The Federal panel investigating the $1.5 billion Hubble Space Telescope has found what appears to be the cause of the instrument's flawed mirror: a spacing error of about one millimeter in an optic device used to guide the making of the telescope's main mirror.
The error, which is considered huge for a complex optical system, was discovered in tests done at the mirror maker's plant on Wednesday, the National Aeronautics and Space Administration said yesterday. A millimeter is about the diameter of the tip of a ball-point pen.
NASA said in a statement that ''a discrepancy of this magnitude'' could account for Hubble's flaw. In interviews, astronomers and physicists went further, saying the spacing error was most likely the culprit.
The finding all but pinned down the cause of the problem as a manufacturing error rather than a design flaw. It also gave astronomers a firmer basis for designing second-generation Hubble instruments, to be lofted to the telescope in three years.
Scientists Relieved
Yesterday scientists said they were delighted the painful saga was drawing to a close.
''I feel relieved,'' said Dr. William G. Fastie, a physicist at Johns Hopkins University who helped oversee the making of Hubble's mirrors. ''The uncertainty was almost as bad as the fiasco itself.''
Dr. C. Robert O'Dell, an astronomer at Rice University in Houston who was the chief Hubble scientist from the late 1970's to the mid-1980's, said: ''An error of one millimeter is a major thing. This indicates a mistake was made. This was not uncontrollable, or something within the range of normal tolerances.''
Dr. Daniel J. Schroeder, an astronomer at Beloit College in Wisconsin who also helped oversee the design of Hubble's mirrors, said the finding was critical to the correction of the telescope's problems.
Cure Now Seems in Sight
''This is important,'' he said. ''It means designers of the second-generation instruments can pinpoint what they need to do.''
New, exacting knowledge of the distortion in the Hubble's mirror, Dr. Schroeder said, will allow a more precise prescription to be drawn up for corrective lenses for the telescope. For instance, NASA scientists are soon to begin grinding eight corrective lenses for Hubble's wide-field planetary camera, one of the telescope's main instruments. A replacement for this camera is to be installed by shuttle astronauts in 1993.
Dr. John Trauger, an astronomer at the Jet Propulsion Laboratory in Pasadena, Calif., who is the project scientist for the instrument, said that the solution in theory can be perfect, eliminating any distortion from the Hubble's flawed mirror.
''We can recover everything we lost, if we know the diagnoses accurately,'' he said.
The Probable Culprit
NASA yesterday cautioned that the finding was preliminary and that further tests would have to be done to rule out contributing factors and to more accurately describe the spacing error.
The mistake was found in an optical device known as a null corrector. In 1981, it was used to shape and channel laser light onto the mirror to measure precisely the surface of the main, 94-inch Hubble mirror during its final grinding and polishing by the Perkin-Elmer optics division in Danbury, Conn. The company was acquired last year by Hughes Aircraft, a subsidiary of General Motors, and is now known as Hughes Danbury Optical Systems Inc.
NASA said yesterday that for two weeks tests had been run at Hughes Danbury to systematically check various aspects of the device. The investigation is headed by Dr. Lew Allen, director of the Jet Propulsion Laboratory, and is to report to NASA.
Faulty Testing Process
The null corrector has been suspect from the start. In theory, there are many ways the telescope's problems -known as spherical aberration - could have been produced in mirror manufacture. The aberration is a perfectly symmetrical distortion that throws off Hubble's focus so that starlight, instead of falling at one point within the telescope, spreads over an area. The result is a blurry image.
Optic experts say such an aberration could have been caused by faulty instructions in mirror cutting machines or by poor physical support of the mirror. But theoretically, any such aberration should have been discovered by test equipment. Since it was not, experts suspected the testing process itself was faulty.
The heart of the testing process was the null corrector. It was placed between a light source and the Hubble's main mirror so an accurate reflection could be captured of the mirror's surface, which was shaped in a complex curve known as a hyperbola. That reflection was then analyzed to gauge the smoothness of the mirror's surface.
Yesterday NASA said that the recent optical tests showed there was ''a clear discrepancy of approximately one millimeter between the design of the null corrector and the device as its exists.''
Sarah Keegan, a NASA spokeswoman, said lens of the null corrector was too far from a mirror.
'A Lot More Testing'
In its statement, NASA said that the investigation would now concentrate on two areas - refining the measurement of the spacing discrepancy and continuing to examine all aspects of the null corrector and associated test apparatus.
The full investigative panel is scheduled to meet at Hughes Danbury next Thursday and Friday, and it will be its first opportunity to review the test procedures and data.
Thomas J. Arconti, a Hughes Danbury spokesman, stressed that the finding of the spacing error was preliminary. ''We just discovered this last night,'' he said. ''There's obviously a lot more testing that needs to be done, and we'll be working with NASA to compete that.''
Errors in the design of null correctors are not unknown. One of the world's newest ground-based telescopes - the New Technology Telescope of the European Southern Observatory - has a spherical aberration caused by a 1.8 millimeter error in the spacing of its null corrector. However, the ''adaptive optics'' of the telescope's deformable mirror allowed the distortion to be readily canceled. In effect, the error was bent out of existence, a feat Hubble is unable to achieve.