Well, my math was pointing to an answer that I did not want to hear, but according to Lunt, you only get about 42mm of inward travel when using a binoviewer with the BF1800 (vs the BF1200) before you loose the fully illuminated solar disk. In fact, my math says that 6mm past this, aperture reduction starts to occur, though to be fair, I do not have exact measurements for the placement and spacing of the blue glass filter, which is the choke point on the system. With the BF1200, the rear filter is the choke point and this filter allows for only a little bit of inward travel before the solar disk looses 100% illumination. With the BF1800, the same 18mm window used on both the BF1200 and BF1800 becomes the choke point but by then the rear filter on the BF1200 is causing aperture reduction.
Unless a Televue Powermate or other telecentric unit is used, many binoviewers are going to take more inward travel than 42mm.
I had calculated this myself in recent days, but I was unsure of the outcome, so I asked Lunt for their figure, and this is what I was given, and again, it kind of matches up with my own computations (I did not have exact measurements for the spacing between the focal plane and the blue glass filter, but I used 75mm and that may be a little more than it really is, but I measured as best as possible using a BF1200.)
Now going in further than this and Lunt says the disk itself becomes vignetted, and my own calculations say that much past this and you may start to suffer aperture reduction. I do not have all of the necessary measurements to give an accurate figure, but we know that the front aperture on the BF1800 is more of a limit than the rear aperture because it is the same diameter but further up the light cone, so my figures say that once the BF is pushed more than 128mm forward of the focal plane (that is at the back of the 75mm light path of the BF housing), aperture reduction begins. This means that if the BF1800 is pushed in more than 53mm from the point where it would be when the system was used with a regular eyepiece, aperture reduction will occur. Again, I do not have exact figures. These are based on my own measurments of a BF1200, and I did not take it apart to ensure exactness, but I feel that I am within 5mm or so.
For people that can't get behind the 53mm forward limit, a 2.5x Powermate may be a good solution. The Powermate will even allow the BF1200 to work well with binoviewers, and this may be a good alternative to selling the BF1200 and fork over the difference for a BF1800. A Powermate is $217, and just about any BV should give a fully or almost fully illuminated disk because the Powermate only takes about 12mm of inward travel to reach focus with a typical binoviewer.
For the BF1800 though, you run out of front aperture in the BF1800 before you run out of rear aperture and if the user has a configuration that requires more than about 2 inches of inward placement of the BF1800, the system might be working with slightly reduced aperture or at the best case, with a disk that has well under half the surface showing with less than 100% illumination (vignetted).