How A Tool For Perfect Human Vision Grew From One Of NASA's Greatest Blunders

Mirror Testing The flight mirrors for the James Webb Space Telescope undergo cryogenic testing at NASA Marshall Space Flight Center. Ball Aerospace

Tech Transfers:  Popular Science

Now a system designed to make sure Hubble doesn’t happen again is being used to build better contact lenses, and to ensure more accurate laser surgery. It’s one of many ways in which NASA technology spins off into new consumer products--but it’s one of few that stems, at least in part, from one of the space agency’s biggest blunders.

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The Hubble Space Telescope, arguably the greatest observatory ever conceived, is one of the crowning achievements of modern science. But its first days in space were not exactly proud. Back in the 1980s, the company contracted to build Hubble’s primary mirror didn’t have a way to test its own testing equipment, and it was ultimately used improperly, resulting in a misshapen mirror.

The now-legendary saga stemmed from a positioning rod in a tool called a “null corrector,” used to create an optical template, kind of like a map for how to grind the telescope’s 96-inch primary mirror. Other tests were not considered accurate enough to measure the mirror’s perfect angle, so mirror-maker Perkin Elmer Corp. set out to design a perfect null corrector. A rod in this null corrector was installed upside down, changing the angle at which its locator-beam reflected light. The rod looked like it was higher than it really was--objects in the mirror may be closer than they appear, if you will. When workers changed the mirror grinding to compensate for this, the result was a 1.3-millimeter error in the curvature of the mirror.

Hubble suffered from a spherical aberration, which means light reflecting off the edge of the mirror focuses on a different point from the light reflecting off its center. NASA engineers were able to figure out how it happened, and three years after Hubble’s launch, spacewalking astronauts installed corrective optics that fixed the problem.

If you are putting a space telescope a million miles up, and will never see it again, it has to be right. The same is true for people. If you are doing laser surgery on someone, and you are working on their eye, it has to be right.
That won’t be possible for Hubble’s successor, the James Webb Space Telescope. Parked in a gravitational three-way among Earth, the moon and the sun, the JWST’s perch 1 million miles from home will never be accessible for space-based repairs. It’s got to be right the first time; it needs a perfect map for grinding a perfect mirror. That's where Dan Neal comes in.

The JWST is so big, its primary mirror is actually 18 separate beryllium hexagons that come together to form one big, shiny surface. Each segment was cast in a rough shape, machined and then ground to perfection. The segments start with a few thousandths of an inch of deviation, but must be ground to optical tolerances, with fractions of a micron in variation, Neal explained.

“There’s a metrology gap. You can measure something larger with mechanical means, but when you need to get to 0.1 micron — that’s going from 40 microns of error to 0.1,” he said. “The measurement techniques don’t exist for something in between, so we had to invent those.”