NASA Committee Finds Root of Hubble Trouble

Faulty Measuring Guide Cited as Cause

WASHINGTON--An error of about one millimeter--called "astonishing" by one expert for its large size--has been found in a measuring device used to guide the manufacture of a flawed mirror on the Hubble Space Telescope.

A NASA committee investigating the defect that has crippled the $1.5 billion telescope announced yesterday that the millimeter mistake was found while testing the measuring device, called a null corrector.

The space telescope, touted as an orbiting observatory that would be able to see objects up to 14 billion light years away, was launched in April.

Engineers discovered two months later that a mirror in the device had been manufactured wrong. As a result, the telescope's views of stars are blurred and of severely reduced value to astronomers.

The investigation committee said in a one-page statement that it found the millimeter error while testing a null corrector on Wednesday at the Hughes Danbury Optical Systems plant in Danbury, Conn., where the mirrors were made.

Hughes Danbury had preserved the null corrector in the exact position that had been used to grind and polish the mirrors in the early 1980s.

Preliminary results of the test, the statement said, "have revealed a clear discrepancy of approximately one millimeter between the design of the null corrector and the device as it exists."

A millimeter is about one twenty-fifth of an inch, or about the size of the very tip of a ballpoint pen.

Daniel Schulte, a senior scientist at the optical laboratory at the Lock-heed Palo Alto Research Laboratory in California, said an error of that magnitude was "astonishing."

"That's gross," he said. "There's no reason for an error of that size to be tolerated."

Schulte said that in normal optical manufacturing, a difference of a 20th or a 50th of a millimeter is considered "standard tolerance."

He said the error was so large "it had to be a transposition of numbers or something like that, that was carried through. It had to be something clerical like that."