Re: [ATM] Fw: Diagonal help

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

From: "Kevin Williams"

With a mirror as small as 3", it is difficult to design a favorable
geometry - the useful field is a large fraction of the primary, and possibly
of the secondary.

The 20% recommendation is just not realistic for such a small scope, and
anyway for use as a finder, a large obstruction is no big deal. I'd say go
for no less than 1.1, but possibly a little more. And make sure the focuser
is not too high profile!

Nils Olof


> I want to build a small scope as a desktop display, but is has to work, I
> am
> building this a display scope, but may use it as a finder on a larger
> scope
> later
>
> I wanted to build a 3" primary, f4, focal length is 12", I plan a wooden
> tube with a dobsonian mount
>
> any recommendations for a secondary size?
>
> I ran this thru newt and Diagonal Calculator, and
>
> diagonal calculator says 1.6 is optimul
> newt says 1.1, but the  help file says 20% of primary max
>
> any opinion on this size?
>
> Thanks
>
>
>
>
>
>
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>


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