The magnetic magic of liquid mirrors

 Liquid mirror

Liquid mirror telescopes are amazing contraptions. They start life as a puddle of mercury in a bowl. Set the whole thing spinning and the mercury spreads out in a thin film up the sides of the bowl.

The result is a fabulously cheap mirror that can be used for a variety of astronomical surveys. If we ever put a telescope on the moon, many astronomers have suggested that it should be one of this type.

It won’t have escaped your attention that liquid mirrors have important limitations. First, they can only point straight up. One or two people  have played with fluids that have a higher viscosity than mercury and so can be tilted a few degrees this way or that but with limited success. And second, they cannot be made adaptive to correct for blurring introduced by the Earth’s atmosphere.

But that may change thanks to some interesting work being done by Denis Brousseau at Université Laval in Quebec et amis.  Their machine controls the shape of the surface of a liquid mirror using a magnetic field. Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.

Instead the team have used a suspension of ferromagnetic nanoparticles in oil. A thin highly reflectivity layer of silver particles can then be spread across the surface of the ferrofluid to create a mirror.

Brousseau and co use an array of tiny coils behind the liquid to create a field that deforms the fluid surface as required.  Their tests show this can be done fast and furiously enough to cope with the usual array of optical aberrations that the atmosphere throws up.

However, it may also be possible to use this technique to tilt liquid mirrors further than ever before.  Ferrofluids can easily be made much more viscous than mercury and so combat the deforming pull of gravity. But they can also be deformed in a way that opposes gravity during each rotation of the supporting bowl. That could make them much more tiltable than mercury mirrors.

Of course, such a mirror would be mechanically more complex than the spinning bowls we have today and correspondingly more expensive. And sending one to the moon seems an unnecessary extravagance given the absence of an atmosphere there.

But here on Earth they could be made much more useful.  It’s a combination of new-found utility and value for money that many astronomy projects on a budget will find irresistible.

Ref: arxiv.org/abs/0807.2397: Wavefront Correction with a Ferrofluid Deformable Mirror: Experimental Results and Recent Developments

10 Responses to “The magnetic magic of liquid mirrors”

  1. ZEPHIR says:

    This invention can open path to development of new sort of adaptive optics – but I’m not sure, if it can still employ the principle of parabolic shape of rotating fluid.

  2. Babak says:

    Is tilting really necessary? Seems to me you could have a panel of flat mirrors guiding an image from any part of the visible sky into the vertically aligned liquid base mirror. Flat mirrors should be a good deal cheaper to build than parabolic ones, so I imagine this approach might be cost effective.

  3. JBBW says:

    “fabulously cheap mirror”?
    I am thinking not so much…
    I would edit it to say “relatively cheap mirror”.

  4. Aidan Karley says:

    [QUOTE BABAK]Is tilting really necessary? Seems to me you could have a panel of flat mirrors guiding an image from any part of the visible sky into the vertically aligned liquid base mirror.[END QUOTE]
    They are used already, in certain specialised telescopes. In solar ’scopes for example, they’re known as a heliostat. There are other examples, but I don’t have them at my fingertips.

    [QUOTE BABAK]Flat mirrors should be a good deal cheaper to build than parabolic ones,[END QUOTE] You’d expect that to be the case, but it’s not actually true. Unless your mirror is flat to a quite exacting degree, then you’ll end up introducing more distortions than you can correct with your rotating mirror (or elsewhere in your optics), which pushes up the overall cost of the system.
    Making a true optical flat is quite hard, because it’s very difficult to distinguish between a flat and a spherical surface of arbitrarily large focal length. You end up having to make 3 (or more) similar flats more-or-less simultaneously, using noticeably different techniques, and repeatedly working one against the other until you’ve got agreement between all 3 (or more) that their surfaces match. Even then, you don’t know if your “flat” is actually flat – you’ll only know that it’s even and of a radius-of-curvature greater than [some value].
    http://www.oldham-optical.co.uk/Testing.htm tells you a little more about the testing, but a brief Google doesn’t reveal much amateur information about how to make one ; compare that with the number of sites about general telescope making and you’ll see that not many people try it.

  5. Tashammer says:

    Would it be of any benefit to use the initial spun out surface (mercury/ferro-fluid) and then to coat that with a rapidly curing film which could then and/or/either act to keep it in place or be used to form a mold/mould?

  6. Ponderable says:

    [Quote]Tashammer on Jul 19, 2008 | Reply

    Would it be of any benefit to use the initial spun out surface (mercury/ferro-fluid) and then to coat that with a rapidly curing film which could then and/or/either act to keep it in place or be used to form a mold/mould? [end Quote]

    In fact this would complicate the matter, since the layer you are adding has to have well known optical characteristics, it has to be VERY uniform. The latter condition is awfully difficult to fulfill if you need a uniformity in opticcal properties.

  7. 1.
    Wouldn’t the mercury freeze and then– tilt…?

    (If frozen-mercury doesn’t reflect, the surface could be uniformly rewarmed by microwave-heat.)

    2.
    And flat panels would be correctable by movable point-laser testing each panel to a stationary-tuned-interferometry-analysis-unit….

    Ray.

  8. Phil says:

    Why not set up the liquid mirror as usual and then reduce the temperature (while still spinning) until the mirror becomes solid ?

    You could make a really large perfect reflector, which being solid, could then be tilted.

    There would be difficulties such as keeping the atmosphere cool and dry (not a problem in a lunar atmosphere on the dark side)

  9. Robert G Bayless says:

    Also consider the ‘magnetic oil’ microencapsulated with a transparent wall. You then will have the same condition as the rotating murcury as a mirror. The encapsulated oil dispersion can be deployed anywhere and achieve the condition sought. Also,any reasonable size of capsule can be made; i.e., from ten micron to 2,000 microns.