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Doug L
super member
Reged: 08/17/08
Posts: 122
Loc: Ottawa, Canada
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I mentioned in another post about something I've only recently noticed. The collimation of my binoviewers changes while altering the inter-pupillary distance. (Thankfully, my binoviewer is more than adequate at my IPD!) I asked a friend to test his binoviewer and his results are exactly opposite to mine - when eyepiece barrels are at their widest they are best aligned, and the collimation worsens vertically as the barrels are rotated closer together. These binoviewers are both the same lower priced brand but I wonder if even premium binoviewers have this "sweet spot" or if they show smaller collimation variances. I encourage others to try this experiment and post the results.
To restate (in my own words) the test (suggested by Glenn LeDrew)...
-remove both eyepieces.
-hold up the binoviewer backwards (single aperture toward one eye and eyepiece barrels away from you)
-look through the single aperture (with one eye) at a fairly high contrast terrestrial object or the moon if it is up.
-perfectly aligned binoviewers will have the two images totally overlapping and only one object will appear. If collimation isn't perfect, two objects will be seen.
-Rotate the barrels together and apart (altering IPD settings) and notice if the image(s) change(s)and by how much. The less desireable change would be a separation in the vertical. 
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Bino reality... 1 + 1 > 2
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Doug L
super member
Reged: 08/17/08
Posts: 122
Loc: Ottawa, Canada
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I just looked at a posting which has some interesting discussions about this topic ("Checking binoviewer collimation.") My post above would have been better placed in that thread.
My apologies, "Newbie" Doug L.  Next time, will do a better search before I post.
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Bino reality... 1 + 1 > 2
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12485
Loc: Cumberland, R I , USA42N71.4W
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You are describing conditional alignment. This is what you get when the prisms are tilted and adjusted while the binocular is set at one IPD. It will never be accurate across the entire IPD range.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Doug L
super member
Reged: 08/17/08
Posts: 122
Loc: Ottawa, Canada
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If this (conditional alignment) turns out to be the rule, not the exception in binoviewer manufacture, then it would be advantageous for prospective purchasers to test a number of binoviewers in a store, trying to find one that best matches their individual IPD.
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Bino reality... 1 + 1 > 2
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