Telescope Reviews: Parallelism, "alignment", and "collimation"

Terminology can be ambiguous , confusing, and context dependent.
The following system seems consistent:

"Collimation" should be used to describe the process of correct angular and locational adjustment of the components of a telescope , in and of itself, such that the nodes of the system are all located on one line.

That is the condition presently obtained, for astronomical telescopes, by use of a sight tube, a Cheshire, a laser eyepiece substitute, and perhaps also an autocollimating eyepiece.

For an industrial alignment telescope, used for turbine bearings, paper mill rollers, etc., such as those by Brunson, Taylor-Hobson, K&E, Leitz, Trioptics, Davidson ..., adjustment/checking of the telescope before use is refined by rolling the accurately cylindrical housing in a vee block, to verify that/adjust until the target image remains stationary during roll. That is truly "true collimation".

A binocular telescope ( a "binocular") should present to each eye a ray, from the center of real or simulated ( from a reversed telescope with an illuminated target in its focal plane, which, as a supplier of parallel rays, is a collimator by definition) distant target,which is parallel to the corresponding ray through the other telescope.

If this is the situation at all interpupillary distances , when the hinge axis is aimed at the center of the target, one enjoys 3-axis parallelism, or so-called "true collimation". But "true collimation" is a misnomer,because attainment of 3-axis parallelism together with genuine collimation of each telescope which would meet optical tooling alignment telescope standards, or even the standard of a well collimated monocular Newtonian telescope, is optomechanically impractical.

Departure from collimation of one or both of the telescopes of a practical binocular telescope (a "binocular"), assuming that each telescope is collimated to start ( which is impractical except as an approximation , in nearly all examples, particularly handholdable/manufacturable ones) , is needed to achieve an approximation to 3-axis parallelism.

3-axis parallelism = At all interpupillary distances, a ray from the target center leaving the left telescope is parallel to a ray from the target center which has passed through the right telescope.

To avoid ambiguity, one should try not to use the unfortunately entrenched terminology "collimation", or "true collimation" for this desirable condition of a binocular telescope.

2-axis parallelism: At one, and only one, interpupillary distance, the ray from the target center which is presented to the left eye is parallel to the ray from the target center presented to the right eye.

That condition has been called " conditional alignment", "hinge ignorant alignment", or even "collimation"(!).

Parallelism is better terminology than "alignment", because "alignment" in optical situations refers to/has overtones of colinearity , as in an optical tooling situation in a shipyard, aircraft factory, papermill, electric power generating steam turbine, gas turbine engine, etc. In these situations, departure from one straight line can have expensive consequences.