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OAG - SCT need one....
In a previous life I used to guide with a piggy back 4 1/2" f10 reflector on top of a SOLID 12" f5 using the old Oly gear and hyper film. No problems with 20-25min exposures, other than the risk of "nose bump" from a cold and tired operator!! When I went over to the dark side and got my first SCT I found the usual problems of guiding vs flexing vs Mirror slumping vs inadequate focal length of guider!
My old rule of thumb was to guide at x3 the photo focal length with a 12mm cross wire eyepiece. So as a minimum for an 8", F10 system this would mean a guide FL of 240", think about it for a moment; a 3" f8 guide scope is 24" FL this ratio would require a x10 barlow! If you can't see the movement then it can't be "corrected"
Flexing between the guide and the main optics due to a variety of reasons is a major cause of headache and the solutions being mentioned ie rigid connections etc etc are certainly a step in the right direction.
Now today, with the current CCD's, the resolution is down about 10-15 micron which puts even more "pressure" on guiding ( the old Hyper 2415 was about 20 micron on a good day!, colour films even less) so my comments are directed towards getting a LONG enough FL to the guider, whether that's a MkI eyeball or a ST-4 CCD guide chip; it doesn't matter.
Yes, you can persevere and get the guide scope nice and tight on the main tube of your SCT, BUT when the main mirror slumps around, and I know it will, then the image on the CCD will definately move.
I have tried it; on numerous occasions, infact many, many times. My latest challenge is to remain "focussed" on a 20 micron slit while a spectroscope which functions at about 30% efficiency builds up an image of a VERY faint spectra.
My frustration is I KNOW the answer..... I successfully use a Lumicon OAG purchased last year because of the flexing/ mirror movement problems on my 12" LX200 for DSO type stuff, and it works 100% for me.
With spectro slits it severely limits the "flexibility" of looking around for a guide star; I must hold my "star" on the slit not just 5 sec arc away.
So for my current work, I'm back to square one. The latest trials are based on an earlier experiment with a beamsplitter, so I can focus and guide on the same star as the spectroscope...........
Summary:
Newtonian/ fixed main mirror = long FL guide scope
SCT(any!) = OAG.
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