I love my mrk5s perfect fov in 24pans. Denki, I dislike the way in lower power arm you get vignetting plus you can see uneven brightness around one edge of the fov... caused by the power switch lens getting in the way! Really annoying
This is why when I talk about the Binotron or Denks, I always reference the D21s. These eyepecies have the largest field stop that you can use in the Denks without vignetting in low power.
Don't get me wrong, I really like the Binotrons and hope to buy a pair one day. I am bothered by the field illumination falloff, but I still feel like in the Dob, the ability to use all three modes of the powerwsitch make it a far better fit than in a large SCT where it is almost impossible to ever use the high power arm due to excessive magnificaiton.
I think for Dobs and Newts though, the value proposition is much better. The only real tradeoff with the Binotron/D21 is the illumination falloff in low power mode, but this may have something to do with having to have the OCS screwed out some to reachfocus. I was wondering if the probelm would be less if the OCS could be fully retracted on the OCS.
I would rather have a little intrusion of the OCS into the light cone in low power mode than have the light fall off so much. The intrusion at low power doesn't matter, and it is out of the way at medium and high power.
But I stick to what I said earlier. If you made me pick just one, it would be the Mark V because it works best with the EdgeHD 8" and I can get pretty close to the same low power in the Newt with the Mark Vs even with the 1.7x GPC because of the ability to run the Ultimas without meaningful off axis ray interception.
Just because I had not done it before, I checked the Mark V with the 1.7x GPC last night on Vega. I mean in normal focus, it i shard to see that the off axis rays are being affected in Mark V/1.7x combo.
I put in the 35mm Ultimas and defocused about 10 waves, and then slewed the pattern to the edge of the field.
About 70% out from the edge, I could see that there was some very slight slicing of the off axis rays. One edge of the pattern started to slightly flatten and I would estimate that at the edge of the field, the flattening would have accounted for about 15% to 20% of the pattern, and this is why it is hard to see in focus. The brighness (or darkness to be exact) of the edge of the field is not easy to see.
The Binotron though was more like 40% of the pattern being cut, and it is clear to see that the sky darkens considerably well before the field stop even of 20mm ES68s in the low power mode of the Denks.
Under a very dark sky, it might go unnoticed though, and it only seemeed to happen in the low power mode.
Again, I wonder is some of this was not caused by the need to have to extend the OCS so much to reach focus (over half an inch).
Now that I have a lower profile focuser, I would like to re-try. I am guessing I could now reach focus with the OCS fully retracted, which might make a big improvement.
It will only cost me $1250 to find out.