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oon 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.
After this discovery, scientists and engineers developed corrective
optics that functioned like eyeglasses to restore Hubble's vision. The
optics of the Wide Field and Planetary Camera 2, which was already under
construction when the problem was discovered, were changed to correct
for spherical aberration.
The original science instruments on Hubble were fixed using the Corrective
Optics Space Telescope Axial Replacement (COSTAR) apparatus, which was
installed during the 1993
First Servicing Mission. By placing small and carefully designed
mirrors in front of these instruments, COSTAR successfully improved
their vision to their original design goals.
All the instruments installed during Servicing Missions 2, 3A and 3B
have internal corrections for spherical aberration.
The COSTAR apparatus was built by Ball Aerospace.
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