It is destined for a point in space 1.5 million kilometres from Earth — too far away for astronauts to visit and fix the telescope if something goes wrong. Hubble required an after-launch repair in 1993, when astronauts used the space shuttle to get to the Earth-orbiting observatory and install corrective optics for its primary mirror, which had been improperly ground.
https://www.nasa.gov/content/hubbles-mirror-flaw
One may assume that maybe the error was simply too big and that's why the aberration. Here's the root cause and the magnitude of the error would be dismissed as nothing by most people on this planet but ultimately turned out to be huge!
"Ultimately the problem was traced to miscalibrated equipment during the mirror's manufacture. The result was a mirror with an aberration one-50th the thickness of a human hair, in the grinding of the mirror."
High tech in those days? Rumor I heard in the mid-1980s was that LLNL had [vertical?] lathes which could mechanically cut mirrors to optical-profile accuracy. Supposedly 100gpm liquid flows were required to keep the part temperatures uniform.