Re: [ATM] Effects of a too small secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Steve,

Steve Chambers wrote:
> My plan is to use a 19"dia 100" FL mirror in a Dob type scope.  However
> as the eyepiece can get a little inaccessible I had the idea of using a
> large secondary (6" minor axis) 69" from the primary.  Then it gets a
> bit strange.  I plan to mount the tube from a 4" f5 refractor complete
> with focuser on the secondary in place of the normal Dob type focuser.
> Finally its finished off with a diagonal and a may be a bino viewer.
> I think this would give a dob that’s fun to use without steps.

A 100" FL scope will not have an inaccessible eyepiece.

I have a friend with a 20" F/5 scope.  The mirror is excellent, and 
let me tell you, F/5 is a nice focal ratio.  The views in this scope 
are wonderful, and you don't have coma coming out your ears.

My recommendation is to keep it simple, and build a standard Newtonian 
with a normally sized secondary and use a ladder.  At zenith, the 
eyepiece shouldn't be much more than 8 feet off the ground.  If the 
mirror is good, when you look through the eyepiece you'll be glad you 
built it this way.

Mike Lockwood
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html	

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