We do like the tintaweb..
Found this article with a great diagram and now I think I get it. Will want to read up a lot more on this but I see John H's point.
http://www.skyandtelescope.com/howto...tml?page=2&c=y
So given my imaging kit, assuming the 60Da still has a well depth of 6.4, and my local seeing (based on being in Sydney and best available weather on the night) will be at best 2, more likely 3, its short focal length refractors for me?
Am I reading this right? If so this thread may have just saved me a pile of cash...
cheers