In terms of contrast, an 8" scope with 47% obstruction is equivalent to about a 4.24" unobstructed scope (Aperture - CO). A 4-5" f/6ish refractor is going to be a lot more satisfying than an 8" RC, while still maintaining similiar portability. That size achromatic refractor is made for DSOs.
Now, funny thing here. Ask people using 4" refractors what they think of the contrast in their scopes and they will tell you that they are amazing! Go to the Refractor forum and you will find 1000s, of post that will tell you how much people can see in a 4" refractor!
High contrast transfer is really only vital for detail that is quite small and fine. As the targets and details within the target get larger, the contrast transfer matters less and less.
An 8" scope with a 40% obstruction can do quite satisfactory observing.
My 6" f/2.8 telescope has a 41% obstruction, but I can resolve a lot of structure in nebula, and I can see extensive faint nebula around brighter nebula. Now I am using an image intensifier, but the point is that if the scope did not have enough contrast to show these, I would not be able to see them even with an image intensifier. The contrast is either there, or it is not.
Here is a picture taken with a scope having a 41% obstruction. Look at this and tell me how the big obstruction has ruined contrast? http://www.teleskop-...on-vorschau.jpg
And for richest field observing, nothing beats a really fast scope with a big obstruction. Look at the observations being done with a small scope with a very large obstruction here:
I am not saying that a big obstruction does not lower contrast, but what I am saying is that for extended targets, (galaxies, and nebula) the effects of obstruction are grossly overstated.
I have provided two examples where it is clear to see that scopes with very large obstructions are still capable of showing a very high level of detail and my own scope that is 6" f/2.8 with a 41% obstruction is capable of resolving considerable detail in extended targets.
(Did you know that M29 actually sits on nebula? The entire field of M29 is covered with nebula, and M29 itself sits on a small peninsula of nebula extending out from Gamma Cygni complex, which itself is nothing short of spectacular. Now, people will say "Hey, that doesn't count because Ed is using an image intensifier" but here is the fact, If the contrast was not sufficient to see these, It would not matter that I was using an image intensifier.. The image intensifier does not change the contrast at the focal plane. Either it is there, or it is not. The image intensifier can't make it appear where it does not exist. Also, look at Mel's drawings and eyepiece impressions. He is seeing things that most people here are never going to see, and he is doing it with fast scopes with very large obstructions, and I can absolutely confirm that he is seeing the structures he is drawing.
Now for planets, I would not be satisfied with the detail I can see even in a 5" Apo, so of course I would not choose this for seeing festoons on Jupiter, but for seeing shapes of Galaxies, or structure in nebula, these big obstructions really don't matter that much. Once the detail gets bigger than about 2 or 3 Airy Disk diameters, you are past most of the serious contrast loss and even at 2 or 3 diameters and 50% contrast loss, you can still see a lot of detail. It all depends on how much contrast is there to start with.
Edited by Eddgie, 22 October 2017 - 08:42 AM.