The length of the light path through your binoviewer plays a role in how it will perform with a given EP.
My new super binoviewer scope, (My William Optics short tube with APO upgrade) can focus any known binoviewer without any needed OCA/OCS in the light path.
With a custom 5.5" extension it can also focus a single EP the way a normal scope would. Tonight I decided to measure the light path through the 5 binoviewers I have been testing with a set of digital calipers. The results were a little surprising.
Using an 18mm Radian and the 5.5 inch extension I brought the scope to focus on Jupiter. I then measured the distance between the edge of the tube and the edge of my 2" diagonal. The base number needed for focus turned out to be 204.39mm.
I removed the 5.5 " extension and focused each of the 5 binoviewers using the same 18mm Radian. I made sure any diopter adjustments were full down and that the EP was fully seated. I then made the same distance measurement I had made with the single EP and subtracted that from the base number to determine the light path of the binoviewer. The results are listed below in order of the shortest to longest light path.
1. Burgess 24 107 mm
2. Burgess BV125C 107.76 mm
3. Denkmeier Dielectric 125.47 mm
4. Siebert 22mm BN with SCD's 136.7 mm
5. Televue Bino Vue 139.08 mm
I can probably give a good guess at why these numbers are off from the advertised numbers. I think the light path was measured on units without Self Centering Diopter adjustments. I know the Siebert SCD's are very tall. The Burgess Diopter adjustments are very low profile.
In any event these are real numbers that were measured on a refractor. Try plugging these numbers into the spreadsheet that Tom Hole supplied in an earlier post and you can see just how much of a change in actual illumination the light path can make.
On a side note, if you view with the power switch attached to your Denkmeier it will add another 12.65mm to the lightpath.
These measurements were taken with the standard nose piece on the Burgess, the 1 1/4" nose piece was placed on the Denks and the power switch was removed.