Saturday, August 02, 2008

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: Wavefront Correction with a Ferrofluid Deformable Mirror: Experimental Results and Recent Developments


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