In your message dated: Wed, 20 May 1998 16:03:39 -0000 you write:
>A catadioptric lens, to the best of my knowledge, is a lens which is
>solid -- the main mirror is the rear surface of the same block of glass
>which also is the front of the lens. So 'solid catadioptric' seems to
>be a tautology.
Ummmm. No. Catadioptric means an optical system with both lenses and
mirrors. Typically, there is a corrector plate, on the reverse of which
(in the Maksutov design) is deposited a spot of silver which acts as the
secondary mirror, which reflects the light from the primary mirror back
thru the central perforation in the primary to the camera or eyepiece.
The Schmidt-Cassegrain design uses a separate mirror mounted behind the
corrector plate, and the corrector plate is of a different design, as
are the secondary and primary mirrors. There is quite a space between
the secondary and the primary mirrors. Here is a rough optical diagram
of a cat, with the light entering it from the left:
______________________________________
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|__ | |
| | ----
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|__| ----
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Unless the lens is of a Maksutov design, there is a lot that can get out
of adjustment, and quite easily. The Mak design removes misalignments
of the secondary, but there is still the parallelism of the corrector
plate wrt the primary to worry about. I've never seen a "solid" cat,
but it is in the minority (if indeed not the only example extant) of a
cat that has absolutely no airspace. As you can see from the above
drawing, there is normally a big empty space in a cat, as one look at
500/8 Zuiko will confirm. It also appears to be a Schmidt-Cass design,
while (I think) the Nikon 500/8 is a Mak design.
Cheers,
--Lee
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