I like that you got your CO down to 18%- Keep me posted and let me know how it works. I got 16% on the 8 f/9 znd I could widdle it down but im told its pointless at that percentage.
I imagine you can get away with 16% CO in an f/9! Per NewtWin, 18% gets me within 1/2 mm of 100% illumination, depending on how accurate my measurements were, so that's about ideal for my 8" f/6.1 scope. Anything under about 20% CO is supposed to be good for planetary. Now I just have to get myself out the door into the cold to give it a try.
I like the stars having less light in the diffraction pattern rings as well as more subtle detail on planets. Im comapring it though to a 35% obstructiuon so the leap is more than youll see - but it has to be evident regardless - that s a drop in CO size!
My Z8 Newt originally had a 50mm ma diagonal and a bit over 25% CO. I should notice some contrast enhancement with a 18% CO! I've seen that some realtively small changes can make a big difference in planetary work. (I should start a new thread on this.)
My new 25mm TV Plossls in the binoviewer should also help a lot. (...right back on topic...)