Telescope Reviews: Garrett® 100mm F/5.3 45º Binocular Telescope

I'd carefully check that clear aperture if I were you. For example, the Oberwerk BT 100 has no limitations on 1.25" eyepiece field stops. It's prism clear aperture is larger than 30mm.

Your assumption that your plossls are all 52° eyepieces is overstated. The manufacturers stated Afov always includes distortion which adds several degrees to the actual that you get, sort of like usable eye relief. Most plossls are really about 49° usable. For instance your Meade 9.7 has a 7.6mm field stop which actually gives you only 48°. I've had two Meade 4000 26mm, one was 48° and one was 49°. Your Meade 40mm plossl really gives you only 39°. There are no 44° 40mm 1.25" plossls on the market.

If you can actually measure your eyepiece field stop then you can determine what actual Afov you will get, and therefore total degrees of sky you will see. It is eyepiece field stop / eyepiece focal length x 57.3° = actual usable Afov.

If the binocular has a prism field stop of 22mm, you won't be able to use eyepieces with field stops larger than about 23mm without getting very noticable vignette.

FWIW, a 26mm plossl has a 22mm field stop and gives a maximum 49° Afov.

If a 26mm WA 60° eyepiece is truly 60°, it has a 28mm field stop, the maximum fs that can be put in a 1.25" eyepiece. The Meade 5000 26mm has a 27.4mm field stop.

A 30mm plossl (Ultima) has a 26.2mm fs and really gives only 50°.

IF this scope really has a 22mm prism stop, then the maximum TFOV that you will get out of this scope is about 2.5° and that may show mild vignette in the outer few % of fov.

24mm Panoptics, 26mm Meade 5K 60° plossl, 28.7mm RKE, 30mm Ultima and 32mm TV plossls would give different magnifications, but would all give the same TFOV. All of them would be clipped by the prism aperture to a maximum of 2.5°.

Does the Clear Aperture affect the effective total aperture?
AGAIN IF the prism clear aperture is 22mm at the back end, it may be reasonable to assume the prism aperture is also 22mm at the front end. And this plays to the question, Where is the focal point of this system?

Take any eyepiece and look inside the barrel of the eyepiece. See the field stop, in long eyepieces quite often in the chrome sleeve. Measure the distance from the plane of the fs to the shoulder of the eyepiece. Now put the eyepiece in the BT and focus it. The eyepiece inserts tight up to the shoulder, and you know where the eyepiece fs is with relation to the shoulder, so now that it's focused (and in general if the eyepiece is any good, it focuses at the plane of the field stop), you know where the focal point location is inside the eyepiece focuser tube.

The light cone of a 100mm f/5.3 aperture needs to be small enough to pass thru the suspected 22mm front prism aperture. You want to see that the front prism aperture is atleast about 410-420mm behind the objective.

Now you need to get into refractive index of the prism, effective focal length path thru the prisms, and I don't even know the shape of the prisms given it's a 45° binocular, so no sense going any further. At any rate, that would be academic, since there are easier ways to measure effective aperture.

edz