With its famous twin orbiting 370 miles above Earth inside NASA's Hubble Space Telescope, the almost forgotten Earth-bound back-up mirror is finally about to step into the spotlight and get some attention of its own.
While Hubble continues to show off the wonders of the cosmos, the 2.4-meter (94.5-inch) diameter back-up mirror goes on permanent display, starting tomorrow, as part of the "Explore The Universe" exhibit in the Smithsonian Institution's National Air and Space Museum in Washington.
Large concave mirrors are the heart of large astronomical telescopes. These mirrors gather the faint light from distant objects, and specially built cameras and other instruments can be used to study those objects in different ways. The larger the mirror, the more powerful the telescope. On Earth, the largest and most powerful telescopes are located on mountaintops where the thin air causes fewer distortions such as the "twinkling" of stars caused by air turbulence.
The largest telescopes on Earth have mirrors measuring 10 meters (33 feet) in diameter, far larger than Hubble's reflector. However, Hubble has a major advantage: it is in space where there is no air to distort the images, giving it a sharper view of the universe than any telescope ever built.
When NASA began building Hubble in the late 1970s, the space agency decided that the building of the primary mirror was so challenging and so crucial to the science program that it was a good idea to build a back-up copy.
Hubble's primary mirror was built by what was then called Perkin-Elmer Corporation, in Danbury, Conn. The company is now Hughes Danbury Optical Systems Inc. The back-up was built by the Eastman Kodak Company, Rochester, N.Y. Kodak, which used a more traditional method of grinding the mirror, completed the back-up mirror in 1980, before Perkin-Elmer completed the primary mirror.
Because Kodak used different equipment for monitoring and testing the mirror during fabrication, that mirror did not have the optical flaw that was unknowingly built into the Perkin-Elmer mirror.
As the space telescope was constructed and launched, the Kodak back-up mirror sat in its shipping crate in Danbury, Conn. It was eventually impounded along with other materials relating to the investigation of the mirror flaw on Hubble.
The Kodak mirror, a "near-twin" of the flawed Perkin-Elmer mirror, proved invaluable as It was used in tests to find out exactly what went wrong with the fabrication of the primary mirror aboard Hubble. Ultimately the problem was traced to miscalibrated equipment.
There was no practical way NASA could swap out the good mirror for the flawed mirror in space, but it turned out that the best solution was to build corrective optics that fixed the flaw much the same way a pair of glasses correct the vision of a near-sighted person.
The corrective optics and new instruments were built and installed on Hubble by spacewalking astronauts during a shuttle mission in 1993, and the telescope has been at the forefront of astronomy ever since.
The back-up mirror, its job done, was returned to storage and relative obscurity while its high-flying sister advanced the frontiers of science and grabbed the headlines. Starting tomorrow, however, that may change as Earthlings have their first opportunity to get acquainted with the twin of Hubble's huge glass heart.
More information about Hubble Space Telescope is available on the Internet at: http://www.stsci.edu
The Explore the Universe Web site is at: http://www.nasm.edu/galleries/etu/
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