Because of wrong information supplied by members of the Hubble Space Telescope investigating team, it was incorrectly reported yesterday that the flaw in the telescope's main mirror arose from a 1 millimeter error in spacing between two mirrors in a test device called a null corrector. The spacing error was, in fact, between a mirror and a lens in the device. (Published 8/11/90)
The $1.5 billion Hubble Space Telescope appears to have gotten its blurry vision because there was a flaw in the device the mirror-makers used to guide their grinding of the huge reflector, the NASA-appointed investigating team announced yesterday.
The flaw consisted of an error in the spacing between two small mirrors inside the test device. The distance was off by the thickness of a dime. That error appears to have led technicians a decade ago to grind the Hubble's main mirror to a curvature that is off by less than 1/50 the thickness of a human hair.
While the errors sound tiny, in the precise world of optical astronomy such bungles are considered huge. Because the mirror produces a blurry image, the scientific usefulness of the telescope has been severely degraded, disappointing a generation of astronomers who nursed the long-delayed project for almost two decades. The Hubble's failure has also evoked the wrath of congressional critics who question the ability of the National Aeronautics and Space Administration to oversee complex and ambitious projects.
If further tests establish that this was the only error, it should guide engineers in making corrective optics to be installed aboard the Hubble during a visit by space shuttle astronauts, now set for 1993.
The error also points to a key revelation in the Hubble investigation -- that NASA officials allowed the mirror contractor to rely on a single test to certify that the Hubble mirrors were correct.
Many optical experts say that NASA should have required at least two, if not three, independent tests. Such tests were repeatedly recommended, but never carried out, according to former employees of Perkin-Elmer Corp., the company that designed, built and tested the mirrors. Perkin-Elmer is now called Hughes Danbury Optical Systems.
The investigating team announced yesterday that tests this week traced the flaw to a device called a reflective null corrector.
The null corrector resembles a miniature Hubble telescope. It contains a pair of small mirrors and a focusing lens. During testing of the Hubble's main mirror, laser light would be shot into the null corrector, where it would bounce between the mirrors, exit to strike the main Hubble mirror and then bounce back though the null corrector. Technicians would analyze the returned image, which would tell them whether the mirror was accurately ground.
The test equipment remains essentially unchanged since it was used during the final polishing and coating of the mirrors in 1981.
Investigators said yesterday that the spacing between the two small mirrors in the null corrector was off by about one millimeter, or the thickness of a dime. The investigative team, led by Lew Allen of NASA's Jet Propulsion Laboratory, said a discrepancy of this magnitude could cause an optical defect called spherical aberration, which is what astrononomers observed from Hubble's main mirror.
Daniel Schroeder, an optical expert at Beloit College in Wisconsin who advised NASA on the telescope's mirrors, said yesterday that neither he nor his colleagues at NASA checked such details as the calibration of the null corrector. "We didn't look over their shoulders," Schroeder said. He and others said they simply assumed that Perkin-Elmer technicians knew what they were doing.
Roger Angel, an optical scientist at the University of Arizona and a member of the investigating team, said yesterday that the error "certainly wasn't a matter of being slipshod." Angel said Perkin-Elmer workers were painstaking in their attempt to build the world's most perfect mirrors. But Angel and other optical specialists fault NASA and Perkin-Elmer for not doing additional tests on the mirrors.
NASA officials reacted to the news yesterday with a mixture of relief and embarrassment. "I guess I would say in a sense it is good news," said NASA deputy associate administrator Sam Keller, who oversaw the development of the telescope. "We should be able to ascertain now exactly what the figure of the mirror is."