1980s:
Hubble Space Telescope Mirror Flight Backup
"The initial shock is wearing off, and people are beginning to think about clever work-arounds …"
"We're stubborn and clever," said NASA's chief scientist, Lennard Fisk, in June 1990 at a packed news conference called to address the discovery that the Hubble Space Telescope, newly in orbit, was flawed. A much anticipated scientific milestone, the telescope promised to provide the clearest views of the heavens ever seen. But after NASA expended billions of dollars and 25 years of effort, the telescope arrived in orbit with blurred vision. The news of the flaw came as a "punch in the stomach" to many astronomers and drew instant recriminations from Congress, the media, and the world of science. As Fisk spoke to reporters, he was adamant, "We're going to make it work."
Fisk was right — but it took many weeks for NASA and astronomers to identify the problem conclusively: the wonderful 240-centimeter (94-inch) mirror at Hubble's heart, manufactured by the Perkin-Elmer Corporation of Danbury, Connecticut, a reliable and prestigious optical firm, was ground and polished to the wrong formula. The press mocked the mirror as a "fun-house mirror," but its flaw was miniscule by everyday standards. A casual observer could never discern a difference between the flawed mirror and its correctly polished twin — a backup copy built by Kodak on display at the Museum. The difference in shape between the two amounted to less than two microns, or 1/50th the width of a human hair. To make this point, the Museum displays a human hair next to the mirror.
As small as two microns may be in everyday life, in optical physics it can make a monumental difference. To make the telescope work properly, this error had to be corrected. As Fisk predicted, NASA found a way, designing a set of corrective optics that space-walking astronauts installed on the telescope in 1993. Since that time, Hubble has made good on its promise to provide the sharpest wide-field optical images of celestial scenery ever seen by humanity. If NASA had not corrected the flaw, Hubble still would have worked but fulfilled only about 60 percent of its planned scientific mission.
The Corning Glass Works in Canton, New York, fabricated the mirror blanks using a special silicon/titanium oxide formula designed to make the mirrors almost completely insensitive to temperature changes. To limit weight and ensure sufficient rigidity to withstand the Shuttle launch, each blank combined two thick disks of glass fused to thin honeycomb support structures.
The Museum's blank then moved to the Kodak Apparatus Division, located in Rochester, New York, and the flight blank went to Perkin Elmer. Engineers ground each mirror to successively finer tolerances using computer-assisted testing routines. For the flight mirror, the final step was to aluminize the mirror's surface, to make it reflective and thereby allow the telescope to capture and analyze incoming light. Kodak did not go through this final step with the Museum's backup mirror-explaining (as seen here) the absence of a shiny surface. If NASA had chosen initially to fly the Kodak backup, in all likelihood the "flaw" would never have been part of Hubble's famous legacy.
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