Yesterday evening, while on work, I suddenly had an idea. I don't know why it came out of the blue, like that, but I suddenly realized that there would be a way to reach focus with a binoviewer on ANY and ALL scopes, no matter what their design:
Yup, that's right. Use an eyepiece in front of the binoviewer, to project its image into the binoviewer, where you then observe the image with normal eyepieces. Would it work? I was sure it would work, but I wasn't sure about how high the magnification was going to be. It would obviously vary with the focal length of the eyepiece used for projection. I also wasn't sure about the image quality.
As soon as I came home from work that evening, I set my 6" f/8 newtonian up and started to experiment. I have two 40mm TS Easyview eyepieces that are made for projection and afocal photography and they have integrated T2 threads, so they can attach directly to a Baader Maxbright binoviewer. The T2 thread is exposed by removing the rubber eyeguard and can slide up and down by about 25mm. Nifty. I threaded one to the front of the bino and put another into one of the eyepiece holders, put it into the focuser and...
It worked! And very well, actually! I compared with a 2.6x GPC, the lowest magnification GPC with which I can reach focus, and the magnification factor with the 40mm eyepiece as close as possible to the binoviewer, was around 2x!! if I extended the working distance, I could increase it to a little more than the 2.6x GPC, so maybe 2.7x - 2.8x. Yes, it worked as a zoom!! And the image was correctly orientated, like in a binocular!
I spend quite some time comparing this setup with the 2.6x GPC and the GPC won slightly on brightness and was a bit sharper as well, but the image in the projection setup wasn't bad at all. Interestingly, there were noticeably less reflections, when the Moon was close to the field stop. The view was slightly soft and defocused near the field edge when I used 25mm Zeiss OPMI eyepieces in the binoviewer, but otherwise it looked really nice. Still, the GPC was always slightly sharper.
I tried removing one of the lenses in the projection eyepiece, to change it into a ~80mm eyepiece and while the image was now MUCH less magnified, it didn't look very good at all. I attribute some of this to the simplicity of a single lens. I also had to use around 6" of extension tube to bring it to focus...
I am pretty sure, that a dedicated projection eyepiece could be made, that would essentially offer a 1:1 projection scale, allowing the same magnification on the binoviewer that you would normally get with a single eyepiece, all without any problems of reaching focus. It would work in a coma corrector, too! Alternatively, coma correction could be designed right into the projection device! A problem might be, that it needs to be at least the same f/ratio of the scope, or faster, lest there will be vignetting. This is easy to achieve in a f/8 scope, but an f/3 is too fast to fit the light cone into a normal binoviewer, so it won't work with a 1:1 projection either. Here a barlow or Powermate remains the only viable solution.
I still think this has something going for it and will merit further investigation.