Posted 04 February 2012 - 04:06 PM
Have you read the very long, locked, then unlocked, then again locked , thread about this topic which developed last year? It must have set a record for thread length. I contributed a lot to it and to its sequels and predecessors, including a low-cost projection laser aligner for binoculars.
You should wish to make the optical axes of each barrel parallel to each other and to the hinge axis. In this condition, the optical axes will be parallel to each other at any interpupillary distance (IPD). If the optical axes are parallel at one IPD, presumably set by you at your personal IPD, without being parallel to the hinge , then they are parallel at only that IPD. This has been called "conditional alignment".
So-called "collimation" has several meanings, depending upon the context. For binoculars, it would be better to call it unconditional alignment, or 3-way parallelism, or 3-axis parallelism, because:
Collimation of a monocular Newtonian telescope, or a monocular refractor or a catadioptric such as a Cass. or a Schmidt-Cass. has been made possible by the Cheshire eyepiece, the sight tube, and recent laser collimators. Here, collimation means proper alignment of the components of the monocular instrument.
Has anyone heard of attempts to collimate the components of one side of a binocular, before conditionally, or unconditionally ( so called "collimating") aligning that barrel to the other barrel?
If a 2-hinge Porro II instrument such as the Nikon 20 x 120,or similar WW II Japanese military binoculars, or the Fuji 25 x 150 or 15 x 80, or some of the straight view Kowa 20 x 120, or the prewar Zeiss mounted binocular telescopes, or the postwar DDR Zeiss Jena versions, or the Chinese copies/near copies have been disassembled in the prism region without retention of the (hopefully) factory correct position of the prisms , it is a good idea to attempt to collimate the affected barrel within itself,and to its own IPD change bearing axis, before attempting alignment of that telescope to its mate on the other side of the body.
A long reach comparator , such as the JTII rhomboidal or something similar,with or without an attached auxiliary telescope, to peek round the prism drum and body anywhere in a large angular swing , even 360 deg., is a big help in attaining an approximation to alignment of one barrel to its own rotating IPD change axis. At the same time, an attempt should be made to keep that telescope optically collimated. One thus gets an idea of the problems confronting the designers, production personnel, the factory assemblers, and ,one hopes, repair people who may eventually have a role in the life of a binocular telescope , be it single hinge bearing, or double hinge bearing.
Once a monocular half of a binocular telescope has been collimated, it is a very difficult mechanical/opto-mechanical proposition to attempt to exactly align the optical axis of a similarly collimated scope to the first one, without departing from collimation within one or both of the telescopes .
Therefore, it is a misnomer to call 3- axis alignment of a binocular "collimation". Though the collimation departures within each telescope may be small in the very best grade of binocular instruments, a departure from collimation within one or both telescopes of the binocular, even if one or both were originally collimated, is inevitably a result of 3-axis parallelism achievement. The difference may or may not be negligible.