A device for measuring the final grinding had been miscalibrated (by a known, constant amount). The result was a spherical aberration, of a kind that is familiar to a lot of users of cheaper lenses and mirrors but on a much smaller scale.

Exactly as azernik says. If you are interested in a lot more detail this reports spills the tea: https://www.ssl.berkeley.edu/~mlampton/AllenReportHST.pdf