Deformable Liquid Mirrors For Adaptive Optics 196
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."
Re:dumb question... (Score:4, Interesting)
Freezing liquids changes their density. In a spinning environment, that causes movement, and there goes your perfectly uniform surface.
Global Vision 2020 (Score:2, Interesting)
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.
If you're looking for a worthwhile charity to donate to, this may be one to consider.
You've built a weapon, Kent. (Score:3, Interesting)
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.
Re:dumb question... (Score:5, Interesting)
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.
Re:All mirrors liquid (Score:3, Interesting)
Adding to parent comment..
I am pretty sure that most large "glass" mirrors used in astronomy actually use a very thin top layer of aluminium as the reflective layer, perhaps only 3 atoms thick... And I recall reading somewhere that this layer is cleaned off and applied every couple of years because of corrosion.
Re:dumb question... (Score:2, Interesting)
What if we used natural gas deposition to produce a layer of diamond a few mm thick then use an atom thick silver layer. Apollo Corp did patent the gas to diamond layering technique. On a side note the Australians came up with a way to produce low grade diamonds by microwaving car exhaust. If we nuke liquid mercury it will boil and froth :-( How warm do polymers have to be to solidify?
Re:dumb question... (Score:3, Interesting)
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 cause severe astigmatism in the mirror. This is only done in cheap mass-produced Chinese telescopes where they just glue the 6 inch mirror to the bottom.
Here's the ESO setup -- http://www.eso.org/public/images/eso9940b/ [eso.org] - you can see for yourself that the mirror is made from one block of zerodur, there is no supporting "bowl" or "frame" between the mirror and the cell.
Re:dumb question... (Score:2, Interesting)