Before I go further, it is important to say that at f/6, all standard binoviewers made today will vignette the light path.
At some point though, what happens if that if the aperture is too small, the vignetting turns into aperture reduction and this is much more concerning than vignetting, which is always present in modern standard binoviewers.
Hi there, and welcome.
The claim that the Baader 45° Maxbright amici prism 1¼", with inbuilt 2x gpc, is not recommended or suitable for the MBII is very surprising.
I use it with my MBII, especially in my 60mm Tak, the FOA-60Q (60/900) or without the Q module (60/630 - but balancing can be a challenge). So my usual setups are slower than yours, which will have some effect.
I agree with Finbarius. If this was a non-amplified diagonal, I would be concerned about vignetting and maybe even aperture loss, but once you put the 2x into the diagonal, this concern goes away.
Here is the way it works. When the light enters the binoviewer, it passes through a 25.5mm opening and it travels 110mm to the focal plane (assuming the field stop is at the top of the eyepiece holder).
This means that a 25.5mm cone of light enters the system (no GPC or Barlow). This cone will converge at a rate of 1mm for every multiple of the focal length that it travels. If the system is f/6, this means that to travel 110mm, the cone will be reduce in diameter by 18.3mm by the time it reaches the focal plane. The fully illuminated field will be reduced to (25.5 - 18.3 =) 7.2mm in diameter. Outside of this circle, there will be vignetting, but all standard binoviewers do this (vignette), and the amount of vignetting is not enough to be easily seen unless the system is very fast (though at f/6, someone that is sensitive to vignetting will see it when using very low power eyepieces with wide field stops).
Let's see what happens with the 2X. I think the aperture is about 21mm or so and I am assuming that the GPC is ahead of the prism, but I don't know this. It does not matter though because here, I am simply trying to show the behavior of the system. So, if the light cone is 21mm at entry, the light cone changes to f/12 when it enters the diagonal.
In this case with the f/12 light cone, given a 30mm light path in the diagonal and a 110mm light path of the BV, that cone will start converging at a loss of 1mm of fully illuminated field for every multiple of f/12. That means that the cone will be reduced by 11.6mm over the path from the 21mm aperture of the BV to the focal plane. If the aperture started at 21mm, and it is reduced by 11.6mm, at the focal plane, the fully illuminated field will be 9.4mm (21mm - 11.6mm =) or 2.2mm wider than the system used without the a GPC.
Running the numbers with the 1.25x GPC between a standard diagonal and the binoviewer, the figures would be that the system is turned into f/7.5. Here, the light cone enters into the 110mm BV and loses 15.17mm so arrives at the focal plane 10.6mm in diameter so using the Amici in the link above would cause a bit more vignetting, but the difference in a fully illumined circle difference of 1.2mm, would be almost impossible to see (2x diagonal vs 1.25x GPC at the BV entry).
Here is a simpler comparison. I run my 130mm f/7 Apo without a GPC (when not observing planets or double stars) and the fully illuminated field is 9.8mm, and even at f/7, as sensitive as I am to vignetting, the field appears to be visually free of any vignetting. It is there, but it is just not enough to see. At f/12, the vignetting falloff would be gentler than at f/7.5, and my guess would be that no observer on earth would be able to detect the slight differences between using the 2x diagonal and the 1.25x GPC.
(f/6 is pushing it, and I would always recommend using a GPC, Powermate, or Barlow in front of any kind of prism diagonal or a Barlow for a refractor this fast.)
As for the Amici, I have not used one that did not show aberration on bright stars, but for non-point sources, this will not be visible, though it does very slightly lower contrast. For terrestrial or daytime use, the aberration does not do enough damage to be of any concern, but the aesthetic damage on stars can make it a show stopper for many.
It is important to note though that I do not have the actual figures for this diagonal, but Baader says the "inner diameter) is 25mm, so if that means the front aperture, my numbers are very conservative being based on 21mm, so if that is indeed the correct figure, the fully illuminated circle would be 4mm larger than the example, meaning that it would be bigger than even when using the 1.25" GPC.
The convergence of the light cone is just basic geometry, and it can be applied to many things, but it is particularly good to know when trying to understand binoviewer behavior.
