[QUOTE=jjg213;1414298]
Slightly like the Bell and Howell M19, developed so that the objectives and eyepieces and the bodies could be replaced in the field while still maintaining acceptable collimation. Light transmission stated on the specs at 70%!
Regards, John[/QUOTE]
Ah, the M19
http://fantao.home.att.net/m19.htm
That comment reminded me of this paper I read a while ago (from the UArizona optics papers collection) from 1981
http://www.optics.arizona.edu/optom...rsar%201981.pdf
Summary report is here for a quick read:
http://www.optics.arizona.edu/optom.../KazReport1.pdf
Introduction in the Army in 1977 (along with that new fangled Bradley that didn't have a name yet!) is shown here
http://sill-www.army.mil/FAMAG/1977...PAGES_44_46.pdf
Interesting evolution from WW2 and the civilian derived M17 7x50 which stayed in service for a long, long time despite a consideration of a redesign in the late 1950s which lead to the T14 7X50 (which never made it into service but acted as a test bed) then to the M19 7x50 and most recently to the M22.
It's interesting how the design requirement has changed especially as regards reliability and maintenance (either field maintenance, army maintenance or contractor maintenance). The paper outlines the three approaches that came up with the T14: piece part maintenance (the M17 approach); modular maintenance (the T14/M19 approach) and "no field maintenance" (the M22 approach now in service).
There is a potentially interesting history of technology paper/book waiting to be written about this interaction between technology and culture of the DoD.
I also find it interesting that the T14 ended up a 0.3 kg (10.5 ounces!) lighter and rather smaller than either the M19 or M22. But it wasn't rugged enough so revised version the T14E1 was design that worked (a little larger than the T14) and that eventually mutated into the M19.
The prisms were minimized in size and glued in place. That seems very modern (perhaps even better than the M22 or modern commercial porros not 1950s US civilian bins!).
The M19 has some similar interesting features shared with the T14. The asymmetric sized prisms with the "eyepiece side prism being smaller than the objective side prism to reduce weight could be used in modern porros (if anyone was making them!). All the items are made with no adjustments so the objective lens when mounted in the objective tube is aligned. So it can be interchangeably with any other similar part. Same with all the other modules. The bin is almost "edge to edge" sharp because it has a field flattener (in the prism housing) with the reticle marked on the flat EP side of the lens (see Fan Tao's photos).
The lack of baffling in the M19 I guess is another modern trend
An interesting read if you are interested in the design of bins and perhaps in the "resurrection" of a modern porro for birders.