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.
Canada's LZT is located at a University of British Columbia research park near Vancouver. As a LMT, it ranks among the largest optical telescopes on Earth. And compared to similar sized conventional-mirror instruments, the LZT was inexpensive to construct, thanks in part, to cannibalized parts from a defunct U.S. telescope.
"The Large Zenith Telescope project began in 1994, as a collaboration between scientists at UBC, Laval University and the Institut d'Astrophysique de Paris," according to Dr. Paul Hickson, LZT's director . "The principal scientific goals of the project are to measure spectral energy distributions and redshifts of over 100,000 galaxies and quasars, and to detect distant supernovae. These observations will allow us to study cosmology, the large-scale structure of the universe, and the evolution of galaxies."
LZT astronomers received used parts from NASA's own narrowband (zenith) Orbital Debris Observatory telescope in New Mexico. The defunct Orbital Debris Observatory ceased operations thanks to the U.S. Congress pulling the plug on its vital space-junk observation program.
In the 22nd century, liquid mirror telescopes may be built on the Moon. The frigid surface temperatures at the lunar poles are ideal for LMTs. LMTs on the Moon would gather far red-shifted light-so-called long-wave infrared light-coming from the extreme depths of space and time.
What's in the Sky: This weekend, after sunset in the S.E., a small telescope will reveal Saturn and its big moon Titan. Look for the tiny orange "star" east of bright Saturn. Titan contains more hydrocarbon material than all of Earth's oil and natural gas reserves.
Lou Varricchio, M.Sc., was a senior science writer at the NASA Ames Research Center in California. He is currently a member of the NASA-JPL Solar System Ambassador program in Vermont. He received the U.S. Civil Air Patrol's Maj. Gen. Chuck Yeager Aerospace Education Achievement Award recently. He is available for public presentations at no charge. You can e-mail him at: email@example.com.