And I am sure that some people are wondering why I say that afocal has a lot of complexities.
Here is a real world example and one that is not readily apparent to someone that has not used both systems as extensively as I have.
Let's say you have a big dob that you use for NV.. You buy a PVS-14 with an afocal adapter and you start using it with 2" eyepieces. Well, first of all, you probably had to buy a 2" filter so you paid $300 for that vs $200 for a 1.25" filter (and this is just the H-a filter, but the long pass 2" filter costs more than the long pass 1.25" filter and you can use 1.25" filters at prime focus)
Now you decide you want to go from your 41mm Panoptic to your 24mm eyepiece (or whatever). You take off the night vision device, remove the eyepiece, pull the filter off, put it on the new eyepiece, put the eyepiece into the scope, and then you put your device on. Hey, that is not so hard.
After doing this 10 or 15 times in a night, you say "Darn, wouldn't it be nice if I had a filter slide in my dob?" So you buy a filter slide ($135 vs $50 for a 1.25" filter wheel) and you mount it into your dob and the next time you use your dob, life is beautiful. You've spend $185 more than prime focus but you got to use that 41mm Pan a bit and well, that is nice.
But one night you don't want to take out the dob. You need your filter though, so you have to go pull your filter out of your dob (if you did not do it before storage, so another complexity... I leave my filters in my filter wheel all of the time).
Now you have your filter but when you use it in your f/5 refractor, you are back to the old filter change dance. Pull the device off, pull the eyepiece, pull swap filter, put eyepiece back in, put device back on. Now using a 2" filter wheel with a refractor and an eyepiece, most refractors won't reach focus, because the 2" filter wheel will usually need a 2" diagonal and because you have to put an eyepiece holder on to the back of the filter, you are adding 60mm to the light path and most refractors will not allow that much inward focuser travel.
For the purpose of this post though, lets say you can use 2" filter. Well, now you need a 2" filter wheel. Ah, no problem, 90 bucks. But now you have to transfer the filter back and forth. Or you could buy a dedicated set of filters for your filter wheel. 60mm is a lot of in focus travel though and a lot of refractors will not have it. A potential solution is to use a Baader T2 mirror diagonal because this has a 50mm light path so you cut off 50mm from the focuser travel requirment (you need the T2 mirror because it has a bigger front opening and the long light path will cause your fully illuminated field to fall below 18mm. That might not matter to you though, but I think a fully illuminated field is very desirable with night vision).
If you can't reach focus though, you are back to having to pull the stack to change filters.
Now you do get to use the 41mm Panoptic though and this does give you lower power in a dob than you can easily get using prime focus. That sounds vital, but in fact, speed comes at image scale, and the biggest problem using dobs is usually image scale and not speed. The built in eyepiece of the intensifier gives 55x in my 12" dob, and for most targets that is not quite big enough, so I often just Barlow up because that is easier than changing eyepieces.
So, while afocal sounds like it is simpler than C mount, it has its own set of complications.
I have used both extensively and I read that people think C mount sounds more confusing or more complex but both methods have their complexities and my own belief is that while the complexities are different, I don't consider one to be any less complex or easier overall, across the full spectrum of scopes that they might be used in. To think that afocal is easier is to deny the fact that it is using a device for a purpose it was not intended to be used for and making it work with all telescope types will have its own issues.
Neither approach will deny the user of the incredible views possible with image intensifiers.
And a lot of these posts make it sound far far far more complicated than it really is. It is just adapters an stuff.. A 15 year old could figure it out.
Edited by Eddgie, 12 February 2020 - 11:57 AM.