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Ask any astronomer if she’d like a perfectly parabolic six-meter-diameter mirror at one-fiftieth the price, and she’ll be unable to answer for all the drooling. Yet, according to the journal Science, the folks at the Alberta Canada Large Zenith Telescope (LZT) have accomplished exactly that. The secret? The mirror is 30 liters of rotating mercury.

When the heavy liquid metal is rotated, the “centrifugal” (inertial) forces combine with gravity to form the liquid into a near-mathematically-perfect paraboliod. This shape is ideal for collecting the faint light from astronomical objects and focusing it into an image. The larger the mirror, the more light gathered, and the clearer pictures you get (within certain constraints).

The LZT is not the largest telescope ever made; many larger telescopes have been made with conventional glass mirrors or flexible “adaptive optics”. However, it is probably the largest telescope in its price range, about 500 000 USD. Nor is the LZT the only liquid mirror telescope in existence. Many others exist, but the LZT is the largest.

Ah, you say, but there must be a catch. Indeed there is. Because the liquid is shaped by gravity, the telescope can only point straight up (the “Z” in “LZT”). This isn’t as bad as it might sound, because one can still observe many objects just by waiting for the right season and time of night. Also, the straight-up orientation is just fine for studying far away galaxies and supernovae and tracking space debris.

To start up the telescope, workers in protective suits and respirators (mercury causes cumulative poisoning, you know) pour 60 liters of mercury into the specially formed bowl. Once the liquid is up to speed, half of the mercury is drained away until only a thin 1 millimeter layer remains⁠⁠—this way, the wind won’t cause ripples. (They tried using the smaller amount to start with, but it kept breaking apart and beading up.) The mirror quietly rotates seven times a second.

Project director Paul Hickson has plans to build a 10- or 12-meter telescope. On the horizon is an optical array of liquid telescopes with an effective diameter of 50 or more meters. Such an array could be very helpful in finding planets around other stars.

The liquid mirror isn’t a new idea. In fact, it’s as old as the concept of a mirror-based telescope, as Isaac Newton came up with both ideas. Performance of rotating liquid mirrors was poor until a landmark 1982 paper by Ermanno Borra that solved significant problems with smoothness. But one group had been using stationary liquid mirrors for years. To set the time, folks at the Naval Observatory used to mark when certain stars were directly overhead by peering into a pool of mercury.

If you want to take advantage of physics to shape a mirror of your own, don’t try it with mercury, which is toxic and tricky. Rotating kilns have been used to shape parabolic glass. An easier way yet is an Amateur Scientist column in the February 1994 Scientific American. The article describes how to make a mirror with resin and a phonograph turntable. (Be forewarned⁠⁠—there are significant caveats.) The same issue has an article about liquid mirror telescopes.