I'll add a bit to John Petrush's description of coma. Catadioptric
telescopes and mirror camera lenses (basically the same) are made with a
spherical main mirror. Left uncorrected, the images would be terrible;
especially since the main mirror is typically a fast optic... like f/2
or even f/1. One of the worst abberations would be coma. Coma is an
off-axis abberation and grows worse the farther from the optical axis.
As John mentioned, a star should be a pinpoint. With coma it will be
rendered with a fuzzy, comet-like tail which will be pointing away from
the optical axis.
Catadioptric scopes (those with a corrector lens at the front) are made
in two optical designs... either Maksutov or Schmidt-Cassegrain. The
Maksutov corrector plate is a very thick meniscus lens with the
secondary mirror being an aluminzed spot on the back of the corrector
plate. The Maksutov design is used for mirror camera lenses, spotting
scopes and astronomical telescopes. The corrector plate gets heavy and
hard to manufacture as the aperture goes up. The Questar 7-inch
telescope may be the largest commercially available Maksutov design.
The Schmidt corrector plate appears to be a flat piece of glass with the
secondary mirror mechanically mounted via holes drilled in the middle.
I say "appears to be flat" since the plate actually has a sinusoidal
looking curve ground into the front which may be only a few thousandths
of an inch deep. Catadioptric telescopes by Meade and Celestron are
typically of Schmidt-Cassegrain design although the very small ones
(like 90mm aperture) may be Maksutov.
If memory serves me right Maksutov and Schmidt were both opticians. I
believe Schmidt was the chief optician at Palomar Observatory and that
the origin of the design was the 48 inch Schmidt (sky survey) camera at
Palomar. He designed it especially for wide field sky photography where
coma was always the worst bugaboo.
Chuck Norcutt
Woburn, Massachusetts
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