Effective aperture of the 100XL-ED is 96mm. 19x on the 82mm, and 25x on the 100mm was as low as we needed, therefore 24mm prism aperture was enough. BTW, no fingernails
Kevin, pardon my persistence but I'd really like to understand BT design philosophies regarding nuisance reflections better. You've been in this business for a long time and have been a driving force in marketing large astro binoculars over the years. I am assuming you have had some input into the design process of your new BTs so I have some questions. I'm not intending to put you on the spot or cause some sort of "BT battle"; just to open an honest discussion of what are real design issues that matter to binocular telescope performance and what may not be. Sadly, I'm not privy to the circle of BT designers/engineers but I'd love to be a "fly on the wall" while they're doing the work.
Regarding the "no fingernails" statement, was this a specific design goal? How have these off-axis reflections been removed from the system? As I pointed out in a post above, I've examined the internals of my Miyauchi BTs and "clones" of these and and there is an aperture stop on the back side of the rhomb turret located midway between it and the Schmidt main prism to prevent off-axis reflections from reaching the rhomb's faces. It appears Kowa, who you've emulated, also uses this design. I don't see this baffle design showing obvious signs of vignette to my eyes, nor does it effect full aperture. These designers thought it was worthwhile. We've discussed this "fingernail" issue since the APM BTs were introduced. Some users don't seem to mind and some are bothered by it, depending on their observing style.
Markus has been very insistent on having his APM BTs being optimized for the best illumination of widest field eyepieces. APM even enlarged the original prism aperture to 28mm from 25mm for this purpose. On visual inspection there doesn't appear to be any baffles within their prism system as it seems the designers didn't want to contribute to illumination falloff from them either. The trade-off appears to be these "fingernail" off-axis reflections. It seems some BT buyers agree (though not necessarily justifiably) that this "fullest illumination" aspect is important in a visual instrument. Perhaps they are way more sensitive to this than I am. I actually expect this to be less important for visual use, though, as our eyes are not nearly as sensitive to these differences in illumination as a CCD sensor.
I've never seen measurements taken of any standard Porro or roof binoculars that are anywhere close to 100% illuminated to the edge of their field, even premium models. Top performers may have the central 50% of area illuminated by 100% of the objective. I've never heard anyone complain about the illumination falloff of their Fuji FMTs, Nikon Astroluxes or Tak 22x60s, imperfect as they may be. FWIW, these standard binos don't show "fingernails" either. In light of this, what do you think about the quest for fullest illumination? Is this a worthwhile design goal for a visual binocular observer or just a quest for needless "perfection" with unintended consequences?
One other thing; maybe at this point in time we should identify our binos based on the actual effective aperture? It seems long past due industry-wide. Perhaps call the new binos BT-80XL or BT-96XL?
I'd love to hear your perspective on this as someone who has surely given this thought, having been in the binocular industry for years. All other comments welcome, too, of course!