History: The Spherical Aberration Problem
The most serious and notorious problem was an optical defect called spherical aberration, which was caused by the malfunction of a measuring device used during the polishing of the mirror. As a result, Hubble could not achieve the best possible image quality, although still outperforming ground-based telescopes in many ways. Analysing the problem and developing an optical correction was a masterpiece of optical engineering and an outstanding example of the valuable collaboration between engineers and scientists from both America and Europe.
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).