Re:ATM Cassegrain as second telescope project.

[William Mc Hale <wmchal1@umbc.edu>]

On Tue, 6 Aug 2002, Steve Beccue wrote:

>
>
>
> Could you please expand on this?  I am considering a RC for a 32".  At
> what f/D is the primary separately useful?
>
>

Well you would have to make the f/D of the Primary pretty long, close
enough that the Hyperbola of the Primary differed from a Parabola by only
a small fraction of a wave of light.  However if you are going to make a
scope with that sort of magnification, you might as well just make the
Classical Cassegrain.  R-Cs are generally pretty fast by Cassegrain
Standards because at longer focal lengths the Coma on the classical
Cassegrain is not really an issue (remember that the Coma of a Classical
Cass equal the coma of a Newtonian of equal f/ratio), field curvature
might be but that is also true of R-C systems.

My personal opinion, R-C really only makes sense for dedicated
photographic instruments.

>
>
> >
> > If you're going to do a variation on the theme of a Cass, go with a Ritchie-Chreitien.  It
> > has no coma, and if you make it with a long enough f/D it will have a
> > separately useful primary mirror.  Difficulty is slightly greater than
> > with a classical Cass, but the optical system is far superior to a DK.
>
>

Bill

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