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Putting Glasses on Hubble
| Article
# : |
20516 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
5 / 1992 |
2,885 Words |
| Author
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Seth Shostak
Seth Shostak, who currently resides in Mountain View,
California, has spent most of his career as a research radio
astronomer. |
In 1929, Edwin Hubble announced one of the most extraordinary discoveries of the twentieth century: that the universe was expanding. Hubble had made his astounding find by meticulous study of faint and distant galaxies, using the biggest telescopes then in existence.
On April 24, 1990, the memory of this formidable scientist received uncommon tribute. The world's most powerful astronomical instrument was launched from the Kennedy Space Center in the bay of the Shuttle Discovery. The telescope bore Hubble's name.
It was a proud moment. But two months later, the famous astronomer might have been rolling in his grave.
The problems with the Hubble Space Telescope developed slowly. Two months after launch engineers were carrying out a series of routine focus tests on their new eye in space. The results were perplexing; the star images were invariably swathed in large, fuzzy blobs of light. No amount of adjustment would sharpen the images, and experts noted that the blobs had the characteristics of a classic optical defect known as spherical aberration. A mirror that is spherically shaped (i.e., has a shape like part of a ball), will not focus distant objects to one sharp point. Instead, it will produce haloed images similar to stars seen through a thin fog. This was the sort of picture Hubble was returning to its dismayed masters, and NASA engineers frantically tried to determine the cause. In early July 1990, the sickening news broke: Hubble's primary mirror was flawed. What was supposed to be the most sophisticated telescope ever build by man was suddenly perceived to be a $2 billion hunk of space junk.
Construction Error
By now just about everyone is familiar with the mortifying error leading to Hubble's blurred vision. A computer-controlled polisher at the Perkin-Elmer Corporation had been used to shape the main light-gathering element, the 2.4-meter (94-inch) primary mirror. This high-tech setup was essential to achieve the very stringent surface accuracy required. Deviation of the mirror from the desired mathematical curve would have to be no more than 1/50th the wavelength of green light, or about one millionth of an inch.
But analysis of images being received from Hubble indicated that the outer areas of the mirror must have been overground by hundreds of times this amount, an obviously gross error notwithstanding the fact that it
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