We have come to take the excellent performance of the Hubble Space Telescope for granted. However, immediately after launch, people were reminded that Hubble was not just an ordinary satellite, but a complex piece of innovative engineering and, as such, liable to teething problems. 

Soon after Hubble began sending images from space, scientists discovered that the telescope's primary mirror had a flaw called spherical aberration. The outer edge of the mirror was ground too flat by a depth of 4 microns (roughly equal to one fiftieth the thickness of a human hair). The flaw resulted in images that were fuzzy because some of the light from the objects being studied was being scattered.

During the first Hubble Servicing Mission in December 1993 a crew of astronauts carried out the repairs necessary to restore the telescope to its intended level of performance. Although the two other servicing missions which have since been performed were at least as demanding in terms of complexity and work load, the First Servicing Mission captured the attention of both the professional community and the public at large to a degree that no other Shuttle mission has achieved. Meticulously planned and brilliantly executed, the mission succeeded on all counts. It will go down in history as one of the highlights of human spaceflight.    

The Wide Field and Planetary Camera 1 was replaced with a second-generation camera (Wide Field and Planetary Camera 2) and the High-Speed Photometer was replaced with COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR is not a science instrument but a corrective optics package that corrects the aberration for the three remaining scientific instruments: the Faint Object Camera (FOC), the Faint Object Spectrograph (FOS), and the Goddard High Resolution Spectrograph (GHRS).

(click to enlarge) Before and after the spherical aberration correction of Hubble

More on Hubble at the Space Telescope Science Institute