Archives - Astronomy: Page 5
Author: paul carson (Sat May 12, 2007 12:35 pm)
Title: Astronomy
Nestled in the mountains of south western British Columbia is a nondescript building that might easily be mistaken for a church or a ski chalet. Little about its appearance would suggest that hidden within is one of the largest astronomical telescopes in North America. Yet on clear nights, the steeply sloping roof rolls back, allowing the telescope to record images of the stars and galaxies passing overhead. More remarkable, however, is the fact that the heart of this telescope, the primary mirror, which collects and focuses light, is a rotating dish of liquid mercury.
Conventional astronomical telescopes, of course, employ glass mirrors for this purpose. The largest, the Keck twin 10-meter telescope atop Mauna Kea in Hawaii, actually use segmented primary mirrors, each composed of 36 hexagonal elements. These mirrors must be carefully ground and polished, to an accuracy of a few tens of nanometers, before being coated with a thin layer of aluminum or silver to make them reflective. Such mirrors also require a complex supports system to prevent temperature changes or the force of gravity from distorting the surface. And for most large, modern telescopes, a system of sensors and actuators actively controls the shape of the mirror on a fine scale so as to counteract the distortions created by the atmosphere, a strategy dubbed “adaptive optics.” These instruments are technological marvels, but they cost an enormous amount to build, roughly $10 million for one with a 6-meter-diameter mirror. Amazingly, comparable precision can be achieved simply by rotating a dish covered with mercury.
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