How Lunar Liquid Mirror Telescopes Work

The Large Zenith Telescope

The Large Zenith Telescope
The Large Zenith Telescope
NASA/Photo by Paul Hickson (University of British Columbia)

The largest LMT on Earth is the Large Zenith Telescope in British Columbia. Its spinning liquid mirror is almost 20 feet across and weighs three tons, making it the third-largest telescope in North America. The dish that holds the mercury is fabricated from hexagonal segments glued together to form a shell. Each piece has a high-density foam core covered with fiberglass. To give the shell a concave shape, it is heated in a large oven. A wall at the rim of the mirror prevents mercury from spilling.

A steel truss and 19 adjustable pads support the dish. The truss, in turn, is supported by a stainless-steel air bearing designed just for the Large Zenith Telescope. An air bearing is a special type of bearing that uses a thin film of pressurized air as the lubricant around the shaft that turns the mirror. Normal bearings that use oil lubricants are less effective, because they produce vibrations and unstable rotations that degrade image quality. As a zero-friction solution, an air bearing eliminates these problems, leading to a perfectly smooth, vibration-free rotation. A built-in brushless DC motor turns the air bearing spindle and can rotate a load up to 10 tons at approximately 10 revolutions per minute.

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­Six support legs attach the primary mirror to a ring at the top of the telescope. The ring supports the detector and a smaller refracting lens that helps focus the image. The detector includes a charge-coupled device (CCD), which gathers photons of light and converts them into picture elements, or pixels. These pixels are transferred to a computer screen and pieced together to form an image that can be manipulated and enhanced to improve the image detail. The computer is not housed in the telescope's observatory structure, but in a nearby building.

The one problem with the Large Zenith Telescope -- a problem it shares with all earthbound telescopes -- is its location. Even at an altitude of 1,295 feet, the atmosphere still shields its view of the heavens. If a liquid mirror telescope could be placed on the moon, where there is no atmosphere to block ultraviolet, infrared and other forms of energy, it could provide even more spectacular results. But, as we'll see in the next section, building an LMT on the moon presents its own challenges.­