Why was Hubble's mirror too flat?

Hubble's main mirror being polished before installation. The edges of the were polished very slightly too flat, leaving the telescope unable to focus perfectly. ... of these was an optical defect called spherical aberration, which was caused by the malfunction of a measuring device used during the polishing of the mirror. read more

It wasn't too flat. It is concave, but it is slightly more shallow than called for in the design. The outer edge was too shallow, by about two microns. A human hair has a thickness of about 90 microns. read more

History: The Spherical Aberration Problem Hubble’s main mirror being polished before installation. The edges of the were polished very slightly too flat, leaving the telescope unable to focus perfectly. read more

Hubble's main mirror was the wrong shape and could not focus properly. Engineers inspected an identical backup mirror and discovered than the central region of the mirror was too flat by just a few nanometers. read more