I am currently using a William Optics binoviewer with 8" f/5 and 12" f/5 Dobsonians. For glass path correction I use the excellent comacorrecting Gerd Düring Barlow lens element from APM, which provides a 3x magnification factor for the optical path length of the WO binoviewer. So far, I use the binoviewer only for planets and the moon with eyepieces in the 15 - 25mm focal length range.
I would like to expand my binoviewing in two ways:
1. use the binoviewer with larger Dobsonians: 16" f/4.5 and 18" f/4.
2. use the binoviewer for the brighter DSOs at magnifications down to about 100x.
I will need a glass path corrector with a lower magnification factor for these objectives. It should also correct or at least reduce the coma at f/4.5 - f/4, since my understanding is that combining the binoviewer with a standard coma corrector is not possible.
After some research I see two options:
(a) Baader 1.7x Newtonian glass path corrector.
(b) Siebert Optics OCA.
Now I am undecided which of the two options might be best:
+ Provides coma correction
- Requires 32mm backfocus, which I can provide by slightly reducing the pole lengths of the (ATM) Dobsonians.
- Has a T2 thread that cannot be connected directly to the WO binoviewer. I need to unscrew the 1.25" nose piece of the WO and try to find an adapter from the unknown thread of the WO to T2.
- It is unclear to me whether the cheaper Siebert OCA #1a will work well at f/4. I got no response from Harry Siebert on this subject. I might need the more expensive #3B OCA.
- does not provide coma correction
Your advice would be greatly appreciated.