Posted 07 June 2014 - 04:04 PM
OK. After having designed and built your own Bino Box, I'd thought your proclivities might lean this way.
The shortness of the optical path length in these rhombs might be the issue. In order to minimize this dimension, the width is also kept minimal out of necessity. There is then no room in which to build in light traps such as transsecting grooves.
Yes, prisms are a liability. But their one advantage comes to the fore when faster objectives are used; they keep the front aperture of the erecting/folding subsystem smaller by virtue of pushing the focus farther rearward by just over 1/3 the optical path length tgrough the glass. This keeps the rear end more compact overall.
However, I get to mulling and wonder if, instead of a rhomboidal prism, a pair of mirrors might not be better employed here. The 'fingernail' would be gone, but the Schmidt prism would effectively be moved farther up into the light con (in real terms, the bino would be a bit shorter.) However, the aperture will not likely be reduced, there resulting a decrease in the circle of full illumination and greater fall-off toward the field edge. I would *happily* trade the 'fingernail' for a reduced circle of full illumination.
Mirrors and prisms both have their pros and cons, with differing balance depending on the mix.