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

[George Anderson <tillerman1@videotron.ca>]

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
>>
>>     
>
> Hi Walter
>   
Have a look at Chris Schur's solution for guiding with a newtonian, it 
eliminates the requirement for a separate guidescope and the cost is a 
small additional secondary and a second focuser.
For the size scope that you are talking about you really need a mount 
like a G-11. If you are using a mount much smaller than that you are 
really pushing beyond the limit of the mount if you are shooting with an 
SLR. If it is a digital SLR you will really need the heavier mount as 
well as a very sturdy focuser.

George Anderson\
Montreal Canada

Clear skies and good health

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