At NASA's request, Riccardo Giacconi, Director of the Space Telescope Science Institute, convened a panel of experts in the summer of 1990 to identify and assess strategies for recovering the HST's capabilities degraded by spherical aberration. This panel included astronomers, optical scientists, astronauts, and engineers from industry, academia, and government. The European Space Agency (ESA)—a partner with NASA in the HST program—contributed significant expertise in optics and space astronomy. The Strategy Panel studied the problem and developed and evaluated solutions during the late summer and fall of 1990 during a series of meetings in the U.S. and Europe. The panel concluded the optical problem was well enough understood to design highly effective optical correctors. After investigating numerous solutions, it was decided that corrective mirrors with precisely the same amount of spherical aberration as the HST's primary mirror, but with the opposite mathematical sign, would pose the best approach.
by James H. Crocker