Glenn, so the primary is oversized a little, larger than the stated aperture. The meniscus being oversized make some sense if it's not mounted in it's cell. If the primary is 5 mm larger, that's interesting.
Ray tracing it in my head, here's what I think is happening.
A ray entering the very edge of the corrector may well fall on the very edge of the primary or maybe even just graze the beveled edge. But very quickly, moving inward from the meniscus edge of clear aperture, the rays will begin striking the primary mirror edge. So, the system works as designed.
That ray then diverges from the primary's edge to the meniscus second surface. The point it strikes the second surface defines the very diameter of the silvered Gregory spot. It is the absolute minimum spot diameter to achieve full aperture illumination at the center of the FOV. I am sure this is how they define how much of the second surface to aluminize. However, Ed's test do not show full illumination of 127mm, nor do mine show full 150mm effective aperture.
On my 150, the secondary baffle is "designed" (surely) to have a smaller diameter than the Gregory spot. The base of the baffle is smaller than the silvered spot, while the widest portion of the secondary baffle is just a tad (~1mm) larger. This means the light from near the full meniscus aperture striking the edge of primary is vignetted at the secondary baffle.
So, even at "prime focus" with 60mm of back focus, the FOV is not fully illuminated as Ed shows because the on axis light cone is vignetted: the secondary baffle is too small for the slower primary mirror. The flashlight test bears this out.
Reversing the ray trace using the flashlight test, the expanding ray strikes the smaller secondary defined by the smaller baffle not the full aluminized spot (on the 150.) The thinner diverging cone from the secondary onto the primary illuminates the primary only partially (unlike the incoming beam which probably illuminates it fully.) From there, the converging beam headed toward and out of the meniscus would be 118mm as Ed shows.
All remaining incident light cones fully illuminating the primary and using full aperture of the meniscus are vignetted, as well. Surely some of those incident rays, though vignetted, use the full aperture and contribute to the image. so, the question remains, how does this affect resolution? Is it at full aperture, or effective aperture on axis?
This is probably a design feature, as Eddgie points out, to keep the secondary baffle obstruction minimal. The aluminized spot is right about 33% of the clear aperture. If the base of the baffle were along this circumference, the wider baffle opening would be much larger resulting in a much larger CO percentage. The FOV would be fully illuminated only at the center, but the CO would be larger by area. The CO would be very large if more FOV were illuminated fully. The scope would be a bit loose in terms of baffling to permit full illumination.
This is much like the Mak Newt design. Some vignetting is accepted in the MCT to keep the CO minimal, at about 33% of clear aperture or about 35% of effective aperture on axis. Surely this is a design feature. Remember, the CO also serves to block longer focusing inner zones to improve SA. It's possible this was also taken into account when deciding on compromise between secondary and baffle, as was the resulting MTF and sacrificial illumination.
So, the result is great SA correction, sharp optics, minimal CO for the design, and some loss of field illumination for scope optimized for lunar and planetary observation. It will have a bit tighter baffle than the more generalized SCT (designed with imaging, planetary, and deep sky in mind for a more generalized observer.) The test results on both designs bear this out. The flashlight test does, too. It explains why effective aperture is less in the MCT, it's vignetted at the tighter secondary baffle. Eddgie was correct.
I have a 5.5" obstructed refractor.
But the more I understand it's specialization, the more I appreciate it.