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Posted: 2/12/2013 4:05:11 PM EDT
I just got my PVS-14 from TNVC. Nice unit specs are decent.

Here is my problem. With the NOD in front of my eye, there is a prismatic error which introduces double vision. For example, if I were looking at a fence, one eye sees the top of it offset from what the other eye sees. I am unable to adjust this out with any of the INVG mount controls or the NOD controls.

I dismounted the NOD and, holding it before my eye, I rotated it on its optical axis. I find that rotating it about 90 degrees either direction progressively corrects the error and nulls it out. The error walks around the null points in an ellipse as if it is an internal prismatic element that is not properly set.

The double vision is a strain on the eyes because the eyes cock out of line to compensate. The effect is independent of distance - it is not a simple matter of parallax.


Has anyone encountered this before? Is there a remedy?
Link Posted: 2/12/2013 5:58:35 PM EDT
I've had this happen on my pair of PVS-7's but not on my PVS-14. With the PVS-14 against my eye even if I pivot the objective lens up down or left right the image is still centered with what the unaided eye sees, although it blurs out of focus because you aren't looking through the center axis of the lens but through the outer edges of the lens.

Not too much to the optics in these things. Some objective lenses and eyepiece lenses. No prisms to go out of alignment. I would suppose that a lens could be gound incorrectly and shifts the image off axis.

TNVC has great customer service. Give them a call. They will help you trouble shoot.
Link Posted: 2/12/2013 6:02:29 PM EDT
By your description, you have thoroughly been through the unwritten enduser trouble shoot list on the glass. I have tried different scenarios with one of my 14's & could not duplicate your results though I'm not an expert.... That puppy would simply have to go back to the kennel for a checkup.. Was it an in house build or factory ITT or L3 device?
Link Posted: 2/12/2013 6:23:57 PM EDT
Thanks for those remarks. Much appreciated.

I am aware of the outstanding customer service at TNVC. I confirmed their enthusiasm before ordering. My intention is to be certain I am not drawing the unreasonable conclusion that there IS a problem before pestering that crew. After the Christmas and Shot Show interruptions, and with the run on equipment these days, they have plenty do do.
Link Posted: 2/12/2013 6:43:31 PM EDT
All PVS-14's suffer from this to some extent. It's caused by a combination of factors, but if you want to record it simply, mount a video camera on a tripod and show it the same scene through the monocular. Turn the monocular around while viewing the scene.

Now review the tape, and place a dot of paper or something on the screen near an easily distinguished feature, like a corner of something. As the monocular moves around, you can see how far the optics move it away from the orginal scene.

This is what you're experiencing and if it didn't occur, you could use your PVS-14 as a clip-on aiming device in front of a dayscope.

As for internally? There's no correction. If the tube is a little skewed or potted slightly off-axis, it can exacerbate the issue. If it has distortion, that can make things worse too. Looking through the edge of the ocular lens is also likely to make things worse.

About all you can do to fix it is to make sure the tube is seated properly. There's no adjustment that will fix it. If it's really bad, or it's bothering you, call TNVC, and send it back. They are good guys and will fix it if it's a serious problem.

But otherwise, using your unaided eye and the monocular at the same time? The issue will always be there. It's common to all monoculars. Short of installing something like a risely prism in them, there's nothing you can do either.

Though on that matter, it is possible to install a risely prism in the ocular if you do end up with a severe enough problem. But custom optics would be very expensive.

Regards
David


Link Posted: 2/12/2013 7:12:34 PM EDT
Thanks, David.
My thinking was that the tube or one of the optical elements might be mounted in an eccentric fitting, one that could offset an optical or mechanical "crookeness" in ant axis at final assembly.
Haven't seen the insides of an NOD since I serviced a few PVS-5 units in a brief training session many years ago.

I happen to have a couple Risley prisms on hand, and although they are not of suitable diameter for long-term use behind the 14, your comment reminded me of those long-forgotten items. A short experiment with one of those will tell me what I need to learn.

Of course, the whole problem is inconsequential when the right eye sees nothing but darkness, however in practicing where light is present, the offset is a problem since my trusted eye (dominant eye) is the unaided one and she's tracking off kilter.
Link Posted: 2/12/2013 7:16:23 PM EDT
@David: interesting test. I tried it just now with my AN/PVS-14

I centered a dot in the PVS-14 in front of a 50mm lens on my DSLR. I took four pictures after rotating by 90 degrees. Downloaded the photos and viewed them in sequence on the computer screen.

Normalizing the image to the screen width of ~18mm I calculate that the center is displaced by about 0.5mm with the rotation of the unit. This poses absolutely no problem under normal viewing for me. On the other hand, I typically use the unit when it's so dark there is no image on my unaided eye.
Link Posted: 2/12/2013 9:26:07 PM EDT
[Last Edit: 2/12/2013 9:26:35 PM EDT by NVGdude]
When you pot a tube you have to line up the optical axis of the tube with the mechanical axis of the housing. On an inverting tube (as used in a PVS-14) this is important, because with the inversion you get a translation in the image.

The spec for most ground systems is an image alignment of 0.020" (roughly half a millimeter). An 18mm (nominal) tube gives you a 40 degree FOV (roughly 2.2 deg/mm) so a max spec image alignment tube would give you a shift of right about 1 degree.


Aviation tubes are normally speced at a .006" alignment.
Link Posted: 2/12/2013 11:21:16 PM EDT

Originally Posted By NVGdude:
When you pot a tube you have to line up the optical axis of the tube with the mechanical axis of the housing. On an inverting tube (as used in a PVS-14) this is important, because with the inversion you get a translation in the image.

The spec for most ground systems is an image alignment of 0.020" (roughly half a millimeter). An 18mm (nominal) tube gives you a 40 degree FOV (roughly 2.2 deg/mm) so a max spec image alignment tube would give you a shift of right about 1 degree.


Aviation tubes are normally speced at a .006" alignment.

MX10160 GS are also 0.006" alignment, so I think that might extend to all MX10160 style tubes.

I hadn't noticed before though that the MX11769 was 0.020" alignment - that's quite a surprise. Do you have any idea why this was the case?

Thanks
David

Link Posted: 2/13/2013 10:17:59 PM EDT
[Last Edit: 2/13/2013 10:19:38 PM EDT by NVGdude]
Originally Posted By cj7hawk:


MX10160 GS are also 0.006" alignment, so I think that might extend to all MX10160 style tubes.
I hadn't noticed before though that the MX11769 was 0.020" alignment - that's quite a surprise. Do you have any idea why this was the case?




Spec writers get lazy, the reason the GS tubes have the .006 alignment spec is because they just copied most of the MX-10160C spec when they wrote it. The GS tubes also have the metalized aviation housing even though there is no good reason for it.

No idea why the 11769 has a 20-mil alignment spec, I've only been working with NVGs since the late 90's
Link Posted: 2/14/2013 8:00:50 AM EDT
Mystery solved. The shift, if it is a fact and NOT my own glasses/eye introducing the effect, is very minor.
After discovering the phenomenon outdoors at night, I investigated more thoroughly in the daytime when objects were more easily seen with the unaided eye.
And the daylight filter itself is the culprit that causes the serious shifting of the image through the NOD. The pinhole is not perfect of course, because it is manufactured in a rubber item, and it is not exactly centered.

So, disregard my ingenious conclusions and my making a mountain out of a mole hill.
And thanks once again for the remarks.
The TNVC PVS-14 is probably as good as one can be.
Link Posted: 2/14/2013 5:46:52 PM EDT
Originally Posted By Feeble-Prize:
Mystery solved. The shift, if it is a fact and NOT my own glasses/eye introducing the effect, is very minor.
After discovering the phenomenon outdoors at night, I investigated more thoroughly in the daytime when objects were more easily seen with the unaided eye.
And the daylight filter itself is the culprit that causes the serious shifting of the image through the NOD. The pinhole is not perfect of course, because it is manufactured in a rubber item, and it is not exactly centered.



Yeah the daylight lens cover can cause all sorts of weird issues. We had a goggle once that was driving us nuts because it looked like it had extensive film damage, but the bare tube looked perfect. Turned out it was a small amount of grease on the objective lens, and the pinhole filter was enough to bring it into fuzzy focus.
Link Posted: 2/14/2013 6:49:06 PM EDT
So obviously wrong to evaluate the optics with the daylight filter there that it never even occurred to me.
I expect one of these days when it conks out, I will tear the device apart to understand why it failed and find the battery is weak.
Link Posted: 2/15/2013 4:46:21 AM EDT
Heh, the daycap causes a lot of interesting optical phenomena... It's also a quick way to check what the T-number is of similar F-number lenses, relative to each other.

Anyway, if your lens is in focus before you use the cap (after, it always appears in focus ) then the displacement will not vary.

This same problem causes all kinds of problems for people sighting up riflescopes with a single-hole daylight cover in place.

Anyway, nice investigation ! :) Figuring that out is not all that intuitive.

@NVGdude... Only since '90 eh? That's pretty much the entire Gen3 era for all practical purposes... About five times longer than I've been playing with the stuff :) Now I see where some of your knowledge is coming from.

Regards
David
Link Posted: 2/15/2013 11:30:06 AM EDT
[Last Edit: 2/15/2013 11:30:47 AM EDT by NVGdude]
Originally Posted By cj7hawk:
. Only since '90 eh? That's pretty much the entire Gen3 era for all practical purposes...


LATE 90's. We had OMNI IV by then.
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