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[ATM] Minimum inner tube diameter

How to determine the minimum inner tube diameter (=inner upper ring diameter
for open truss tube design)?
I thought, this is a simple question, but  it seems not.

My method so far is:
- no vignetting of light path with the widest field eyepiece
Rule of thumb:
Inner Tube Diameter= Mirror Diameter + Field Stop Diameter
more exactly, like shown in Andy's Excel sheed:
http://www.stathis-firstlight.de/atm/kyklopas_fov_and_baffle_calculations.xls

Others say, that the secondary mirror is usually sized to illuminate the edge
of the field of such a widest field to 75% or 70% only, so the upper ring can
be made smaller as well. Going this way, it's sufficient to make the upper
ring only as large, as is needed for the 100% illuminated field determined by
the secondary mirror.

In David Kriege's book (page 154+154) he says, that no vignetting of 0.5°
field is sufficient.

What is right?


Stathis Kafalis
www.stathis-firstlight.de

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