Liquid Mirror Telescopes Set For Magnetic Upgrade

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."

Comments

See Dune novels...

[zarozarozaro]

for more info about oil based lenses.

Smooth Magnetic Field

[MBCook]

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?

Re:Smooth Magnetic Field

[bcrowell]

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, 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

[jd]

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.

Re:Interesting design

[techno-vampire]

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.

I love ferrofluid

[Xelios]

So many interesting things

Laval University

[geogob]

These works are done by a group from the Centre d'Optique et de Photonique Laser (COPL), 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.

Re:Mercury vapor

[geogob]

As I said, it was an earlier design of the liquid mirror they are currently working on. Not the current one which is discussed in TFA. But it was a similar design, by the same research group, at the same location.

I have seen only photographs of the current design, using Ferromagnetic fluids, but have not seen it in person.