And here is a very simple way to determine the amount of field vignetting that occurs from a given aperture.
For example, the Denk Standard has a light path of about 116mm (at least this is the figure I seem to remember seeing posted, I have not measured it directly). This does not include the distance to the opening of the nose piece screwed into the power switch. I think that total distance adds another 25mm or so (I have measured this, but I forget the measurement).
If the opening at the front is 26mm, then given the focal ratio, the fully illuminated field circle will shrink 1mm for every multiple of the focal ratio.
So this means that if the Denk II were used in an f/7 system (with no GPC or Barlow) then for every 7mm, the fully illuminated circle will be reduced by 1mm so by the time it passed through the system, the 26mm fully illumined circle will have shrunken to (116 / 7 = ) 16mm less than the size it was at the front aperture. If the front aperture was 26mm, then at the focal plane, the fully illumined field will be only 10mm.
Now remember this though. Most reflectors will only have about a 10mm fully illuminated field to begin with, but then most refectors are considerably faster than most refractors. Even in low power mode, an f/4.9 reflector in low power mode would still be working at f/6.37 and if you added the light path and aperture of the power switch itself (needed to get to 1.3x) here are how the numbers would look.
If the light path through the entry of the low power lens in the power switch adds 10mm, then the you would have 126mm of distance at f/6.37 so you would reduce the light cone by 19.78mm, and since the diameter of the low power lens is something like 24mm, then this would reduce the 24mm fully illuminated circle to (24 - 19.78 = ) 4.22mm, and everything outside of this is vignetted. So, move 2.12mm off axis and the vignetting starts. It is soft though so unless skies are moderately bright, it would go undetected. I used such a configuration in my 12" f/4.9 dob and with the D21s, I could live with the visible vignetting, but with the 24mm ES 68s, the outside edge of the field was very dark as compared to the center when viewing under my red zone skies.
Again, not picking on the Denks. Just used them because this is the unit in the tread.
The message here was how to calculate fully illuminated field size and this method can be applied to any binoviewer used in any scope.
(Note as well that since the fully illuminated field is so small, once a planet drifts out of the center of the field, it quickly drifts into an area of the field that is essentially working at reduced aperture and contrast is lowered accordingly. For binoviewing planets with a fast dob, the best result will occur when the planet is kept near the center of the field.).
None of this is intended to say that you are making any kind of mistake or that what you are doing with your own configuration is bad, or anything like that. I am simply providing information on how you can determine the performance envelope of your own configuration. What you use is fine with me. I make a lot of compromises in my own configurations and with binoviewers, it is often simply a fact of life. Do what works best for you. I don't condemn anyone for what they do as long as it meets their needs. Now, if they want to improve their performance envelope, this kind of info can sometimes help.
I have become a much bigger fan of the Televue 2x Amplifier for binoviewers because it retains almost full field illumination but this comes at the expense of the high (2x) magnification. People using un-driven dobs for planetary views that wish to let the planet drift though the field would probably do well to use the TV 2x amplifier. I now use it in my dob. For planets, the 2x is desirable, but again, this provides for almost full field illumination and corrects for spherochromatism. Is it easy to see the difference? No, but this is CN, the home of the free, brave, and most anal-retentive hobbyist known to man. LOL.
Edited by Eddgie, 04 October 2019 - 11:38 AM.