Re: [ATM] Classical Cassegrain or Dall Kirkham ?????????

[Mike Lockwood <melockwo@uiuc.edu>]

Vimal,

Vimal Mamania wrote:
> First to introduce my self, I'm Vimal. M. From Mumbai (India) and
> I'm interested in ATM very much. Currently I'm working on 6" f/8
> and 15" f/5.
> 
> One of my friend and I'm also planning to build a catadioptric,
> preferably around 12" or 14" diameter. Anybody with previous
> experience on the same, pls. tell us which design to go for out of
> the three?
> 1) Ritchey Chrétien  2) Dall Kirkham  3) Classical Cassegrain
> We understand it is difficult to figure the secondary. The easiest
> & safest type would be a Dall Kirkham but the focal ration has to
> be around F/15 + .  we would rather prefer to make a Classical
> around F/12 .

If you only want to look at planets with the Cassegrain (it's not a 
catadioptric - there is no corrector plate), then go with the 
Dall-Kirkham.  You will probably want to use interference testing to 
check the secondary mirror against a spherical (concave) test plate.

If you want to use the telescope for objects larger than planets, or 
at F/12, as you say, then I recommend a classical Cassegrain.  It 
could be made with an F/4 primary and a secondary with a magnification 
of 3x.  You will probably want to interference test the hyperbolic 
secondary on this design, too, unless you have another testing method 
in mind.  Making the secondary is moderately difficult.  1/2" plate 
glass should be sufficient for the secondary mirror and test plate, 
though the figure of both the primary and secondary are going to 
change quite a bit during cooldown in the evening.

An R-C would be more difficult to make, so you might save that design 
for your second Cassegrain telescope.

You can see my 12.5" F/12.5 classical Cassegrain on my web page under 
the telescopes heading.
   http://bi-staff.beckman.uiuc.edu/~melockwo/index.html

	Mike Lockwood

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