Liquid-mirror telescope set to give stargazing a new spin
|Biggest of its breed, the new Large
Zenith Telescope harnesses simple physics to probe space at a fraction of the cost of
In an isolated building deep in a forest east of Vancouver, Canada, lab-coated technicians maneuver sealed containers of toxic liquid, their faces covered by breathing masks to avoid inhaling the hazardous vapors that slowly damage the human nervous system. No, this is not a group of terrorists preparing a poison gas attack in North America. It's the Liquid Mirror Observatory, where a revolutionary 6-meter telescope with a surface of liquid mercury is about to peer into the heavens for the first time. One of only a handful of such liquid-mirror telescopes in existence, the Large Zenith Telescope (LZT) will be by far the largest. In fact, it will be the third-largest optical telescope in North America. Yet hardly anyone knows of its existence.
"We don't have a large number of people," explains project director Paul Hickson of the University of British Columbia in Vancouver, Canada, "so our PR budget is comparably low." In fact, the project's whole budget was a pittance. Building the telescope and observatory cost a mere $500,000--a fiftieth as much as a conventional 6-meter telescope. Little wonder that Hickson is now planning a giant successor: an array of liquid-mirror telescopes with an effective diameter of 50 meters, which could be completed in 8 years for less than $100 million.
The savings stem from the novel way telescopes such as the LZT collect and focus starlight. Astronomical telescopes traditionally do that with curved mirrors, which must be painstakingly ground and polished into a parabolic shape with nanometer accuracy at great expense. In the LZT, the laws of nature take care of the mirror's curvature. Centrifugal forces smooth out any rotating liquid into a parabolic shape. By using a reflective liquid, such as mercury, it's easy and cheap to spin the universe into focus.
Although the idea dates back to Isaac Newton, the first small liquid-mirror telescope was not built until 1909. Even then, the technology didn't work well until Ermanno Borra of Laval University in Quebec City, Canada, improved the design in a landmark paper published in 1982. Borra built a 1.5-meter prototype, and in the late 1980s, Hickson decided to start working on a 2.7-meter liquid-mirror telescope. "I had to build it in a garage," he says. "It was too big for my lab." A slightly larger follow-up effort formed the centerpiece of NASA's Orbital Debris Observatory in New Mexico, and in 1994 Hickson started work on the LZT, which is due to see its first light later this month. "I've been a bit skeptical because the technology started off looking pretty clunky," says Tim Hawarden of the Royal Observatory in Edinburgh, U.K. "But they now seem to have got most of the bugs out, so no tech-skepticism is allowed any more."
The 6-meter mirror consists of a segmented parabolic dish, shaped to an accuracy of about a tenth of a millimeter, made of expanded polystyrene and polyester in an aluminum and Kevlar frame and filled with 30 liters of mercury. The dish spins around some seven times per minute, precisely enough to distribute the mercury into a 1-millimeter-thick parabolic layer. The outer edge of the mirror has a velocity of just more than 2 meters per second. "If you stand next to the mirror, you feel a bit of wind," says Hickson, "but you don't hear anything at all. It's very quiet."
The wafer-thin mercury layer is essential to prevent surface ripples due to wind. In developing their scopes, however, Hickson and his team found that it was impossible to pour out a 1-millimeter-thick layer, because surface tension breaks the mercury into droplets. Their solution, repeated every few weeks when the mercury is removed for cleaning, is to fill the dish with 60 liters of mercury to make a thicker layer and then slowly drain half of it away through a hole in the center.
Liquid-mirror telescopes suffer from one big limitation. Because the dish of rotating liquid must lie flat, they can look in only one direction: straight up. But that's fine for studying objects such as remote galaxies, distant supernova explosions, or pieces of space junk: After all, they are found all over the sky. "For cosmology, liquid-mirror telescopes look very attractive," says Hawarden. The next generation of liquid-mirror telescopes will have movable secondary mirrors so they can see up to 4 degrees away from the zenith. Depending on latitude, that modification will bring a few percent of the whole sky into the telescope's view. Nevertheless, says Torben Andersen of Lund University, Sweden, "my guess is that [the successor of the LZT] will remain a 'niche' telescope, but that it has the potential of becoming a very good one." Gerry Gilmore of Cambridge University, U.K. agrees: "[These telescopes] can do a very few things well but can never be general purpose."
Still, Hickson has big plans for his niche scopes. Together with a number of institutes in New York state and Australia, the University of British Columbia is now trying to raise up to $8 million to build a 10- or 12-meter liquid mirror telescope. Even that will still be just a prototype for their ultimate goal, the Large-Aperture Mirror Array (LAMA): 18 identical telescopes in a 60-meter-wide circular array, to be sited in either Chile or New Mexico. LAMA is expected to cost between $50 million and $100 million. "It's a lot of money," says Hickson, "but it's still only 10% of the cost of the currently planned 30-meter telescopes" such as the California Extremely Large Telescope and the Giant Segmented Mirror Telescope (Science, 8 November 2002, p. 1151; 20 December 2002, p. 2311).
Although distant galaxies and supernovas will be the staple diet of both the LZT and LAMA, the proposed array is also ideally suited to hunt and study extrasolar planets crossing in front of their parent stars and maybe even to measure the composition of their atmospheres. Says Hawarden: "It's likely to be so much cheaper [than other extremely large telescopes] that it might get going faster and cream off most of the good stuff."
© Govert Schilling
Voor het laatst bijgewerkt op 15 maart 2003