"cat" is short for "catadioptric"... a lens composed of both mirror and
refractive elements. There are two main types; Maksutov or
Schmidt-Cassegrain. Lenses/telescopes up to about 90mm diameter are
typically of Maksutov design (as are almost every 500mm mirror lens I've
ever seen) while larger ones tend to be Schmidt-Cassegrain. I know of
no Maksutov larger than 7" diameter which is the largest made by
Questar. Check the price of a Questar 7 and you'll understand why
Maksutov's don't come larger. A Maksutov has a very thick and heavy and
deeply curved meniscus lens as its front element. Casting such a large,
thick piece of glass without bubbles is difficult and expensive and
grinding and polishing is tough too.
A Schmidt design uses what's called a "Schmidt corrector plate" which is
a thin, nearly flat front element. Although it looks flat it actually
has a somewhat sinusoidal looking curve on one side that may be only a
few thousandth's of an inch deep. The Schmidt design was invented in
the 30's but remained rare since the corrector plate wasn't able to be
ground and polished by machine until Celestron, working with a Japanese
company (late 60's or early 70's I think), figured out how to do it by
machine. So, for anything over 90mm the Schmidt design is cheaper to
manufacture and probably performs better except for those superb things
that Questar makes.
In small lenses like a 500mm f/8 the Maksutov design offers another cost
saving in that the back side of the Maksutov corrector can be configured
to have the same curve as the secondary mirror and thus the secondary
mirror is nothing but an aluminized spot on the back side of the
corrector lens. A Schmidt design requires a physically separate mirror
which is mounted in its own cell and fastened on thorough a hole in the
corrector plate.
Probably more than you wanted to know but there it is.
Chuck Norcutt
Scott Peden wrote:
> Blue Bird, CAT... and new to DSLR, I assume the CAT isn't what the bluebird
> was watching?
>
> Is the 500/8 CAT different than the 500/8 Zuiko?
>
> -----Original Message-----
> From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
> Of Walt Wayman
> Sent: Monday, February 19, 2007 8:29 PM
> To: olympus@xxxxxxxxxx
> Subject: [OM] 500/8 Zuiko
>
> Got the bluebird pictures taken with the 500/8 Zuiko. Another list member,
> who may wish to remain masked and anonymous, downloaded and saved them and
> sent them back to me. I still can't find the originals, but this should give
> an indication of what thhe 500/8 Zuiko CAT can do. These are just little
> JPEGS.
>
> http://home.att.net/~hiwayman/wsb/media/192375/site1161.jpg
>
> http://home.att.net/~hiwayman/wsb/media/192375/site1162.jpg
>
> Walt
>
>
>
>
>
> --
> "Anything more than 500 yards from
> the car just isn't photogenic." --
> Edward Weston
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