First, we have to differentiate between true field and fully illuminated field. The true field of a C8 can be made a couple of degrees but that does not mean you would want to do it.
We should differentiate between the largest true field and a field that is vignetted to only the degree that the user would find acceptable.
See, even with the factory visual back and diagonal, the field of your telescope is already vignetted. Only a small circle at the center of the field of your eyepiece is fully illuminated (100%) and the moment you move something outside of the field, form there on, it is all vignetted and the further you go away from the center of the field, the worse that vignetting gets.
And this has nothing to do with the rear opening size of the baffle. I could make that rear opening larger, but it would not change the size of the fully illuminated circle because this vignetting is caused by the opening at the front of the baffle. The reason you don't see it is because it is so far away that it is out of focus.
Now the 32mm opening in the front of the diagonal does not reduce the fully illuminated field size (which is nothing to do with the true field size, only the size of the unvignetted true field) because at the point where it passes through the opening, the cone that produces the fully illuminated field would be narrower than 32mm.
And here is how you calculate that. Ray traces suggest that the fully illuminated or unvignetted part of the field of the C8 is about 8mm. Outside of this, the field starts to vignette and continues to loose brightness over whatever size true field you can see.
Now we know that the size of the cone will grow for 1mm in diameter for every multiple of the focal ratio that you move ahead of the focal plane. For example, if the fully illuminated field was 8mm, and I measured the diameter 10mm in front of the top of the eyepiece holder, it would be 9mm in diameter.
Now in your case, the light path length to the front restriction in the diagonal is the distance from the restriction to the top of the eyepiece holder. The T2 diagonal has a light path from that 32mm opening that is about 31mm in front of the top of the diagonal flange and as I recall, the eyepiece holder is 30mm, so this means you have a light path of 61mm from the restriction to the top of the eyepiece holder.
This means that if your fully illuminated circle is 8mm at the focal plane, it will be 6.1mm larger at the point where it passes through the opening at the front of the diagonal, so where it passes through that opening, it will be 14.1mm, so the 32mm opening will not further reduce your fully illuminated field size.
Now the rear baffle size does not affect the size of the fully illuminated field, but what it does affect is the point at which illumination falloff becomes more severe. If you could measure the illumination of your 28mm field stop, you would find that at the edge of the field, the illumination would have fallen by about 16% or 17% at the edge of the field.
Now if you used a 2" diagonal, with a 130mm light path (including visual back) we see from the above chart that at 46mm with a 130mm flange to field stop distance, the field will be vignetted to about 63% or so at the edge of the field. The very outermost part of the field would show enough of a sharp transition to see, but it would not be glaring.
Now that is with a 2" diagonal.
What will the 32mm openening do? Well, here we have to figure out if the off axis light cone is has gotten small enough to clear the 32mm opening. First, let's go back to the 100mm spacing.
Here, the light cone would be illuminated to about 68%. We can work forward to see how big it gets as we get closer to the 32mm opening. We know that it is 46mm at about 68% illumination here, so we know that as we go further forward it will get bigger and we know from our earlier example, it will be 6.1mm larger when it gets to your 32mm opening, so it has expanded from 46mm to 52.1mm. We also know that it only started at about 68% and now we see that it will not fit through the hole, so the chart above would show the falloff that we now see as happening at 46mm mm would be occurring over an image circle about 32mm in diameter and that this falloff would be about 60% (because the circle is wider in proportion to the opening than it was at the rear baffle.).
Now because the source of the vignetting is much closer to the focal plane and the vignetting increase is quite severe, this would not escape detection. You would see very sharp vignetting in the very outer part of the field of view.
So, essentially you would get something like this:
But that is not the true field limit! That is just the part of the field where the vignetting starts to be more noticeable as you transit from the part of the field with mild vignetting to the part where the vignetting becomes more severe.
So, you scope is always vignetted. With a 2" diagonal, you would see about 63% illumination at the edge of the 2" eyepiece, but because it is very close to the field stop and because the drop is still not wide and sharp enough to see the transition, this would still be usable and the use of this kind of eyepiece in the C8 is pretty common and most report that vignetting is not really an issue.
In the case of the 32mm opening that is 61mm in front of the field stop, the situation is much worse. The transition from about 75% illumination to maybe 40% illumination occurs over a wider area and you would see a distinct vignetted circle at the edge of your field of view..
But this is not the size of the true field. This is just the amount of illumination at the edge of the true field. As the above diagram shows, there is still plenty of light there, but things are dimmed by 60% or more.
The maximum true field one could get and have the telescope work at full aperture wold be 1.97 degrees (as we see in the plot above, we could attain that with a focal reducer and a 2" eyepiece if the diagonal box was mounted directly to the focal reducer with no 2" nose piece of visual back). Now the field would be 100% only at the center and the vignetting would be a very smooth transition form the glow of sky to jet black of fully aperture aperture loss.
But it would be an almost 2 degree true field, just not one that is working very well.
So, your diagonal is not may not be suitable with the eyepiece you want to use. (Never say never... )
The 35mm Panoptic might be OK, but my advice is to upgrade to a 2" diagonal.
Now I could have saved us both a lot of time and maybe you skipped this, but the goal was to help you understand how vignetting works and how to make this kind of analysis in the future if there are other applications (like binoviewers) where it can be important.
The practical true field of the C8 is pretty much exactly what you get if you use a 2" diagonal and and an eyepiece with a 46mm field stop. The practical true field of your scope using your diagonal would probably be with something like a 31mm Hyperinn Aspheric used with the 2" barrel. Maybe a 35mm Panoptic, but I think the vignetting would be much more noticable than in the 31mm Ashpheric. But you see, at this point it starts to get subjective because it depends on how much how wide and sharp the the vignetting is and that is really a personal call.
Edited by Eddgie, 05 May 2020 - 10:47 AM.