Pellerin went and spent a week in Japan during which he was out of contact with NASA. He was unprepared for what awaited him. "I land in St Louis airport and I made my first mistake: I called Washington," he says. His secretary asked him if he had spoken to his boss lately, and when he replied in the negative she put him straight through. "I'm thinking, 'What good news could this be?'" Pellerin recalls.
"He says, 'What do you know about spherical aberration?' I said all I know is that when amateurs build mirrors and do it sloppily they get what's called a downturned edge, and on different radii of the mirror it focuses the light on different parts of the optical axis and it's physically impossible to ever focus a telescope; they're useless."
Pellerin's boss responded: "Well you launched Hubble Space Telescope with a spherically aberrated mirror."
"This was two PhDs and this was the maturity of the conversation," Pellerin says: "I said, 'Did not.' He said, 'Did so.' 'Did not.' 'Did so.' 'Did not.' 'Did so.'" Finally his boss told him to go and pick up a national newspaper and read out loud the front page headline above the fold.
"So I find the St. Louis Post Dispatch and I bring it back and I pick up the phone and I read it. It says 'National disaster: Hubble launched with flawed mirror'. So he said to me, 'Now what do you say?' And I said — 'You guys are too much; how'd you get a fake newspaper printed in this airport lounge?'"
Pellerin returned to Washington DC for a crisis meeting with NASA's top administrators. What followed were congressional hearings into the disaster. "It's a misnomer," Pellerin says. "There's no-body hearing at congressional hearings; they should be called congressional browbeatings or congressional yelling sessions."
Pellerin was appointed NASA liaison to the failure review board that was formed. The mirror was fabricated in 1977 and he didn't take his position until 1982, so "the thinking was I could have had nothing to do with whatever problem the mirror had. It turns out that was wrong."
The technical error related to the null corrector used to test Hubble's aspheric mirror. Using a null corrector, imperfections in a mirror can be found and fixed by the person fabricating it. The mirror was balanced on what NASA described as a 'bed of nails': A series of steel pins with little springs that maintained the mirror in the shape that it would have in zero gravity. NASA built a reflective null corrector, because even optical glass has inhomogeneities that can cause refraction; this isn't a problem with a mirror. "Looked foolproof," Pellerin says. The null corrector was used first used to build a 60-inch mirror. NASA tested the mirror extensively. "It's just a throwaway mirror, we did everything to it. Perfect mirror."
After that, to build the telescope's 96-inch mirror "all you had to do was respace and put a new field lens on [the null corrector], which is really simple. But because we're building the world's most perfect mirror we didn't use a more normal process like micrometers or something." Instead, they obtained precise metering bars from the US National Bureau of Standards (now called the National Institute of Standards and Technology). The metering bars were used to respace the reflective null corrector’s two mirrors to produce the larger flight mirror.
NASA's Hubble Space Telescope Optical Systems Failure Report (known as the "Allen report", after Lew Allen, director NASA's Jet Propulsion Laboratory who headed the review in Hubble) states: "The ends of the metering rods were rounded and polished because the very precise positioning of the optics in the RNC [reflective null corrector] used an interferometer, rather than a mechanical measurement. This procedure involved auto-reflecting a focused beam of light off the end of a rod and observing an interference pattern from the beam that came back on itself. Centering the light beam on the rod end was essential for the measurement.
"To prevent the metering rod from being misaligned laterally with respect to the interferometer axis [so that the light hit the precise centre of the rod's curved end — RP], P-E decided to attach 'field caps' to one end of the rod… The field caps were fitted over the rod ends and had a small aperture in the center to ensure centering of the rod on the beam."
"The instructions were take the cap and spray paint it," Pellerin says.
"The guy was working under great stress because we were angry about costly delays, and were threatening to put Kodak's mirror in the telescope, which would have been humiliating for our contractor [Perkin-Elmer Corporation was responsible for the mirror]. So he's working, he's really hurrying, he can't find the spray paint. So instead he puts black tape on [the cap] and he takes his X-Acto knife and cuts a hole. He doesn't notice he made a shiny little burr. So he puts the thing on, the light hits the burr and goes back up.
"His instructions were to move the thing in every direction to see if the intensity of the light went down. So he moves it. Light goes onto the black tape, onto the black tape, onto the black tape, down the hole [in the field cap]. He assumes he got it centred."
However, the light had hit the burr instead of the top of the rod, which led to a gross misplacing of the two mirrors.
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