Binoviewers almost always force some meaningful compromises on the user, and one of the most meaningful of those is the size of the fully illuminated field..
I just got a new to me telescope and I thought I would share the analysis so that others would know how to very accurately calculate their own fields size. It is actually easy to do, and takes nothing more than a ruler and some simple math.
The first very simple formula needed is the reduction in image circle for distance traveled and this is pretty easy. One starts with the size of the fully illuminated circle at some point in the light path and subtracts 1mm of circle for every multiple of the focal ratio of the instrument.
Here is an example of this. The telescope I am working with has a 2" focuser, and the focuser tube is 134mm long. Since it is 51mm at the objective end, this would be the light cone size as it entered into the tube. The scope I am working with has a focal ratio of f/6.5, so for every 6.5mm of length, the 51mm circle is reduced by 1mm, so doing the math, the fully illuinated circle shrinks from 51mm at the front of the focuser tube and loose 20.6mm of diameter, so when it comes out, it is (51-20.61) 30.39mm in size.
Now, my home made T2 diagonal has a 2" nose with a 12mm flange, so by the time the light gets to the end of the flange, it will have traveled 12mm and lost 1.85mm, so when it reaches the face of the diagonal, it is now 28.54mm.
Now what I find here, is that when it reaches the opening in the front of the mirror box on the diagonal, the illuminated field is actually .54mm bigger than the hole in the mirror box, so at this point, I have to star to use this new figure as the fully illuminated circle size. Had the hold been larger (Baader T2 diagonal) then the light cone would have retained its full 28.5mm.
You might say "well Ed, get a T2 diagonal so you can use the full light cone!" Well, this is why we are doing the analysis.
The light path through the mirror box of the diagonal is 58mm. This means my newly sized 28mm light cone will be reduced by 8.9mm so the 28mm light cone is now 19.1mm. Now the clear opening of the RAF adapter is 22mm, so here, the light cone passes without restriction and continues to converge.
Because the RAF adapter adds 3mm to the light path, my light path through the binovewer is 99mm and this means that in this distance, the light cone shrinks by 15.3mm!!! The fully illuminated circle that was 19.1mm is reduced to 3.8mm in size!
In its journey, the light cone has been reduced from 51mm to a tiny 3.8mm circle!
Now this is not terrible, but it is not really all that good either. The illumination falloff is not huge when using eyepieces with smaller field stops as is the case with short light path binoviewers, and because it is gradual and the field is narrow, this is probably OK, but at the same time, for best planetary performance, the planet should be kept very near the center of the field of view. Outside of this circle, and in effect, you are looking at the planet with reduced aperture. The loss is small, but loss is loss.
Suppose I went to the Baader T2. Here, the light path is about 12mm shorter and this is due to the fact that I would not have a flange in front of the diagonal. Once again, at the point where the cone leaves the focuser tube and would enter the T2 diagonal, the cone is 30.39mm, so it would pass though the 34mm opening at the front of the diagonal and travel 50mm before getting to the opening in the RAF adapter. This would mean that it would be 22.44mm at the entry to the BV.
The BV though has the 22mm openning, so this now becomes the light cone size opens up to a whopping 6.77mm (which is actually not terrible. Some SCTs don't have a fully illuminated field much larger than this!).
So for the cost of the Baader T2 mirror diagonal, I would increase my fully illuminated circle size only a tiny amount. Would it be worth it?
Well, If I were doing white light solar and wanted a fully illuminated disk, then maybe it would be. See, in this scope, the sun would be 6.33mm in diameter, and this means that with the T2 mirror diagonal, I could fully illuminate the suns disk.
But how about another approach? For the same price, I could get the Televue 2x Amplifier. Now since the light path is shortened by the length of the binoviewer (the TV amplifier makes the BV parfocal) I would now get the 17mm illuminated circle provided by the Televue 2x Amplifier, and as a plus, I would still get a full disk sun, but I would not have to rely on very short focal length eyepeices to get high power.
So, what this analysis tells me is that there is little value for me to buy a T2 mirror diagional. The size of the fully illuminated field is not that much bigger for general observing, and while it would allow the suns disk to be fully illuminated, my zooms would not give enough magnfication to reach the high powers I would use for sunspots, so I would have to add some form of Barlow anyway.
Now, I already have a 2x Amplifier, so for me, there is no extra cost, where the T2 mirror would be a lot extra.
What about a bigger aperture binoviewer? Excellent question.. Wish I had asked it myself.
As the prisms get bigger, the light path gets longer, and in this particular case, I only have 3mm of focuser remaining so I would not be able to reach focus. If I went to the T2 mirror I could gain myself 10mm of light path, but it would be very close. If I could not reach focus, then I would have to use the 1.25X GPC, and now I negate the wide field advantage of the bigger prisms, but I do get bigger illuimated field because the GPC shortens the light path though the Binoviwer. A BV with a 110mm light path (Maxbright) would now have a light path of only 92mm, and since the front aperture was larger, the fully illuminated field would be larger as well (and I would let you do that math on that now that you know how). As can be seen though if the goal is to increase the fully illuinated field size and preserve image sharpness over the largest circle possible, then a lot of amplification that shortens the light path of the binoviewer is the probably the most optimal path. This is one of the many reasons that for viewing planets, lunar, or solar, I always suggest people use a Barlow, GPC, or specially designed amplifier like the Televue. All of these will greatly increase the fully illuminate field size when working with modern, fast refractors, and amplifiers like the GPC or TV amplifier correct aromatherapist to boot.
I realize this has been a long post and if you slogged your way through it, I hope that it has given you some valuable insight in to how to analyze a binoveiwer configuration. Is all it takes is some measuring, simple division, and simple subtraction to accurately calculate the fully illumined field size you will be presented with in your own configurations.