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on the mum its the eyepiece that holds the tube tight so i would not loosen that - also if you tighten it back up dont overtighten it as the lite pipe can be broken if its overtightened,
i did hear of a mum that was dropped on the front lens and it was mis-alighned - have you dismantled the front lens unit to make sure none are cross threaded ? allthough it sounds like you say its the rear lens at fault - are the actual lens elements tight and fitted properly?
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No I haven't taken it apart, I'm not an expert so I'm a little cautious when doing these operations on my precious equipment. I could take a look at this too though. There might be small misalignments everywhere, what you see trough the tube is all of them combined of course. Thanks for the tip on the light pipe and overtightening.
I have spent a good deal now reading about monocular collimation problems and have found that generally none of them are collimated, they usually have a slight misalignment somewhere. Some PVS14 models seem to have a way to collimate them, and there was a patent for that method which described the process. Very similar to that of ANVIS. This was to allow binocular use. They can't be collimated in the sense that it's dead straight regardless of orientation, but they can be collimated relative to each other so when mounted on a bino mount, they show the image fully overlapped. It's just to make the optical path offsets align, not to remove the offset alltogether.
All the monoculars I have previously owned had the same problem in varying degrees, but I never used them head mounted that much that it bothered me. When it's dark enough the problem obviously goes away, but if the sky is even bit lit the brightness showing trough trees is annoyingly separated between my eyes. When it comes to urban environments streetlights far away show as double which again is not nice. Not a big deal in other words, but would love to fix the issue.
If someone here has a MUM and is willing to do a quick test I would greatly appreciate. Just to know whether my unit is actually far from what it should or if this is just how they are. How I see the issue the clearest is by looking at a street light or some bright pinpoint object far away, keeping my both eyes open, and then turning the MUM with my other hand. Then try not to align your stereo vision, just stare as a drugged person to infinity, and if you see the pinpoint object doing a full circle relative to where you see the object with your naked eye, then it's not collimated. If you could describe how bad the effect was I'm interested. For example, can you align your stereo vision regardless of the MUM rotation angle, or can you do it only in certain orientations?
Edit: To add to the testing methods, stars in the sky is probably the easiest way. Look up and see how the images differ between your naked eye and the one behind the NV.