ATM Folded Refractor Question

[Adam Perkins <scopeguru@yahoo.com>]

Hi everyone,

I just bought Stephen Tonkin's "Amateur Telescope Making" and highly
recommend it to you all.  The chapter by Klaus-Peter Schroder got me
thinking...

The scope he wrote about is a folded refractor.  The use of a flat
tertiary mirror at the back of the scope allows for a tube roughly half
the focal length of the objective (obviously). The focal plane is
located near the side of the objective lens. 

My question is this; if I take a large focal ratio achromatic objective
and bounce it off a large focal ratio concave spherical mirror in the
back, I'll get a light cone equaling a much shorter f/l ratio when it
leaves the mirror.  The drawback to this is a comatic image resulting
from the tilt needed to make the focal plane accessible at the side of
the main objective.  The advantage of the design would be a
substantially shortened OTA.

The idea is appealing, a 4" achromatic objective of maybe f/20 or so
would be a great planetary scope, especially if you could crush the
unobstructed optical train into a short tube.

Oscar Knab's 4.25" schiefspiegler (shown in one of Texereau's
appendices) is an instrument built around the idea of accepting coma
below the resolving limits of the objective. That's where my line of
thought originated, I started grinding optics for just such a scope
long ago and never completed them. If I choose an objective diameter
with low enough resolving abilty to mask the comatic star images, will
I still suffer from other defects?

I need a critique, this idea has me losing sleep...

Adam Perkins
Covington, Louisiana
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