Telescope Reviews: MrBill's review of Gemini 25x100 IF MkII binos

" .... very likely have to compromise the system by tilting a prism anyhow" "You now have a conditionally aligned binocular".
Instead of prism tilting, consider prism sliding of Porro II prisms ,which is the factory alignment adjustment method for the Fuji 25 x 150 II. The heavier earlier model, I, including the 90 deg inclined astronomy version , has double eccentrics around each objective lens. If 1972 to about 1989 eccentrics are hopelessly corroded, such as from galvanic brass-aluminum-salt water sacrifice of aluminum, I have converted them to sliding the Porro II prism cluster(s), by adding fine thread setscrews/grubscrews through the wall of the prism drum.

For an early Fuji 25 x 150 so modified, one neutralizes the eccentrics first. The current Fuji II has no eccentrics. In a modified model I, or any model II, the technician disconnects the link between the prism housings, and swings one prism housing through a large arc, even a full circle. During this, the position of a direct "peek around" view of the distant target is compared, via a beam combining comparator such as the handheld JTII or equivalent, to the view of the target through the binocular half. Alternatively, a telescope large enough to see the eyepiece throughout a large swing, and to peek around the body, or a beamsplit laser, or other possibilities come to mind. The Porro II cluster is slowly pushed, via fine pitch setscrews/grubscrews , in an appropriate direction, until the image through the barrel under test remains superimposed and stationary atop the peek-around direct image, throughout the swing.

This technique requires repeated removal and replacement of the prism housing cover-eyepiece assembly, to unclamp the prisms, adjust with the setscrews/grubscrews, and reclamp.

The Nikon 20 x 120 I and II can be similarly treated, after setscrews/grubscrews are added as pushers. Incidentally, the Nikon 20 x 120 I and II have eccentric pairs around only one of the objective lenses. The other objective is surrounded by a concentric cell and ring.

This tiltless process , if combined with stable, well machined bodies and rotating mechanisms with parallel axes , and well-centered lenses and tilt-free mounting of them , goes a long way toward the goal of intra-barrel optical collimation combined with optomechanical parallelism of the two optical axes and the two axes around which the two prism housings rotate.

I am probably not alone when I say that I like the clear thinking and exposition which you have repeatedly demonstrated.

Edited by Gordon Rayner (04/20/12 01:30 AM)