An old idea for astronomical imaging is undergoing a technology-driven renaissance
Shooting for the Moon
What does the future hold for liquid-mirror telescopes? Probably a lot. There are two new efforts under way right now. The International Liquid-Mirror Telescope, led by Jean Surdej of the University of Liège in Belgium, will be a 4-meter instrument equipped with a large-format CCD camera. Unlike the LZT, which of necessity had to be built near my university in Vancouver, where observing conditions are poor, the new telescope will be located at a superb site for astronomy on a Chilean mountaintop. The second project, led by Arlin P. S. Crotts of Columbia University, aims to build an 8-meter liquid-mirror telescope, also in Chile. Dubbed the Advanced Liquid-mirror Probe for Astrophysics, Cosmology and Asteroids (ALPACA), this telescope would have a wide 3-degree field of view and be equipped with a drift-scanning camera containing as many as 240 CCDs. With full funding, these instruments could begin operation in as little as three to four years.
Further in the future, the possibilities become really intriguing. Using active correcting elements at the focal point, it is possible for liquid-mirror telescopes to follow objects in the sky for as long as 30 minutes. Such tracking capabilities would allow these telescopes to operate at infrared wavelengths, which is impossible at the moment because infrared detector arrays cannot drift-scan. Advanced liquid-mirror telescopes could include adaptive optics to compensate for atmospheric turbulence. Also, it should be possible to combine the light from many separate liquid mirrors, just as is being done now for some conventional telescopes, providing very high sensitivity and resolution.
Perhaps the most remarkable idea yet voiced is that of building liquid-mirror telescopes on the Moon. The Moon is an ideal location because of its dark sky, stable gravity and the absence of any atmosphere that might disturb the liquid and distort the images obtained. Located near the Moon's north or south pole, a zenith-pointing telescope could observe the same area of sky for months on end, detecting and studying the most distant and faintest objects in the visible universe. On the Moon, lightweight rotating liquid mirrors could be deployed with diameters perhaps as large as 100 meters. These giant reflectors could not use mercury, which would freeze, but they could employ low-temperature ionic liquids that do not evaporate. The liquid would have to be made reflective by depositing a metallic film on its surface while the mirror is spinning. Because the Moon has no atmosphere, an air bearing would not be practical. Instead, the mirror would probably float on a magnetic field stabilized by superconducting elements.
Far from being a wild dream, the concept of a lunar liquid-mirror telescope has already won NASA support: An international team lead by J. Roger P. Angel of the University of Arizona is currently developing plans for such a powerful instrument. If this concept is carried forward successfully, liquid-mirror telescopes might help usher in a whole new era of astronomical imaging.
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