[ATM] Focal Plane Location

[Gilbert Gagne <ggagne@clearwire.net>]

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?
2. Eyepiece specs specify their focal length; what is the location of 
the focal plane relative
to the field lens? How can this be measured?
3. For star images to be "in focus", does the focal plane of the 
eyepiece have to be
coincident with the focal plane of the primary mirror?
I'd appreciate any insight on these questions.
Gil Gagne
Port Orange, FL
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