Telescope Mirrors from Antifreeze?

Today's "Word of the Day" is ferrofluid. It's what chemists call a suspension of extremely small iron-based particles in some kind of liquid. The result is a room-temperature fluid with magnetic properties.

So why are you reading about this here and not in the Chemistry Junkies Forum? Because someday soon you might be ogling amazing images of deep space taken by a telescope with a ferrofluid mirror.

Astronomers have toyed with liquid-mirror telescopes for decades. In theory, a shallow vat of fluid, when spun slowly, assumes the paraboloidal shade needed for a reflecting telescope's primary mirror. This quirk of gravity is the key to the highly successful Steward Observatory Mirror Lab in Tucson, Arizona, where spinning ovens cast glass blanks for some of the world's largest telescopes.

The only suitably reflective liquid for spin-table mirrors has been mercury — not exactly the kind of material you want to spend any time with. Yet, despite mercury's toxicity, curious opticians continue to experiment with it. In fact, an international team is building a giant mercury-mirror telescope on a mountaintop in India, with the hope of getting "first light" sometime next year.

Experimental liquid mirror
An aluminum container 2.7 inches (7 cm) across holds a liquid mirror created by floating silver particles atop a ferrofuid of iron-oxide particles suspended in ethylene glycol.
J.-P. Déry, E. Borra, and A. Ritcey / Chemistry of Materials
Yesterday I learned of a remarkable finding by three researchers at Université Laval in Québec, Canada. In their work with ferrofluids, Jean-Philippe Déry, Ermanno Borra, and Anna Ritcey have chanced upon a concoction consisting of ethylene glycol — the antifreeze in our cars — mixed with maghemite, an iron oxide. The maghemite particles are no more than 10 nanometers (100 angstroms) across, and they get coated with a type of acetic acid that prevents them from clumping together while in suspension.

The team's real breakthrough has been to add (by spraying) a small amount of similarly tiny silver particles. These then float atop the ferrodfluid to create a mirror-smooth surface that's more reflective than liquid mercury. Better still, the shape of the surface can be altered by placing electromagnets beneath the container and adjusting the voltage applied to them — no spinning is needed.

As the trio reports in the November 25th issue of Chemistry of Materials, a biweekly journal of the American Chemical Society, so far they've created a lab-bench liquid mirror 2.7 inches (7 cm) across.

What's most amazing is that its surface is accurate to 1/20 the wavelength of red light (624 nanometers) — easily accurate enough for telescopic optics. They haven't yet tried to deform the silvery surface into a paraboloid, but Anna Ritcey told me that she's confident that can be achieved with the right combination of electromagnets.

You won't likely see antifreeze-and-silver reflectors in backyard settings — among other things, they have to be pointed straight up. But it's certainly a technology that bears watching closely.

5 thoughts on “Telescope Mirrors from Antifreeze?

  1. Wondering

    Why would the ferrofluid mirror have to be pointed straight up? If its curvature can be determined by electromagnets, shouldn’t it be possible, at least in principle, to alter the paraboloid to compensate for deformation caused by tilting the mirror through small angles, depending on the container?

    Not mentioned in the article: this sounds like the perfect adaptive-optic mirror.

  2. Roy RobinsonRoy Robinson

    With its high reflectivity and a 1/20 wave surface, the ferrofluid mirror sounds promising for use as an optical flat, without the complications of spinning or magnets. See, e.g., “Autocollimation Test for Schmidt Cameras” in ATM-3 (p. 342 in my 1974 edition), which utilizes an oil flat. Adding appropriate magnetic adjustments made possible with the ferrofluid mirror, we may even make a practical “dial-up” surface for use in a host of commercial or home-brew optical testing applications.

  3. Steaphany Waelder

    A fluid mirror telescope does not move the mirror to direct the view, instead the instruments at the prime focus are offset from being directly above the mirror and their position results in where in the sky the telescope points.

  4. Fred ShumanFred from Laurel, Md

    This trick was used long ago (19th century?), spinning Hg to make a zenith instrument. I suppose to ‘steer’ it, you could add one or two optical flats the size of, and ahead of, the fluid ‘primary,’ flats being easier to make than a paraboloid or sphere. A small, flat diagonal mirror à la Newton, et voilà! A fixed-paraboloid reflector scope! It’s pretty hard to see this taking over even amateur scope design, but human ingenuity has surprised us many a time already.

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