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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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  • dumb question... (Score:4, Insightful)

    by girlintraining (1395911) on Thursday June 17, 2010 @05:14PM (#32606864)

    Why not just spin it, and while it's spinning, lower the ambient temperature so that it freezes? And if you remember your thermodynamics, you'll remember that raising or lowering *pressure* raises or lowers the temperature of a gas -- seal it up, spin it, then freeze it. Easy peasy.

    • by DarkKnightRadick (268025) <the_spoon.geo@yahoo.com> on Thursday June 17, 2010 @05:19PM (#32606918) Homepage Journal

      Some liquids possibly lose the reflectivity as a solid that they had as a liquid.

      • by Jenming (37265)

        It doesn't have to be mercury (they are not currently using mercury). So if you pick something that is reflective as a solid, melt it, spin it, freeze it. Though it is possible that the freezing process would distort it.

        • Re: (Score:2, Insightful)

          by Quartinus (1678656)
          That is how they make glass mirrors. I believe the purpose of making liquid mirrors is not only to get a good reflective surface, but to also use Adaptive Optics (nearly infinite possibilities for reflective surfaces, so it would be really easy to correct for atmospheric anomalies). So, a frozen mirror (e.g. a glass mirror) would not work nearly as well as a liquid mirror for this.
    • by gninnor (792931)

      My guess, changes in temperature causes changes in density, the frozen stuff will sink and cause deformation.

    • Re:dumb question... (Score:4, Interesting)

      by fotbr (855184) on Thursday June 17, 2010 @05:23PM (#32606976) Journal

      Freezing liquids changes their density. In a spinning environment, that causes movement, and there goes your perfectly uniform surface.

      • And yet, this is how they make glass mirrors used in astronomy -- melting the glass in a large shell inside a furnace, then letting it cool while spinning...
        • Re: (Score:3, Insightful)

          by Sperbels (1008585)

          And yet, this is how they make glass mirrors used in astronomy -- melting the glass in a large shell inside a furnace, then letting it cool while spinning..

          While you can do this to get the approximate shape of the mirror, you still need to do additional grinding and polishing to get a nice reflective surface. I'm not sure if anyone does this, but all of us amateur mirror makers usually grind the desired shape into the glass with abrasives.

        • by fotbr (855184)

          And that somewhat-mirror-shaped chunk of glass then required a LOT of grinding and polishing, and then coating, and then (sometimes) polishing that coating.

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

      by Bigjeff5 (1143585) on Thursday June 17, 2010 @05:24PM (#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.

      • There is already a liquid mercury telescope at UBC in Canada - the Large Zenith Telescope. With a 6 meter diameter, it's one of the largest telescopes in the world. Of course, it's limited to viewing only a narrow range of angles near the zenith, due to gravitational constraints. Even so, it was stunningly cheap compared to other telescopes of its size, and provides decent value for money.
        http://www.astro.ubc.ca/lmt/lzt [astro.ubc.ca]
        If freezing the mercury would help, you can be sure they thought of it. It's not just
    • Re: (Score:2, Insightful)

      by Moblaster (521614)

      Probably because it would be almost impossible to assure a complete uniform freeze -- you have all kinds of complicated temperature and phase transitions between the underlying bowl material, the ferromagnetic fluid and the reflective film/fluid. The stuff would inevitably crystallize and distort in patches, creating a mess of a surface.

    • by mea37 (1201159)

      I guess I'm not seeing how the relationship between temperature and pressure in a gas is to be applied to liquid mirrors.

      In any case, I think there are likely to be problems with freezing the mirror. Do most metals naturally freeze to a polished surface? Might the shape deform significantly during the freezing process, so that even if it remains "shiny" enough it still doesn't suit the application? Won't the act of spinning the liquid interfere with the process of freezing it?

    • by Grishnakh (216268) on Thursday June 17, 2010 @05:35PM (#32607110)

      I've got a better idea. Just keep using spinning liquid mercury, but put it in an artificial gravity field so that you can point it in any direction with "down" still being at the base of the mirror. This only needs some small advances in the field of physics.

      • by tlhIngan (30335) <<slashdot> <at> <worf.net>> on Thursday June 17, 2010 @06:08PM (#32607456)

        I've got a better idea. Just keep using spinning liquid mercury, but put it in an artificial gravity field so that you can point it in any direction with "down" still being at the base of the mirror. This only needs some small advances in the field of physics.

        What advance in physics is needed? Gravity works by the attraction of objects to each other, so all you need is a really, really, really massive object at the base of mirror. Such objects could easily range from a planet to a small black hole.

        Problem solved.

        • by sammyF70 (1154563)
          why don't we just use a warp drive to fly where the telescope would be pointing at? Tss .. silly scientists always complicating things!
        • by decoy256 (1335427)

          So... we're going to solve with a black hole, what could be solved with some good magnets... do you work for NASA?

        • by ultranova (717540)

          What advance in physics is needed? Gravity works by the attraction of objects to each other, so all you need is a really, really, really massive object at the base of mirror. Such objects could easily range from a planet to a small black hole.

          So just put it on the back of an airplane and fly the plane to a location where whatever direction you want to look at is straight up. Or would some kind of helicarrier be better?

          You could use a ship or a barge, but then most of the atmosphere is goign to be between y

          • by Grishnakh (216268)

            The problem here is that the airplane has a lot of vibration, which would not only cause problems with a regular telescope, but will screw up the spinning-liquid lens badly. So you'll need some sort of antigravity or stasis field to remove all such forces. Inertial dampers might do the job.

        • by EdZ (755139)
          If you've got a black hole, just sit your telescope at it's gravitational focus and use IT as your lens. Of course you can do this with any massive body, the focal point is just much further away (the sun's is out in the Oort cloud), and you have to move evenfurther in or out to change the focal distance.
        • Just put your mirror on a mobile platform on the moon...
        • Such objects could easily range from a planet to a small black hole.

          Problem solved.

          How about, oh say, Mercury?

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

      by siddesu (698447) on Thursday June 17, 2010 @05:36PM (#32607124)

      Probably becaue you lose the biggest advantage of the "liquid mirror". With a liquid, you can make a very large, very thin spinning surface which will keep its perfect shape because of the motion. Now, freeze that, peel it off --- how do you keep the shape? If it is thick, it will pose the same problems as any large mirror - heavy, unwieldy, needs lots of time to come to equilibrium with the environment, etc. If it is thin - keeping the shape is probably hopeless.

      Even if you could keep the shape somehow, freezing isn't a uniform process. As the temperature is lowered, crystals, lumps and whatnot starts forming in the melt. Some of these will inevitably go to the surface and spoil the figure of the resulting surface. And we're talking really, really small lumps here - on the order of less than quarter of the lightwave the surface is supposed to reflect. So, you'll need to work on the surface afterwards, just the same way you'd work on a surface of a "normal" mirror.

      I am not sure enough effort will be saved by making the initial figure in this way vs. the traditional methods of preparing a surface for polishing to justify the spinning. Speaking from experience, "pregrinding" a piece of glass to a rough sphere with a piece of pipe (or, if you're hi-tech, a diamond saw) does a good enough job. And the professional mirror makers have more than that at their disposal.

      • The error is in your additional implications. Nobody said anything about peeling it off. You obviously make it in-place on the supporting bowl and frame, and then movie the whole thing to wherever you need it.
        That’s already how it’s done. Because even huge glass mirrors deform under their weight, when not properly supported..

        • Re: (Score:3, Interesting)

          by siddesu (698447)

          That’s already how it’s done.

          Nope it isn't, not for "normal" mirrors anyway. Monolithic mirrors are made of uniform piece of material, properly annealed for stress. After polishing, the mirrors are mounted on a specialised structures, called mirror cells. These are designed with the assumption above, and with the goal of making the mirror behave as if it was floating freely.

          Adding of a bowl or a frame between the reflecting surface and the mirror cell (which you imply) will induce stress and

      • I am not sure enough effort will be saved by making the initial figure in this way vs. the traditional methods of preparing a surface for polishing to justify the spinning.

        You have to heat and cool less glass, meaning easier heat management and lower strains.

        The University of Arizona's Steward Observatory Mirror Lab [arizona.edu] has been spin casting big (6 meter) blanks for quite a while now for an impressive array of customers.

    • Adaptive Optics (Score:5, Insightful)

      by c++0xFF (1758032) on Thursday June 17, 2010 @06:12PM (#32607500)

      I thought one of the points was that you don't want to fix the shape permanently. Adaptive optics [wikipedia.org] lets you adjust the mirror to account for atmospheric distortion. Think of it like being able to change the prescription of your glasses. A liquid mirror would allow for near-infinite possibilities to adjust how the light is reflected, with greater precision than current adaptive optics systems.

    • by StikyPad (445176)

      They already do this for many lenses and mirrors, except that the freezing process occurs well above room temperature. It's called spin casting [wikipedia.org]. If you wear soft contacts, there's a good chance they were produced through spin casting. The hexagonal mirrors [arizona.edu] of telescopes referenced in the article are also produced through spin casting, followed by polishing. I can only assume that keeping the mirrors in liquid form offers some benefit, otherwise there would be no point.

      • t's called spin casting [wikipedia.org].

        I thought it was called spin forming. Regardless, I'll take a liquid lunch over a liquid mirror any day.

    • by Khashishi (775369)

      try it in your freezer, and maybe you'll see the problem.

    • Why not just spin it, and while it's spinning, lower the ambient temperature so that it freezes?

      I don't know for sure (chemistry isn't my field) but several potential issues occur to me immediately:

      • How does one accomplish this and keep the frozen mirror open to the sky? Got to take it out of the "fridge" at some point and I can't think of a way to do this.
      • Not clear to me if the physical properties of mercury remain useful for optics in a solid state. Maybe they do but a chemist would have to elaborate.
      • I suspect freezing the mirror simultaneously and consistently would prove very challenging. Defor
    • by Dishevel (1105119) *
      I believe that at least one issue with doing that would be controlling how the liquid freezes. Unless all the liquid freezes at the exact same time the solid portions and liquid portions will behave very differently while spinning and probably make for a very distorted surface.
    • by geogob (569250)

      On of the current main idea behind liquid mirrors is to make adaptative optics; that is optics that can be deformed to shape or correct a wavefront. That is in part why they moved from simple mirrors made from spinning mercury or mercury-like liquids around to ferromagnetic fluids that can be shaped at wish (within limits, of course).

      So, even if the freezing idea could work, you’d loose one of the key advantages of this technology.

  • Sounds like the oil lenses from Dune.
    • by Bigjeff5 (1143585)

      Which sounded friggin cool.

      There are (retarded looking) glasses that are similar, but they use water pumped into a bladder to alter the lense.

    • Re: (Score:3, Funny)

      by SnarfQuest (469614)

      I know where you can get a lot of oil, but it might be a bit salty...

      • by StikyPad (445176)

        Sure, but do you really think everyone with latex sheets is just going to... Oh, right. That oil.

  • bowl? (Score:2, Funny)

    by RapmasterT (787426)

    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..

    so our bowl making technology exceeds our bowl shaped mirror technology? seems like we could just hire the bowl makers and fire the current crop of mirror makers, problem solved.

    • by Bigjeff5 (1143585)

      It seems you missed the part where the mercury provides the smooth surface, not the bowl.

      The bowl gives it shape, the mercury gives it the ultra smooth mirror.

      • It seems you missed the part where the mercury provides the smooth surface, not the bowl.

        The bowl gives it shape, the mercury gives it the ultra smooth mirror.

        It seems you missed the irony of the supposition.

        The immaturity of ideas like this lies in stating that " 'x' technology isn't good enough, so we should use 'y' technology", while overlooking the fact that 'x' technology actually exists, while 'y' does not.

        Spinning mercury may theoretically make better mirrors than grinding glass, but grinding glass actually WORKS BITCHES! Suckit mercury!! (note: do not suck mercury)

      • But does a spinning liquid give you a parabolic surface? I wouldn't think so. We did this with liquid in a spinning globe in science class. At 0 rpm the surface is going to be flat. As you increase the rate of rotation, the liquid will attain a concave surface. As you keep increasing the rotation rate, the concavity will increase. At some point, the pool of liquid will be so concave that it touches the bottom of the globe. If you increase the rate of rotation from that point, you no longer get a simpl
    • Re: (Score:3, Insightful)

      by NonSequor (230139)

      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..

      so our bowl making technology exceeds our bowl shaped mirror technology? seems like we could just hire the bowl makers and fire the current crop of mirror makers, problem solved.

      The liquid takes on a shape that minimizes its surface tension. Small imperfections in the bowl don't affect the surface tension and are smoothed over.

    • by selven (1556643)

      The side of the mercury touching the bowl will be just as imperfect as the bowl. It's the inside. The centrifugal force will push most of the mercury to the sides, and the resulting shape will look like a bowl with a precise curve going from the center to the edges.

    • Re:bowl? (Score:5, Funny)

      by goodmanj (234846) on Thursday June 17, 2010 @05:35PM (#32607104)

      The perfect shape comes from the spinning liquid: the bowl doesn't have to have any particular shape. You can even use a flat-bottomed bowl, you just need more mercury.

      "Flat-bottomed bowls, you make the liquid scope go 'round..." -- Freddie Mercury

      • by geekoid (135745)

        "Flat-bottomed bowls, you make the liquid scope go 'round..." -- Freddie Mercury

        Bravo - *Applauds*

      • “You spin me right round baby, right round, like a flat-bottomed bowl baby, right round right round...” — Mike Percy [wikipedia.org]

  • Global Vision 2020 [gv2020.org] is doing something similar to this, creating eyeglasses for people in third-world countries.

    They have glasses with special lenses that can be filled with oil. The oil changes the shape of the lens.

    The client puts the glasses on and fills the lens with oil until he can see clearly. Then the technician seals the glasses so the amount of oil (and shape of the lenses) won't change any further.

    $10 per set of glasses, and no optometrist required to issue them.

    I

    • Re: (Score:2, Informative)

      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 t

  • why can't we just spin it up then freeze it solid?

    As long as its kept cold you can use it at any angle, and even make a precision mold from it then make a less temperature sensitive version.

  • by goodmanj (234846) on Thursday June 17, 2010 @05:28PM (#32607028)

    Laslo: Well what would you use that for?
    Ick: Making enormous Swiss cheese?
    (Chris laughs.)
    Mitch: The applications are unlimited.
    Laslo: No. With the fuel you’ve come up with the beam would last for what15 seconds. Well what good is that?
    Chris: Oh Laslo. That doesn’t matter. I respect you but I graduated.
    Mitch: Yeah, let the engineers figure out a use for it. That’s not our concern.
    Laslo: Maybe somebody already has a use for it. One for which it is specifically designed.

    PS: I'm serious.

  • by RodRooter (1835462) on Thursday June 17, 2010 @05:29PM (#32607044)
    Sounds like something perfect for the next generation Hubble (or the next next one - the next one is getting ready for launch). Why fight gravity, when you can just spin it in space?

    Course - making it spin for a long time between maintenance visits (on who knows WHAT vehicle) could be tricky.
  • Some Typoes (Score:3, Funny)

    by RevWaldo (1186281) on Thursday June 17, 2010 @05:48PM (#32607214)
    MIT's Technology Review is 'nalyzin a team's success in combating problems with a-bringin' liquid mirrors into the practical applications of astronomy.

    FTFY

    .
  • Wobble wobble (Score:4, Insightful)

    by blair1q (305137) on Thursday June 17, 2010 @06:07PM (#32607446) Journal

    Seems to me that liquid mirrors would be orders of magnitude more sensitive to vibrations than solid ones. (Experiement: fill a glass with water; tap the glass; which has a greater amplitude, the ripples on the surface of the water, or the ripples on the surface of the glass?)

    And rotating something large and heavy with a motor, moreso while simultaneously manipulating its surface with several dozen actuators, is a huge source of vibrations.

  • Step one, manage the forces on a liquid to make a constant bowl -- this allows you to make a telescope and point it anywhere you want.

    Step two, manipulate that bowl to alter the parabola. No more adjustable mirrors on solar collection systems, for starters, but this also allows you to direct sunlight on a quickly moving target, like a solar powered space elevator crawler. This gets particularly interesting in space. It allows you to focus sunlight on a satellite, or an object on the ground. A small mirr

    • and a big one could burn tiny little holes in even the fastest moving target. A big enough lens could do the same gag on people's heads on the ground.

      Dude, that's...duuude!

  • The key problem in this situation is that the bowl will always have to point straight up.

    That, and the fact that mercury is an extremely toxic and hazardous element that has to be carefully handled and disposed of. Accidentally disperse some into the air (splatter) and you could face a very costly cleanup. In short, mercury is a little bit beyond something your average hobbyist should be playing with – not that it would likely deter them, unfortunately.

    • Re: (Score:3, Informative)

      by geekoid (135745)

      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 Toonol (1057698)
      That, and the fact that mercury is an extremely toxic and hazardous element that has to be carefully handled and disposed of.
      No more than a hundred other compounds commonly found in a house. Less than many, in fact. There's been a bit of an overreaction against mercury.
  • Yes, there will be the earths gravity, but the temp of space would freeze it? Then you have an awesome replacement for the Hubble's lens, if it needs to be upgraded (not from what I've seen though).

    Or a whole new telescope all together.
  • The key problem in this situation is that the bowl will always have to point straight up.

    Not if it's at the bottom of a tube in a centrifuge. Of course, spinning the end and dealing with the gyroscopic forces is a new problem, but you can't have everything. Assuming you can figure that out, you could take snapshots every X microseconds (whenever you're pointing at something you want).

    • by goodmanj (234846)

      Unfortunately, exposure times for astrophotography need to be seconds to an hour long, even with modern digital imaging.

      • by Culture20 (968837)

        Unfortunately, exposure times for astrophotography need to be seconds to an hour long, even with modern digital imaging.

        What if we built this large wooden badger?

  • Rather, that should be spelled a-bringin'.

    MIT's Technology Review is annal-ma-lyzin' a team's success in combatin' problems with a-bringin' liquid mirrors and such t'the pract'cal applicationin's of the astronomy.

  • So why are we still using mirrors? Why not a big CCD?

    • Pointing a detector (CCD, CMOS sensor, film or whatever) straight at your target doesn't work. Light from all sorts of angles will hit every pixel and what you get will not be an camera but merely a sensor for ambient light levels.

      To get an image of an object you need an optical system that will create a mapping between the angle at which the light enters the system and the position the light ends up in on the sensor. There are many ways to do this (the simplest being a pinhole) but mirrors tend to work bes

  • ..every word coming out of the US these days that applies to particular challenges HAVE to be war related? Why are the "combating" this problem? Really guys lighten up?

  • Now if we can just get these stable when faced horizontally, I see an awesome revisiting of the Archimedes Death Ray.

  • It's a bit rube-goldbergian but why not have two axis of spinning on the mercury pond? The first rotation to give you the concave, the second off-side of that to centrifugally aim the center of the concave to whatever angle with respect to the vertical. Then you just need a "chopper circuit" to capture the image when the second axis has the azimuth of the concave puddle pointing where you want. Hell... don't waste the other azimuth directions and have three or more cameras to get images in other directi
  • by Doc Ruby (173196) on Thursday June 17, 2010 @10:19PM (#32609170) Homepage Journal

    How about spinning the bowl of mercury facing up, then swinging the bowl around a horizontal axis? Like a bucket of water swung at arm's length over one's head then back down, then back up, in a cycle, the way kids show each other "centripetal force"? The momentum would keep the concave surface intact as the whole contraption spun and swung around. Then the contraption could be rotated on the other horizontal axis, pointing the concave mirror at whichever direction was desired.

    The mirror would point in that direction only intermittently, as the mirror swung past that point in its arc. But the image sensor could be sampled only at that moment, as the position synced with the desired direction.

    All of that swinging would have to be calibrated to compensate for the interaction of the various axes of spin. But that all sounds like a set of DSPs could do it, with a laser interferometer keeping the cycles synced and sampling at the right timeslot.

  • Here's the link to website of the research group at Laval University.

    http://wood.phy.ulaval.ca/ [ulaval.ca]

    They've been publishing on liquid mirrors since the '80...

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