Re:ATM Cassegrain as second telescope project.

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

On Tue, 6 Aug 2002 kwhitefire@juno.com wrote:

>
> I guess I could go for a Dall Kirkham design, but the thought of using the
> one primary as an f/4 rich field instrument and an f/15 planetary telescope
> is quite appealing.  Besides, I'm after a challenge.
>
> Decrease the primary f/D to around 3.25 to 3.5.
>
> 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.
>

Correct me if I am wrong, but the coma in a Cassegrain is the same as a
Newt of equal f/ratio.  Since coma in a Newtonian is negligable for most
applications by the time you reach an f/8 or slower scope, it does't seem
to me that it would make much difference in going for a Ritchie-Chreiten
unless you wanted to make a fairly fast photographic instrument.  In other
words for at least 95% of the things an amateur will want to do with an
Cassegrain, a classical Cassegrain will do fine.

Bill

***************************************************************************
There's One In Every Crowd And They Always Find Me.
---------------------------------------------------------------------------
Home page - http://www.gl.umbc.edu/~wmchal1
***************************************************************************