Hubble's mirror was ground to the wrong shape. How do you fix a telescope in orbit?

The mirror's outer edge was too flat. Not by a visible amount — by roughly ten times the tolerance the design allowed, which is still far finer than anything the eye could detect. The consequence was spherical aberration: light striking the outer edge of the mirror came to a different focus than light striking the middle, so no single position produced a sharp image. NASA announced the flaw on 27 June 1990, two months after launch.

The mirror was not measured carelessly. It was measured with a device called a null corrector, built specifically to verify this shape, and the mirror was polished until it matched what that device reported as perfect. The device was wrong: a spacing inside it was off by 1.3 millimetres. Every subsequent measurement inherited the error and confirmed it, because they were all made with the same faulty reference. The mirror is an almost flawless example of the wrong surface.

Replacing a primary mirror in orbit was never realistic. Instead, engineers characterised the error precisely — an advantage of a flaw that is consistent rather than random — and built optics that introduced exactly the opposite distortion. In December 1993 astronauts installed two things: a redesigned camera, WFPC2, with the correction built into it, and COSTAR, a package of corrective mirrors that acted, in NASA's description, like a pair of eyeglasses for the remaining instruments.