Spinning lunar telescopes

This NASA story got my attention today. It might not be all that difficult to build big telescopes on the moon after all:

October 9, 2008: A team of internationally renowned astronomers and opticians may have found a way to make “unbelievably large” telescopes on the Moon.

“It’s so simple,” says Ermanno F. Borra, physics professor at the Optics Laboratory of Laval University in Quebec, Canada. “Isaac Newton knew that any liquid, if put into a shallow container and set spinning, naturally assumes a parabolic shape—the same shape needed by a telescope mirror to bring starlight to a focus. This could be the key to making a giant lunar observatory.”

see captionBorra, who has been studying liquid-mirror telescopes since 1992, and Simon P. “Pete” Worden, now director of NASA Ames Research Center, are members of a team taking the idea for a spin.

Right: An artist’s concept of a spinning liquid mirror telescope on the Moon. Credit: Univ. of British Columbia.

On Earth, a liquid mirror can be made quite smooth and perfect if it its container is kept exactly horizontal and rests on a low-vibration low-friction air bearing that is spun by a synchronous motor having one stable speed. “It doesn’t need to spin very fast,” says Borra. “The rim of a 4-meter–diameter mirror—the largest I’ve made in my lab—travels only 3 miles per hour, about the speed of a brisk walk. In the low gravity of the Moon, it would spin even slower.”

Most liquid-mirror telescopes on Earth have used mercury. Mercury remains molten at room temperature, and it reflects about 75 percent of incoming light, almost as good as silver. The biggest liquid-mirror telescope on Earth, the Large Zenith Telescope operated by the University of British Columbia in Canada, is 6 meters across—a diameter 20 percent larger than the famous 200-inch mirror of the Hale telescope at Palomar Observatory in California. Yet when completed in 2005, the Canadian Palomar-class liquid-mirror telescope cost less than $1 million to build—only a few percent the cost of a solid-mirror telescope of the same diameter–and, for that matter, only a sixth of Palomar’s original cost in 1948…..

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