [ulaval.ca], at Laval University in Quebec City. This research center is one of the largest player in the field of optics research in North America.

    I've seen this liquid mirror myself while it was in its early stages. At that time it used only mercury. It's a very impressive (and beautiful) sight. This research group, working on liquid mirrors, has been quite excited with the recent talks about lunar-based telescopes. This has always been one of the aimed application for their liquid mirror.
  • Too dense? (Score:5, Informative)

    by imsabbel (611519) on Thursday July 17, 2008 @05:15PM (#24234075)

    "They use a ferromagnetic fluid of iron nanoparticles in oil instead of mercury which is too dense to be easily manipulated in this way."

    Well, that and the fact that a ferrofluid (== ferromagnetic or antiferromagnetic, ir depends) is a little easier to influence with magnetic fields than an weak diamagnet like mercury...

    • by kesuki (321456)

      the real question, is does covering the fluid with silver, after it's shaped really make a practical liquid mirror? or are there some ferro liquids that would provide as good a mirror as mercury?

      it seems that some of the advantages of a liquid mirror, are lost if the reflective surface is not the portion that's controlled with magnets.

      on that note, every time i've opened a HDD platter, i've always noted how the polished shine is as reflective as a mirror... is that a protective layer? or is the magnetic lay

      • by kesuki (321456)

        whoops, small mistake, my 2.5" hdd had a screw 'hidden' by the labels, at the time just forced the drive open, because i didn't care. and none of the bottom needed to be removed, ah well, always look for a hidden screw if you can't open a 2.5" laptop drive that dead..

      • by imsabbel (611519)

        It really depends on the HD.
        For once, HDs dont use such basic magnetic stuff like mangetite anymore.

        They use metal or glass (but i think the latter is not common anymore) discs with a sub-um thick vacuum-deposited coating on it. The magnetic storage is in some magnetically really hard-as-rock stuff like FePt.
        What you see is the perfectly polished base metal.
        If you take a closer look, you might see that its not as reflective as a real mirror

  • by WalksOnDirt (704461) on Thursday July 17, 2008 @05:55PM (#24234501)

    That question immediately came to mind, since as wild guess I would expect something more like a catenary. At http://www.math.iupui.edu/m261vis/LMirror/mirrorproof.html [iupui.edu] they show that it really is a parabola.

    • Re: (Score:3, Informative)

      by againjj (1132651)
      Yes it is. Wikipedia talks about catenary [wikipedia.org] and parabola [wikipedia.org]. Basically, when you have a cable-like object that has evenly distributed mass and support only at the ends, you get a catenary. When you get support over the entire object, you get the parabola. This is talked about in reference to a suspension bridge [wikipedia.org].
    • by expatriot (903070)
      Typical slashdot. The erroneous comment is modded +3 informative and the correct response is at +1.
      • Are you claiming my comment is wrong? If so, why?

        My comment and the only other comment to it both claim the mirrors are true parabolas, and I have a link to a physics proof that this is so rather than to a side comment in a Wikipedia article.

  • sexy as a project/ion... magnetic...

  • so....it's like a giant set of Blue Blockers?
  • Former site, I should say. It's just outside of Cloudcroft, NM, and was decommissioned a few years ago. I've been there, the site is owned by NASA but reopening with a 1 meter remote-control telescope.

    I've always thought LMT's were cool, but mercury can be dangerous stuff. It's good to see alternative liquids.

  • For the moon project, ship up some epoxy, aluminum vacuum deposition equipment, and materials to build spinning pan. Mix the epoxy, pour in spin pan, spin until sets. Buff surface. Allow spinning shelter to go to vacuum and sputter on a few layers of aluminum and you have a cheap, low-mass mirror. Tilt as you wish on lunar surface. Make more, create arrays. Have neighborhood mirror-making parties! ;)

    For atmospheres, go to military and pry some of that "Star Wars" developed phase-conjugate mirror technolog

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