I had this discussion on another site, but it was so fascinating I wanted to share it here.
Abbe-Koenig roof prisms are weird, and expensive - they have long prism elements in a sort of "boomerang" layout, and they are famous for (contrary to common knowledge) being high light transmission roofs. Historically they were only used in large primary, low light applications, like 9x63 or 8x56. And there are only a small number of builders who make these, mostly European - Zeiss, Docter, Zwarovski, Optolyth. Nikon make a set - the 7x50 WX, all yours for only $6000 dollars! Yikes.
Enter: my Orion 9x63 roof prisms! In 1991 I purchased the last pair on clearance at my local Orion dealer - 9x63 roofs! Everybody knows that won't work, right? Still, they were/are super cool, and nobody has a pair like them. Then, this weekend, I learned about Abbe-Koenig roof prisms on another site...
Now, for the record, I am 99% convinced that my Orion Mini-Giant 9x63 roof prisms are Abbe-Koenig prisms! Here's why: upon analysis, a telltale sign of AK prisms is that, due to their design, they are "bow-legged", i.e. the primaries are not in the same plane as the eyepieces. Standard SP roof prisms (which are overwhelmingly common BTW) are essentially perfectly straight tubes between the primaries and the eyepieces.
There IS one example of AKs being "straight" - the Nikon WX's, which are $6000+ (!) and are probably a unique/ tailored design (you can see the schematics online); virtually all other AK prisms have the "bow legged" bend when you study them.
In particular, the Orions appear to be a direct Japanese knockoff of the Optolyth 9x63's - virtual dead ringers. The only differences appear to be (1) my pair have green coatings, whereas the Optolyth's have scarlet/purple coatings; and (2) the mystery Japanese builders moved the neckstrap anchors from the side to the back.
BTW I love these! They are cool, unique, have super long eye relief, a strangely large apparent
I took these with me when traveling to Australia, to observe the Southern skies, as they were the biggest optics to fit in my luggage. I can also say they are RUGGED (another attribute of roofs); these are in perfect shape, but I have used them and knocked them around in my travels. And NO, you can't have 'em! ;-)