Re: ATM Cassegrain

[Dave Sopchak <das@po.cwru.edu>]

>I seem to be missing something regarding the Cass design.
>
>What is the reason the primary has to be so fast?  What's wrong
>with an F6 or greater?

Nothing. It just makes for a longer tube, and this kills one of the
benefits of a cass, a short tube.

>
>It's not the tube length that bothers me, it's climbing up on
>a monster ladder to look through a long Newt.

You're OK then.
>
>Next.. in light of the difficulty of making the fast primary and
>wierd secondary, what makes the Cass better than a Newt with a
>Barlow other than the length?

Not much, as far as I can tell, it has a bigger secondary. Maybe the ladder
is all you avoid. IMHO, I've never seen a design that will give you as much
satisfaction for as little effort as a newt- anybody knows one that does,
let me know.

Getting ahead of myself, I am hoping to make a cass secondary for my 18.5"
scope (first gotta finish the primary!), simply to see if I can. Having a
cass secondary will shorten your tube length a bit and will allow you to
avoid the dreaded ladder, and I won't have to pop in a barlow to look at
planets. I am interested in seeing how the bigger secondary will reduce my
contrast, but I suppose I could get a preview by making a cardboard cutout
the same size as the cass secondary and putting it behind my diagonal.

Climbing up ladders never bothered me- after climbing Grand Teton, I could
sleep  on top of a tall ladder :)

Dave

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Great minds think alike, but fools never disagree.