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Deformable Liquid Mirrors Could Revolutionise Astronomy

Liquid mirrors are cheaper and easier to make than glass ones. A new prototype shows how they can also make adaptive optics systems.

  • June 17, 2010

Put a puddle of mercury into a bowl, set it spinning and the liquid will spread out in a thin film across the surface. The result is a concave mirror with a surface so smooth that it rivals anything that astronomers can make out of glass but at a fraction of the cost.

So it’d be easy to imagine that liquid mirrors have revolutionised astronomy. Not quite. These mirrors have important limitations not least of which is that they can only point straight up, lest the liquid slosh out of the bowl. That makes them useful only for surveying telescopes that sweep the night sky looking for near earth asteroids or artificial satellites.

But there’s another reason why astronomer still prefer glass: the revolution in adaptive optics that has swept through astronomy in recent years. Instead of building one big expensive mirror, astronomers build lots of small cheap hexagonal ones and fix them together, honeycomb-style, on actuators that can tilt them in different directions.

By changing the angle of these mirrors, astronomer can compensate for the degrading effects of the atmosphere on starlight. These telescopes produce pinsharp images even on the most turbulent of nights. Astronomers have never been happier.

But liquid mirrors are fighting back. A couple of years ago, we looked at the work of Denis Brousseau at Université Laval in Quebec and friends who are developing a way of manipulating the surface of a liquid mirror to acheive exactly the same kind of correction.

Instead of mercury, these guys use a ferromagnetic liquid and then distort its surface using powerful magnetic fields. (Ferromagnetic liquids do not have very reflective surfaces so they have to be coated themselves with a thin layer of metal-like film.)

At that time, this technique was fraught with problems. The magnetic fields, for example, could only be cycled at a rate of about 10Hz, severely limiting their application in astronomy.

The most serious problem, however, was that the deformation of the surface depended on the square of magnetic field strength. This non-linear response meant that a mirror could only be controlled with algorithms that would have to be built from scratch and were far more complex than anything used before in adaptive optics.

Today, Brousseau and buddies reveal a next generation liquid mirror that gets around these problems. The proof-of-principle mirror is just 5 cm across but sits atop a honeycomb of 91 actuators that can deform the liquid.

The team say they’ve found a way to cycle their actuators at rates of up to 1 Khz, much more useful than before. And they’ve overcome the the non-linear control problems by superimposing a strong uniform magnetic field on top of the field created by the actuators. This has the effect of linearising the response of the liquid.

The big advantage, of course, is that instead of having to develop their own exotic control algorithms, they can now use off-the-shelf algorithms developed for conventional adaptive optics.

The result is a mirror that ought to be able to compete with the vary best conventional adaptive optics but that can be built at a fraction of the price.

That could have a big impact on disciplines like astronomy, not because the optics are any better than glass alternatives but because they are so much cheaper. These liquid mirrors will appeal to astronomers the world over and also to any research groups requiring top quality mirrors for optical testing. Neat!

Ref: arxiv.org/abs/1006.2843: Linearization Of The Response Of A 91-Actuator Magnetic Liquid Deformable Mirror

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