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[OM] Re: 500/8 Zuiko

Subject: [OM] Re: 500/8 Zuiko
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Tue, 20 Feb 2007 09:18:36 -0500
"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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