[ATM] Can one build a Newtonian stiff enough to use a Guide Scope?

[Walter Belhaven <wbelhaven@yahoo.com>]

Greetings,

I'm considering building a Newtonian for imaging and have a fundamental
question before I proceed.  Is there a consensus of the folks on this
list as to whether a 9" f/4.5 Newtonian (give or take an inch or a few
tenths of an f/ratio) CAN be built rigidly enough to allow 30min
exposures with a Guide Scope; i.e., with sub-pixel differential flexure
at fl=1200mm or so?  If it can be done, would it require materials that
are prohibitively expensive (or heavy), making it impractical to carry
such a beast with a modest GEM?  In other words, is one all but forced
to use an Off-Axis Guider with a Newtonian, or can a Newtonian be built
stiff enough to use Guide Scopes?

I've done lots of imaging with a very cheap 8" Celestron Newtonian.  It
has a woefully undersized secondary for imaging; primary retaining
clips that create ugly diffraction artifacts; and it flexes like a
Romanian Gymnast.  Seems to me that there are some simple solutions to
two of these problems -- a "right-sized" secondary, and perhaps the
conical mirror sans retaining cilps to name a few -- but my query here
is really whether I can "solve" the flexure problem(s) (and use a Guide
Scope) or whether I shouldn't even bother trying (and use an OAG).  To
this end, does anyone have a design for a super-rigid 9" f/4.5 that
they'd be willing to share, especially in regard to secondary sizing,
materials, focuser placement, etc., etc.?

Thanks,
Walter Belhaven

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