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Through a mirror-fluidly

A reader recently e-mailed this writer wanting to know more about liquid-mirrors replacing conventional mirrors in major telescopic observatories.

Here's the simplest way to explain this exotic mirror technology for use in telescopes: a liquid reflective metal, such as mercury, is spin in a container around an axis. The liquid must be 99.99 percent free of impurities and its surface must be dust free to be effective for deep-sky observing. As the liquid metal rotates, its shape can be altered for the purpose of focusing. So, when astronomers get just the right shape for their fluid mirror, they can focus it on distant starlight as good as-and sometimes better than-a conventional glass mirror. These mirrors are used for narrowband (zenith) telescopes.

Land-based liquid-mirror telescopes (LMTs) use either mercury or liquid gallium alloys in very tiny amounts-up to a millimeter in thickness. While these liquid metals are toxic, the small amounts used are protected and present no overt hazard to observers.

At the heart of a liquid-mirror scope is a cylindrical vessel made of kevlar that holds the liquid. This vessel is spun so that the mercury (or other liquid metal) assumes a paraboloid; this is accomplished via a motor that rotates the vessel up to several revolutions per minute (rpm). Looking at a liquid mirror, you'd be fooled at first glance-its shape looks exactly like a conventional telescope mirror.

The real advantage of a LMT over a glass-mirror telescope is its low manufacturing cost; the cost of a large liquid mirror telescope is nearly 95 percent less than a conventional mirror scope. Unlike conventional mirrors, liquid-metal mirrors do not have to be cast, ground, and polished; this fact alone eliminates considerable time and labor in construction.

The largest liquid-mirror telescope on Earth is the Large Zenith Telescope in western Canada. With a diameter of six meters (over 9 feet), its kevlar container rotates at 6 rpm to maintain a mercury mirror inside as a nearly perfect paraboloid.

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