You have a fine collection of gear, but you will find there are limitations to all systems. To answer your questions:
1. 1.25" filters are typically used on the front of small objectives like the Envis lens; adapters are made for that purpose for Envis lenses. If you have a small 1x objective, you need to determine the filter thread size at the front of the objective. Then search for standard camera adapters that can be used to connect the filter to the objective lens. This can involve more than one adapter. It would help to know what 1x lens you are using.
For objectives having a larger lens like the Cosmicar 75mm, typically 2" filters are applied to the front of the lens. Attaching a 1.25" filter to the front of a larger objective lens will result in severe vignetting. There is no adapter that allows a 1.25" filter to be used behind a C-mount lens. The only way to use a filter behind a C-mount lens is to use an unmounted filter of 24mm diameter inside the C-mount opening on your Ovni-M.
Since your Ovni-M can be used in prime focus, you can use a 1.25" nose attached to the Ovni-M housing (in place of the 1x lens) and attach filters to that nose for use in your telescopes.
2. In afocal, since you have a TV 55, a 2" filter screws onto the bottom of the eyepiece. This means that the light cone is coming to it from your f:5 system. The focal reduction between the telescope eyepiece and the 1x lens occurs after the filter, thus the filter is working in an f:5 system. You do not need filters made for a very fast optical system.
3. Yes, Chroma is very expensive. I have owned many H-a filters made by each of the filter makers and did comparisons between them when used in prime focus with my Mod 3C. I found no difference between results using an 8nm Chroma compared to my 7nm Optolong. I can see no difference between H-a filters separated by 1nm. Even when comparing differences between a 5nm and 7nm filter (2nm of difference) the visual impact is very subtle. If you are primarily a visual observer, a 6-7nm H-a filter is very useful. A 3nm or 3.5nm may be a better choice for use with an afocal system. But there are tradeoffs with more scintillation and fewer stars seen. A narrower 3-3.5nm also suffers more band shift than wider pass band filters.
I currently use a 7nm Optolong and a 3.5nm Antlia filter. I am satisfied with their performance. But I use my NVD in prime focus and most of my filter use is for NV Phonetography, using my iPhone. You should get opinions from others if you intend to use the 55/67mm eyepiece in afocal. I have tried many afocal eyepieces and consider the TV 55/67 the best, but I have sold them all because I prefer using my NVD in prime focus. This is a matter of personal preference.
If you are using a Single Lens Reflex camera to take photos with NV, you will probably want to use your NVD in a prime focus system. The following is one page from the BEST of NV thread found near the top of the forum:
Within the BEST of NV, there is more information concerning filters, and differences. between prime focus as compared to afocal use of your NVD. The BEST of NV is an index. Click on any of the topics to find a list of threads with information about the selected topic. Wishing you all the best.
Edited by GeezerGazer, 01 September 2021 - 10:24 PM.