Flaw in Hubble linked to misuse of measuring rod


HARTFORD, Conn. -- A misused measuring rod has been identified as the reason the main mirror in the Hubble Space Telescope was made to the wrong shape, National Aeronautics and Space Administration investigators said yesterday.

After a two-month study, scientists have determined that the problem with the measuring rod resulted in the improper assembly of an instrument used to control polishing of the telescope's 94.5-inch mirror.

The findings will be reported to members of NASA's investigative committee during meetings today and tomorrow in Danbury, Conn., where the mirror was made a decade ago by the optical division of Perkin-Elmer Corp., now Hughes Danbury Optical Systems Inc.

Since the committee last visited Danbury in August, investigators have concluded that technicians misusing the measuring rod built a spacing error into an optical instrument called a null corrector, said Robin Laurance, a European Space Agency official who has observer status on the committee investigating the flaw in the Hubble mirror.

The lens within the null corrector was placed 1.3 millimeters too far from the instrument's two mirrors. Because the null corrector was used to read the curve of the mirror's surface as it was polished, the spacing error resulted in the telescope mirror being manufactured to the incorrect shape.

Committee members also are expected to review additional evidence that Perkin-Elmer officials ignored warnings of a problem with the null corrector.

Committee member John D. Mangus, head of the optics branch of NASA's space technology division, said yesterday that tests conducted with a device called an inverse null, which was used to check the alignment of the reflective null corrector, indicated that there was something wrong with the measuring system.

The company, however, was so confident that the reflective null corrector had been assembled correctly that it dismissed the discrepancy, said Mr. Mangus and a retired Perkin-Elmer scientist who worked on the Hubble project.

The measuring rods now believed to be the source of the problem were used to measure the space between the mirrors and the lens in the reflective null corrector.

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