The HST’s mirror was originally defective and required a Shuttle mission to correct it.
http://www.scienceclarified.com/scitech/Telescopes/Hubble.html
When the HST was launched, an optimistic NASA spokesperson called the telescope a new window on the universe. Entering orbit, all systems functioned properly when astronomers sent the remote signal to open the door that covered the telescope optics to take the first picture. The so-called first light occurred on May 20, 1990.
The photographs transmitted back to Earth successfully, yet experienced astronomers found the images disturbing. They were the wrong shape. Engineers attempted to adjust the lens, but after several weeks they recognized that something was seriously wrong. As more blurred photos poured in, astronomer Eric Chaisson inspected the faulty images and later recalled, “I sensed a total deflation in my gut.” 24
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. This mistake severely reduced the resolution of the telescope so that when focused, it was able to gather only about 15 percent of the light of a very distant star instead of the 80 percent needed to produce a clear image.
The mistake was devastating to the $1.5 billion project. The mirror itself could not be repaired or exchanged deep in space, so NASA engineers went to work to develop corrective optics for Hubble’s mirror.
