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."

All A Cover Story...

This is all a secret cover up for research on how to make a robot transform into liquid metal... Sarah Conner better keep on her toes.

Power outage

I hope they have this thing on a serious UPS. Imagine the thing upright losing power, and all that mercury spilling onto the ground. (assuming that they get to the point that they can use real mercury)

Re:

TFS says they're not using mercury in the tilting kind since it's too dense to work with effectively.

Re:

TFS says they're not using mercury ... since it's too dense to work with effectively.

You accidentally hit Submit before you made your witty connection to the poster and too dense to work with effectively. I look forward to your followup.

Re:Power outage

Wow.. Readers are getting pretty lazy these days. Not only are they not RTFA, they aren't even RTFS!

Alternately

they could still use mercury [1] and magnets while taking advantage of the fact that it's a moving conductor.

[1] Despite facing the wrath of Jenny McCarthy

Re:

Mercury is too dense. From TFA:

Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.

Eddy currents

Since it's a single conducting mass, whether or not it's spinning won't matter.

Tell that to my exercise bicycle.

See Dune novels...

for more info about oil based lenses.

Smooth Magnetic Field

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

From their abstract they say they corrected the residual wavefront error to 0.05-microns, which for the 659.9-nm light they tested works out to lamda/13, which is not bad for the imaging surface for a mirror. Then again, the mirror was only 37-mm in diameter, so it would be interesting to see how it scales (especially with actuator number).

Re:Smooth Magnetic Field

The ferromagnetic liquid will always try to achieve an equilibrium point between gravity, surface tension and the surrounding magnetic field. Gravity and surface tension will make it try and remain flat. As a magnetic field is continuous, it should be possible to have a large number of small but powerful magnets to make the liquid adopt whatever position is desired.

There are a good few videos on youtube: Magnetic sculpture [youtube.com]

Re:

Well, optical-grade corrections are extremely small. The magnets would be nowhere near to making something look like a polyhedron. Max deflection from these suckers would probably be millimeters if that....

Re:Smooth Magnetic Field

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 [wikipedia.org], 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

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

When you talk about space, everything changes. If the ferrofluid has a volatile base liquid, it will all evaporate/boil away in the vacuum (and make a heck of a mess of the rest of the telescope). I couldn't tell from the ferrofluid manufacturer web site, but the material doesn't make any claims about vacuum compatibility (the stuff is used to make seals but those look to be hermetic and not vacuum seals).

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. If the weight of all the actuators, actuator support structures, electronics to run the actuators and the control system, etc. weigh more than a proper piece of glass of comparable diameter, then you're better off going with a nice stable piece of glass.

As an aside, I'm not so sure it makes it easier to build larger interferometric arrays. Everything behind the primary telescope mirrors stays the same and you are only talking about how much gain you get building these mirrors over glass. For interferometric arrays what is important is the "filled" area vs the area of the effective diameter, and unless you're talking about these mirrors being an order of magnitude larger (and much cheaper than the glass ones), I'm not so sure it impacts your "filled" vs "unfilled" area ratio.

Re:Interesting design

Ferrofluids are generally dispersed in water. They consist of coated magnetite (Fe3O4) nanoparticles. It doesn't take a particularly strong magnetic field to distort them. If you place a small, good quality permanent magnet (NdFeB), which has a surface field ~ 150 mT, under a dish of a ferrofluid, the ferrofluid grows peaks (it look slike a hedgehog) to minimise its surface area.

With small electromagnets, it is possible to generate fields of this magnitude, on this scale. The magnetic field inside a solenoid is
B = mu_0 * (N/L) * I
where mu_0 permeability 4*PI e-7
N - Number of turns
L - Length of solenoid
I - Current in solenoid

Typical Values of N = 5000, L = 1 cm, I = 0.5 A, B = 314 mT at the center (so ~ 150 - 200 mT at the edge).

Re:Interesting design

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

So [youtube.com] many [youtube.com] interesting [youtube.com] things [youtube.com]

Re:

I don't know why it stains so well, but you spill even a drop of it and it will never come off.

Iron is very common in ink such as the classical iron gall [wikipedia.org] and black tattoo ink [about.com].

Laval University

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

Too dense?

"They use a ferromagnetic fluid of iron nanoparticles in oil instead of mercury which is too dense to be easily manipulated in this way."

Well, that and the fact that a ferrofluid (== ferromagnetic or antiferromagnetic, ir depends) is a little easier to influence with magnetic fields than an weak diamagnet like mercury...

Is it a real parabola?

That question immediately came to mind, since as wild guess I would expect something more like a catenary. At http://www.math.iupui.edu/m261vis/LMirror/mirrorproof.html [iupui.edu] they show that it really is a parabola.

Re:

Yes it is. Wikipedia talks about catenary [wikipedia.org] and parabola [wikipedia.org]. Basically, when you have a cable-like object that has evenly distributed mass and support only at the ends, you get a catenary. When you get support over the entire object, you get the parabola. This is talked about in reference to a suspension bridge [wikipedia.org].

Re:

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.