Liquid Eyes

By Tim Folger|Monday, February 01, 1993
Must all mirrors be made of glass? Not at all, says Ermanno Borra. A spinning pool of liquid mercury makes a perfect mirror for collecting starlight.

From Galileo’s first spyglass to the new 400-inch Keck telescope in Hawaii, optical telescopes have grown considerably in size and light- gathering power. But they’ve all had one thing in common: lenses or mirrors made of glass. In contrast, the device sitting in astronomer Ermanno Borra’s laboratory at Laval University in Quebec is an entirely new type of telescope. Its primary mirror is an eight-foot-wide, shallow pool of liquid mercury. Liquid mirrors have really outstanding optical quality, says Borra. They’re essentially perfect.

As the designers of the flawed Hubble Space Telescope know all too well, perfection is hard to achieve in a glass mirror, and even a small flaw can render a telescope myopic. To convert a hunk of glass into a perfect paraboloid--the right shape for collecting and focusing light to a point--requires painstaking grinding and polishing. Then the glass has to be coated with reflective aluminum. Borra doesn’t have to do any of that. He just pours his mirror.

He begins by pouring liquid polyurethane into a slowly spinning dish made of Kevlar (the stuff of bulletproof vests). The centrifugal motion naturally distributes the polyurethane in a parabolic shape over the Kevlar. After it solidifies, Borra pours liquid mercury on top, until the entire polyurethane container is coated with a thin layer--typically less than a tenth of an inch deep--of the silvery chemical. (Mercury is good because it is highly reflective, cheap, and liquid at room temperature.) The rotation also spreads the mercury into a smooth parabolic sheet, more smooth even than the most finely polished glass mirror. To maintain that shape, the entire telescope has to rotate constantly.

The eight-foot mirror now spinning in Borra’s lab is his largest yet, and it’s slightly bigger than the Hubble’s mirror. It cost around $30,000 to $40,000, versus $1 to $2 million for a comparable glass mirror, says Borra. Yet although liquid mirrors are cheap, and perhaps optically superior to glass mirrors, they do have one fundamental drawback: they can’t be tilted, because the mirror would spill. Thus they can’t be pointed at just any object; they can only look straight up at the narrow band of sky that passes overhead as Earth turns on its axis.

But that limitation hasn’t deterred Paul Hickson, a cosmologist at the University of British Columbia who recently finished building a liquid-mirror telescope with Borra’s help. Liquid mirrors are very well suited for cosmology, he explains, because what you need to study are large numbers of galaxies or quasars, which are all over the sky--it doesn’t matter where you look, you’ll see these things. And straight up is the best place to look because it’s through the least amount of atmosphere.

Also because they can’t be tilted, liquid-mirror telescopes can’t track a celestial object as it moves across the night sky--the standard time-lapse technique astronomers use to image very faint objects. But Borra and Hickson have found a way around that problem. Instead of observing a quasar one night for a whole hour, say, Hickson observes it for two minutes when it passes over his telescope, and stores the observation in a computer. The next time the quasar comes around, you can observe it again and add the observations to the computer, says Borra. In 30 nights you can get one hour of exposure.

Another use for liquid-mirror telescopes is in tracking space debris, which tend to be confined to a limited band of orbits. Borra and Hickson are helping NASA build a telescope for that purpose. Our research budget could never support building a ten-foot glass telescope mirror, says Andrew Potter, an astronomer at NASA’s Johnson Space Center. But the liquid mirror is probably at least one order of magnitude cheaper. And it should allow us to detect objects a half an inch or so in size.

Borra, who has been pushing liquid mirrors for a decade now, would ultimately like to see the construction of a truly huge one. With the Keck, he thinks, glass mirrors have gotten about as big as they’re going to get--larger ones could conceivably be built along the Keck’s segmented design, but they would be prohibitively expensive. Liquid mirrors might be a way to break the size barrier. My guess is that a 35-foot liquid telescope is reachable, even a 100-foot one, says Borra. But that is looking so far ahead I’d say it’s more of a hope.
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