What was Hubble's major optical flaw discovered after launch?

Hubble’s major optical flaw discovered after launch was spherical aberration. The primary mirror had been polished to the wrong shape, causing light reflected from its edges to focus at a different point from light reflected near its center.

The problem became clear in 1990, within weeks of launch, when Hubble’s first images appeared blurred instead of meeting the telescope’s demanding design specifications. The mirror was extremely smooth, but its outer edge was too flat by roughly 2.2 micrometers. A faulty test device had been assembled with one lens in the wrong position, allowing the mirror to be manufactured precisely—but incorrectly.

NASA repaired the problem during the first servicing mission in December 1993. Astronauts installed COSTAR, corrective optics that acted like prescription glasses, while the replacement Wide Field and Planetary Camera 2 included correction in its own optical design. Hubble’s reputation recovered after the repaired instruments produced sharp images.