Telescope Reviews: Insight about collimation

drl,
In my bino invistigatin' and buildin' I've also done just what you've described, to see whether it would be a good further test for collimation. But I didn't arrive at the same conclusion. The image of the field stop circles, as seen through the objectives, is angularly quite small. If the bino's true field is, say, 5 degrees, looking into the objectives will show the stop circles as 5 degrees across (and that's irrespective of your viewing distance, given that the objectives are acting as collimators.)

The small apparent size therefore makes what might be significant mis-collimation at the eyepiece end beyond one's ability to detect in this reverse-viewing arrangement.

Another thing to be aware of... Due to the inevitable manufacturing tolerances, both mechanical and optical, collimating a bino may require that one or even both optical axes NOT pass axially and/or perpendicularly through the eyepiece(s). Perhaps more significant than this is the case for those binos which have fixed field stops within the body, but eyepieces that not only move fore/aft for focusing, but which can "jiggle" a little bit laterally. These factors can cause, even for an otherwise perfectly collimated bino, the field stop circles to be not coincident. How do you know when this is the case? Look into the eyepieces and carefully observe both field stop images when gazing at a VERY distant object. More often than not you'll find that the field stop circles are not merged into one, but instead slightly overlapping (more often, and to a greater degree, in the horizontal.)

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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV

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