A few posts ago, someone suggested a pentaprism feeding a Porro arrangement ( Porro II preferred).
Instead of a pentaprism , a penta mirror composed of two mirrors arranged as are the reflecting surfaces of a pentaprism. ( Not a roofed penta, as in SLR cameras)
Such pentamirrors have been used for many years in rangefinders for ships, armored vehicles, and fixed fortifications.
With a new, shorter, fatter body of plywood and/or aluminum/magnesium sheet/plate, and/or carbon fiber, WW II f/5 20 x 120 Japanese military, or postwar Nikon/Nippon Kogaku or Kowa, or Fuji Meibo/Fujinon/Fujifilm 15 x 80 or 25 x 150 I or II would be right angle viewing, while retaining erect, unreversed images.
Because the very long glass path in the large Porro II prisms, retained, is the same, no readjustment of the objective two elements' spacing between each other , which otherwise is the most convenient variable, would be needed. See Selby "Aberrations of the prism diagonal" in ATM III older version , or in ATM II (?) from Willmann-Bell, for an applicable analysis.
There would , in some /all cases, be the need for shorter IPD bearings. Fuji II 25 x 150 already has very short bearings there, much shorter than the long, heavy ones in the Fuji I 25 x 150.
If for only one person, IPD swivel bearings might be eliminated.
Another approach to the topic of this thread, is( DEJA VU numerous times): Two mirrors arranged as seen in Hopkins in Mil Handbook 141 ( pops up easily in an online search, courtesy of Univ. of Arizona) Replace the Amici II beamprints for a prism by beamprints on mirrors ( which do not need to be ellipses to pass elliptical beamprints).
I wrote a 3D vector program,to handle the compound angles involved, for HP 48 , using the lateral motion of the virtual roof to be half the lateral motion of the one moving eyepiece, for IPD change, ff. WW II CZ Jena 25 x 100, 12 x 60, etc. Various mirror shapes are included in a routine to minimize the intermirror spacing without edge interference. That spacing is fixed. In the Matsumoto approach, the mirrors' axial spacing is varied for IPD change. That change in the "prism" size requires a refocus. In the M-San setup that seems to be between the objective lens and the 'prism'. That seems to eliminate the nose clearance problems with 2 inch barrel oculars and 55-56 mm. IPD.
2 inches is 50.8 mm . 56mm/2 minus 50.8mm/2 equals 5mm/2, =2.5mm wall thickness available, about 0.10 inch for a male threaded tube to hold the eyepiece, together with a female threaded mate whose exterior touches, or is very close to, the nose. This is
a very tight construction problem and use situation for someone with a 56mm IPD. (I am 65mm), if the focuser surrounds the eyepiece. Two inch eyepiece barrels waste valuable space.
It would be possible to move both eyepieces laterally, still using the invariant "prism" approach I chose, but that is more complex . It would have no effect on nose clearance.
The invariant "prism" approach eliminates the possibility of image rotation which might occur in the variable "prism" (Matsumoto-San) method.
In these 2-mirror erect and unreversed image "prism" designs, the line of sight can be chosen and fixed to deviate from perhaps 60 deg ( long, narrow beamprints) to 180 degrees. At 180 degrees , the angles are simple, not compound, and the beamprint is the familiar 1.41 major/minor axes ratio Newtonian diagonal shape.
Edited by Gordon Rayner, 26 February 2015 - 01:36 PM.