Re: [ATM] Focal Plane Location

[Mike Lockwood <melockwo@uiuc.edu>]

Gil,

Gilbert Gagne wrote:
> I'm building a new truss dob telescope and need to determine the 
> length of the truss tubes. I'd like to avoid tedious iterations of 
> cut & try I went through in a previous telescope.

> 1. Where should
> the focal plane of the primary mirror be located relative to the 
> mounting base of the focuser?

A friend once told me that he puts the focal plane of the mirror 1/4" 
above the top of the racked-IN focuser.  That positions it fairly well 
for most eyepieces, I have found.

If I have to adjust from here, I usually end up trimming off another 
1/4" to 1/2" from the poles.  (One iteration is not bad, I think.) 
Further trimming will depend on your eyepieces, how closely the FL of 
the mirror is measured, and the travel of your focuser.

	Mike L.

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