Obscure Firm Took Task to Focus Hubble : Optics: Others declined when faced with grinding mirrors to billionths of an inch. Tinsley Labs finished below budget and met all NASA specifications.
When NASA needed small corrector mirrors to restore sharp focus to the Hubble Space Telescope, it recruited some of the nation’s biggest high-technology corporations to make them.
Kodak declined to take on the task of grinding such convoluted mirrors, and Hughes and United Technologies could not meet the agency’s strict schedule and billionths-of-an-inch specifications.
Almost as an afterthought, NASA turned to Tinsley Laboratories.
The obscure company, hidden behind a shopping mall in this blue-collar San Francisco suburb, mostly made custom lenses for the electronics industry. But scientists at the Jet Propulsion Laboratory in Pasadena knew it also supplied optics for Voyager spacecraft and mirrors for the Keck telescopes in Hawaii.
Despite the daunting complexity and uncompromising requirements of the job, Tinsley, using computer-controlled grinding and polishing machines built by its own employees, turned out 36 mirrors on time, under budget--and well within NASA’s stringent design standards.
Eight of the mirrors, tucked inside a new wide-field and planetary camera, were installed by astronauts Monday night. Ten more, part of an instrument called Corrective Optics Space Telescope Axial Replacement, or COSTAR, were to be put into Hubble Tuesday night. The remaining 18 mirrors are duplicates stored at NASA’s Goddard Space Flight Center in Greenbelt, Md.
Tinsley executives here--and NASA officials in Washington--are confident the corrector mirrors will fully restore Hubble’s sight and permit astronomers to complete the orbiting observatory’s full scientific mission.
The two repair packages in the camera and COSTAR each are roughly the size of a telephone booth, but the mirrors inside are only as big as the coins required to make a call. Since even tiny defects may significantly distort images in such small mirrors, NASA decided the mirrors must be ground to within 10 Angstrom units (an Angstrom is four-billionths of an inch) of perfection.
Tinsley’s mirrors are perfect to within six Angstroms.
“That means there are only a few atoms out of place,” said James Crocker, the National Aeronautics and Space Administration engineer in charge of designing one of the two packages. Tinsley President Bob Aronno said such tolerances test the limit of today’s technology, both in polishing fused-silicate glass to such precision and being able to detect such minuscule flaws.
The difficulty was compounded because the Hubble repair team needed mirrors that were aspherical--that is, their concave surface is slightly steeper at the edges than it is toward the center, to compensate for an opposite error in the space telescope’s infamously flawed 96-inch main mirror.
At the same time, several mirrors must be asymmetrical--their surfaces must be steeper on one side than the other--to compensate for other complex optical factors.
Although Tinsley has produced such complicated optics in the past--for giant super lasers at nearby Lawrence Livermore National Laboratory, for example--the company had never before attempted to make them as small as NASA required.
“The smaller you make a mirror, particularly an asymmetrical (and aspherical one), the harder it is to do and to get smooth,” said Tinsley Vice President Dan Bajuk. “These were definitely a challenge.”
Crocker said NASA estimated it would require a full year and $3 million to produce 36 mirrors for the Hubble repair mission. Tinsley turned them out, fully tested, in 11 months for less than $1.6 million.
The space agency first approached Tinsley two years ago with a prescription for each mirror needed to correct for the Hubble’s too-flat primary mirror.
The 96-inch-wide main mirror, made by Perkin-Elmer Corp. of Danbury, Conn., was designed to concentrate starlight from large parts of the sky into a small area, letting astronomers see dim and distant galaxies. Because it is slightly too flat, light from the outside edge and the center of the mirror come into focus at different places, blurring objects and obscuring distant galaxies.
Bajuk said Tinsley wrote some new software to accommodate the difficult prescriptions and confirmed their accuracy in mathematical models, then loaded them into the computer-controlled grinding and polishing machines.
NASA’s Goddard Space Flight Center then applied an extremely smooth coat of highly reflective aluminum to the mirrors, and they were tested repeatedly for accuracy using laser-light patterns. Every one was perfect, Bajuk said.
“It was a step forward in size and accuracy,” said Aronno.
