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Hubble
operates on the same principle as reflecting telescopes
invented in the 17th century by Newton, Cassegrain,
and Gregory. A Cassegrain telescope is a reflecting
telescope in which light is reflected from a large
primary mirror onto a secondary mirror, which
then focuses the light back through a hole in
the primary to a point behind the mirror. The
science instruments are located directly behind
the primary mirror, which, for Hubble, makes instruments
easier to replace on-orbit. In Hubble the primary
mirrors are exactly configured to eliminate the
optical aberrations of ordinary telescopes.
The
main mirror in Hubble is about eight feet in diameter.
Light enters the telescope and strikes the main
mirror. The light is reflected forward to a smaller
(12-inch) secondary mirror where the light is
reflected again, returning down the telescope
through a two-foot hole in the center of the large
mirror where the image forms. The science instruments
record the images and analyze the incoming light
stream.
After
launch in 1990, NASA discovered that the large
mirror was flawed. The flaw was tiny — about
1/50th the thickness of a piece of paper — but
significant enough to distort Hubble's vision.
During the First Servicing Mission, astronauts
added corrective optics to compensate for the
flaw. The optics acted like eyeglasses to correct
Hubble's vision.
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