I have read many threads in this forum, as well as other material, but I remain confused on filter sizes. For context, I ordered the Televue afocal setup. Mainly because I did not feel comfortable figuring out all the intricacies of picking the right tube criteria, getting the right casing, etc. This way I can always swap cases in the future if I decide to go to prime focus. And I already own many TV eyepieces. I have an 18" f/4.3, a 10" f/5, and several refractors f/6 to f/7. I figure I will spend a lot of my time with a 55mm Plossl on the 18", which is basically serving as my reducer when paired with the NV objective lens.
My question is what filter size do I need for an afocal setup? It seems a 2", assuming I use a 2" eyepiece like the 55mm Plossl. That places the filter into the telescope's native focal ratio, which should be fine in my case. In theory, a 1.25" would fit after the eyepiece before the NV objective, but I guess the problem is that the effective focal ratio will be much shorter and therefore go off band at the edges?
And for 1x use in front of the NV objective, I see people using 2" filters and TV offers a 2" adapter. But it seems to me that a 1.25" would work as well. The edges in a 2" filter will be off-band anyway, no?
And for prime focus, there a 1.25" filter seems obvious. But if you are using a reducer, then you might be better off putting the filter before the reducer, which means you might need a 2". After the reducer, you will have a shorter f-ratio and go off band on the edges, I think.
I ask because the difference in cost between a 1.25" and 2" is large. It seems that many people like a 5nm H-alpha filter, with some preferring 3.5nm and some preferring 6 or 7nm. And some also have a 12nm for special cases.
(Does the high transmission and sharp shoulders of a filter like an Astrodon--especially a big one--make a big enough difference to warrant a very costly purchase?)