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.
The biggest advantage of an LMT is its relative low cost. Liquid telescopes cost much less to build than polished aluminum mirrors of similar size. For example, the Large Zenith Telescope carried a price tag of $1 million. A comparable glass mirror telescope would cost 100 times that much to build. And LMTs cost less to maintain, mainly because the liquid mirror doesn’t need to be cleaned, adjusted or aluminized.
Of course, there are some drawbacks. Mercury is extremely toxic, so working with it poses some long-term health risks. Not only that, the dish holding the mercury can only be tilted so far before the liquid spills out. This limits the view of an LMT, which can only look straight up.
Supporting the dish is a steel truss and 19 adjustable pads. 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 to act 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.
Photo courtesy of Professional Instrument Company
The air bearing spindle at the bottom of a liquid mirror telescope
The one problem with the Large Zenith Telescope -- a problem it shares with all Earth-bound telescopes -- is its location. Even at an altitude of 1,295 feet, its view of the heavens is still shielded by the atmosphere. If a liquid mirror telescope mirror 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.