If you find the moon a little too bright, pop the lens cap back on minus the smaller 2" lens cap and presto, you've got a 50mm f10. I also use the smaller opening in the lens caps when I'm viewing the sun as I've attached some baader solar film to the inside - that actually works pretty well.
You will need some sort of barlow for the binoviewer to come to focus. I have a 4" F5 that I cut the tube a couple inches shorter and attached a low profile crayford focuser - this allowed the binoviewer to reach focus without having to use a barlow. I probably ended up cutting into the light path/light cone ??? by shortening the system this way with an F5 scope but I can't tell visually and it worked just fine. (see attached pic)
You certainly lose a bit of image brightness with the binoviewer. If you've ever switched from a mirror diagonal to an erecting prism and noticed the image was noticeably duller......well, it's twice as worse with the binoviewer. You can always buy a second 100mm F5....
