Re: Intermediate ATM project

[Michael Peck <mpeck1@ix.netcom.com>]

Thanks to everyone for their helpful responses to my rambling post from last
weekend. It looks as though most of you regard a cassegrain as a reasonable
ATM project, and consider the range of designs I'm looking at to be
manageable. So, pending a bit more research a cassegrain it will be.

One specific comment. Steven Lee wrote:

<< nice discussion of cass secondary discussion snipped>>
>
>There's my views to help with your confusion - you may treat them as
>you wish.  I would point out (as others have already done) that the
>title of one of the chapters in ATM is "How to make a Cassegrainian -
>and why not to".  I would never try to dissuade you from making one,
>but I will warn you that the choice of this design may contradict one
>of your earlier requirements of starting a project that you want to
>finish.
>
>(I would also point out that the Gregorian design offers some advantages,
>although compactness isn't one of them.)
>
>

Well, I didn't really think I was confused, although a week's worth of
browsing through a couple books and plugging numbers into a spreadsheet
isn't enough to have a real feel for the practical issues involved. One
positive aspect of trying a cassegrain is that at least one crucial design
issue can be put off until late in the game -- since I'm starting from
scratch there's no need to decide which of the infinite family of possible
cassegrains I want to do until I'm ready to start figuring. Also, if the
main risk of failure comes from the difficulty of getting a satisfactory
secondary then I'd say there's no problem. That's not being arrogant - just
recognizing that if worse comes to worst it's always possible to finish off
the primary with a paraboloid, declare victory, and retire with dignity. The
cost of the small mirror blanks is nothing, relatively speaking. 

Thanks (to Mel also) for suggesting taking a look at the Dall-Kirkham.
Anyone for a Pressmann-Carmichael ??

Mike Peck
mpeck1@ix.netcom.com