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

[Bob Lombardi <boblombardi@cfl.rr.com>]

> Date: Fri, 4 May 2007 17:41:29 -0700 (PDT)
> From: Walter Belhaven <wbelhaven@yahoo.com>
> Subject: [ATM] Can one build a Newtonian stiff enough to use a Guide
> 	Scope?
> To: atm@atmlist.net
> Message-ID: <20070505004129.39248.qmail@web37910.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
> 
> 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
> 

Not enough information.

Of course a Newtonian can be built stiff enough.  Can _you_ build it 
stiff enough?  That depends on you.

What do you consider a modest GEM?  What do you consider too heavy?  A 
9" f4.5 would probably need something like the Synta EQ-6, also known as 
Orion Atlas.  The EQ-5 or Orion Sirius might work, but I'm less sure.

Is your Celestron in a metal tube, or is it cardboard?  Are you sure 
it's your tube flexing and not your mount?  A narrow dovetail bar, like 
so many use, may be the problem, not the tube.  Have you tried something 
like a Hargreave's strut?

It's easy to say something like "use an aluminum tube with a thick wall" 
but that doesn't do you much good.  How thick does the wall need to be? 
  What about a steel tube?  Graphite?  It is possible to calculate how 
much the tube will bend under load and how much flex you can stand.

I personally don't have a design for a 9" f4.5 sitting ready.  Perhaps 
someone else does.



Bob
http://www.amateurtelescopemaker.com/


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