The encyclopedia · Engineering & Operations · Technical decision · 1990
Hubble launched with a mirror ground perfectly — to the wrong shape
A 1.3mm error in one test rig made the world's most precise mirror precisely wrong. Two cheaper tests caught it — and were overruled.
NASA · Perkin-Elmer · 1990-04-24
What happened
The Hubble Space Telescope launched in April 1990 carrying the most precisely figured large mirror ever made. Its first images came back blurred. The mirror's edge had been ground too flat by about 2.2 microns — a spherical aberration that scattered starlight into fuzzy halos and crippled the flagship instrument of a $1.5 billion program.
The investigation traced the flaw to a single instrument: the reflective null corrector Perkin-Elmer used to guide the grinding. A field lens inside it had been positioned 1.3mm off, because a technician had measured against a metering rod whose end cap was worn — the laser had reflected off the cap instead of the rod. The polishing team then shaped the mirror to match the corrupted reference with superb accuracy: a perfect answer to the wrong question.
The part that turned an accident into a scandal: two other, cruder null correctors both showed spherical aberration during testing. The discrepancies were dismissed on the grounds that the fancy instrument must be right and the cheap ones wrong. In 1993, astronauts installed COSTAR — corrective optics, effectively spectacles — in one of the most celebrated servicing missions ever flown, and Hubble went on to remake astronomy.
Why it happened
- The entire manufacturing process trusted a single reference instrument with no independent end-to-end test of the finished mirror.
- Two independent tests disagreed with the primary rig and were explained away instead of investigated.
- Schedule and budget pressure on a delayed program made 'the expensive instrument is right' the convenient conclusion.
- A worn end cap — physical wear on a measuring aid — sat below the threshold anyone thought worth checking.
The lesson
When two instruments disagree, that disagreement is the finding. Precision without an independent cross-check is just confidence, beautifully machined.
Aftermath
Post-repair Hubble produced the Deep Field images and reshaped cosmology; the flaw's paper trail reshaped how NASA treats dissenting data. The telescope outlived every prediction and was still observing three decades after launch.
Sources
- The Hubble Space Telescope Optical Systems Failure Report (Allen Report, NASA 1990)
- Hubble's mirror flaw — NASA history
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