In April 1990, Hubble reached orbit. The $1.5 billion observatory sent back blurry pictures instead of the sharp views everyone expected.
The outer edge of the main mirror was about 2.2 micrometers too flat. That is roughly one-fiftieth the thickness of a human hair.
The mirror was not cracked. It was not roughly polished.
It had been made with extraordinary precision, to the wrong shape.
How did a mistake that small pass every check?
The device used to test the mirror on the ground had itself been built incorrectly. The test treated the flaw as a correct result. The error showed up only after launch.
In December 1993, astronauts aboard the space shuttle Endeavour caught the telescope with a robotic arm and installed a new camera, WFPC2, plus COSTAR, a set of optical parts that corrected the flaw for the other science instruments.
Hubble got something like a pair of glasses, not a new main mirror.
On January 13, 1994, NASA showed the same star before and after the fix. The pictures matched what the engineering models had predicted.
The original mirror is still in orbit, with the same defect.
The glass was never fixed. The path of the light was.
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