Cause of Hubble Telescope Defect Reportedly Found

The defect that blurred the Hubble space telescope’s view of the heavens apparently was caused by a flaw in an instrument used to measure the observatory’s mirror as it was being built, officials said Thursday.

Preliminary results of tests conducted on a testing device called a “null corrector” found one spacing in the instrument was about one millimeter off from specifications, which would account for the deficiency, officials said.

“This is certainly the smoking rifle,” said National Aeronautics and Space Administration project scientist Edward Weiler. “There may be a few peashooters around. But this is the rifle.”

The null corrector was used to measure the surface of the telescope’s primary mirror in 1981 when it was being ground in Danbury, Conn., by what was then Perkin-Elmer Corp.

Officials were uncertain how the error in the null corrector occurred and withheld any final conclusion until the possibility of additional problems is eliminated.

Although upset that such an error occurred, Weiler said he was pleased that the probable cause of the telescope’s defect had been found because it will enable engineers to be more certain that repairs being planned will correct the problem.

Engineers are developing a “prescription” for mirrors that will be built into a replacement camera officials hope to install in the telescope in 1993 to restore the telescope to full capability.

The long-awaited, highly publicized $1.5-billion telescope was expected to revolutionize astronomy by peering more deeply and with greater clarity into space than ever before from its perch high above Earth’s atmosphere.

But, after the 12-ton instrument was ferried into space in April, engineers discovered that the telescope’s supposedly perfect optical system was seriously flawed because its 94-inch primary mirror suffered from spherical aberration, meaning its curvature is slightly incorrect.

One millimeter is about 1/25 of an inch, or about the diameter of the point of a ball-point pen.

“Preliminary analysis indicates that a discrepancy of this magnitude could cause spherical aberration similar to that observed in the (telescope’s) primary mirror,” a NASA statement said.

The observatory can still conduct valuable research, but the flaw sharply reduced the telescope’s abilities by preventing the device from bringing starlight to a sharp focus.