I'm looking for advice regarding setting up my Dob for binoviewing while anxiously awaiting the arrival of a pair of Denis' fine Leica binoviewers. This will be my first experience using any binoviewer. They will be primarily used for medium to high magnification only as I have binocular telescopes for low to medium power wide field views.
The BVs have 23mm clear aperture on all sides. They will be used on a 13.1" (332mm) truss Dob with 1478mm focal length. The diagonal is 80mm. I currently have only about 25mm of backfocus. Denis suggested 170mm for the Leicas. A quick raytrace with the secondary moved down enough to give that amount of backfocus shows the rays just passing by the outside edges of the secondary. This is with the rays coming to a point at the focal plane. Obviously I'll lose a small amount of aperture (they almost touch the secondary) and have no diameter of fully illuminated zone.
So the question, is it better to go that far or make new trusses that move the secondary some part of the way down and use a Barlow or GPC? I don't really know exactly what a Barlow does to the light cone so I'll try to do some research on that as well.
Also please correct any misunderstanding about what the three devices do to the light cone. The scope has a focal ratio of 4.45. If the BVs have a 23mm clear aperture and 146mm focal length that works out to a focal ratio of 6.34. Do I need to Barlow, GPC or Powermate the 4.44 native focal ratio of the telescope to make them work best? 6.34/4.45 = 1.42. If one of these devices is needed how does that change the amount I need to move the secondary?
I'd like to use the BVs without adding more glass but with no experience I don't know if that is the best plan given the restrictions described above.
Thanks in advance for any help.