Re: [ATM] couder cassegrain

["David Harbour" <scarab2@cox.net>]

Eric,

A "true" Cassegrain has a CONVEX secondary, to amplify (make the effective
focal length longer). What you are talking about is, strictly speaking, a
"folded Newtonian": , although it seems that everyone uses language loosely
in the world of telescopopes and telescope making.  Astrometry, the precise
measuring of star positions, has long favored a "Cassegrain", with a HUGE
FLAT the best way to measure star positions accurately; once this was the
province of large refractors.

As a "true" Cass., your Newt would have a much longer EFL.

I remember when JMI brought out, briefly, a huge folded Newt. I think no one
could afford it.

The reason "folded Newtonians" are not popular is that one pays the price
for the HUGE secondary required: lower contrast, less resolving power
(bigger, fatter diffraction ring), and last but not least, loss of light
grasp.

Dave

P.S.- You will also need a tertiary mirror, another liability, for the
Nasmyth focus.


----- Original Message ----- 
From: "Eric Torraca" <eric.torraca@gmail.com>
To: <atm@atmlist.net>
Sent: Saturday, April 23, 2005 12:08 PM
Subject: [ATM] couder cassegrain


> Hello All.
>
> I searched the archives and didn't find anything in this vein, sorry if
> I missed something that's been gone over thouroughly.
>
> Anyway, has anybody tried the Coude Cassegrain as shown by Texereau?
> I've got a Coulter 13.1" at about f4.5 and I'd like to shorten the tube
> using this design. I've heard that it might not give the best results
> at the eyepiece but I can't figure out how it would be much different
> than the standard newt. design as you're only adding a single optical
> flat. I'm quite a novice at this so pardon the ignorance. Any tips
> would be greatly appreciated.
>
> Also, does anybody know of any good simulators (freeware or shareware)
> that would let me test the design on a Mac?
>
> Thanks!
>
> Eric
>
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>


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