Quote:
Originally Posted by RyanJones
Thanks for the link Bo. Interesting reading.
Given that my setup with this particular OTA isn't ideal, I will practice on my newt first. With the C9.25 , I have to accept it will have its challenges. As you mentioned before, I will try to learn guiding before expecting it to work with the 9.25. This returns us back to the original question then is what guide setup I should use. Will my 102mm refractor be good ? Should I get a smaller 80 / 400 setup ? Or as Ken suggested should I scrap those ideas and go for an OAG ?
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Rule of thumb is you can do a guide scope for under 1000mm FL. I have a 50 mm guide scope (modified from a straight through finder) with my 770mm refractor and it works pretty well.
Once you go over 1000mm, then an OAG will be better because if differential flexiture etc.
Your 120mm f5 will work well with a guide scope given its 600mm FL. Being an achromat, you’d be expecting significant fringing, esp at f5. So it will be a test scope to fine tune your guiding and you won’t be able to image with it.
An OAG can be quite fiddly to set up as Paul said, plus you will need a sensitive guide cam due to dimness of stars caused by the prism and restricted number of stars to guide with.
My suggestion (and a suggestion only) is to get a 70-80mm doublet (or triplet if budget allows) and start with a guide scope to get the hang of guiding. A lot of DSOs are surprisingly large so you can get plenty of shots at around 500mm FL, plus it will be a good match to your 350D.
Hope that helps.
Bo