So, I've been looking around for a while for a decent sac window that will eliminate me having to remove it when I use the 3X magnifier or not having to clean the objective lenses every time I accidentally touch the lenses or when it rains or snows outside... etc... I didn't want to spend 200 dollars for sac windows either. Most of the low profile ones are priced abusively and using a LIF wasn't an option since they are expensive too and don't allow me to screw the 3X magnifier either.

I already have 6 NVDs and IMO a sac window should be in the 3-5 dollars a piece range, not 30, 50 or 100... I know... think what you want...

Anyhow... so after much thinking I came up with this idea and it has worked very well for me so I figured I'd share it. This sac lens has very minimal reflections from the input side, but under some circumstances you'll still see a reflection, especially from very very bright light sources; however, it is unnoticeable under most conditions and I didn't notice much difference for stargazing; but in short , for 5-6 dollars you can't beat this thing and it will prevent your 200 dollar objective lens from getting dirty, dusty or scratched while walking in the woods around thick bushes or accidentally touching it with your dirty hands or gloves, you name it... This sac lens is watertight too, so your main lens will never get wet or fogged inside due to water entering the back of the sac lens and will still allow for the 3X magnifier to be screwed into the threads. Doesn't require any modification to the objective lens either and its 100% reversible (not that you really want to anyways...)

Things that you'll need:

Something like this 29mm rubber o-ring
Rubber O-ring

Something like this 30mm UV filter. (You can get a high quality multicoated UV filter that will eliminate the internal reflections, but it will be more than 2.99 a piece... you can also get the 30mm IR filter instead)
UV filter

First is to clean the main lens of your NVD: Blow the dirt off first with compressed air (make sure you don't get the compressed liquid inside the can over the lens!! DON'T SHAKE IT!! ) Then use 99% isopropyl alcohol and a Q-tip and in slow circular motion go from the center to the end of the lens. After that, then hit the lens with compressed air and you are ready to go. Put the cap on after you're done.

Take the UV filter and gently unscrew completely the lockring that holds the glass. Leave the lens in there until you're ready.

Remove cap from the NVD lens and place the 29mm rubber o-ring on top of the front lens lockring.

Now, take the UV filter glass out of the UV filter frame and gently place it on top of the o-ring; make sure you don't get your fingerprints all over it... I recommend using nitrile gloves for doing this... or cleaning any optics for that matter...

Place the UV filter lock ring on the NVD objective lens threads and gently tighten it until the filter is secured against the o-ring.  (Do not overtighten it, doesn't need to be tightened to 1000 foot pounds of torque... )

You're good to go!

This is how it should look after you're done.



NOTE: Do NOT try placing the UV filter without placing the O-Ring first: the objective lens protrudes further than the lockring holding it in place so you'll need the o-ring to make this work without damaging the front lens.

NOTE 2: If you haven't thought of it, you can also do the same with the 30mm 720 IR filter as well... just instead of the UV filter use the IR glass filter. you might be able to do this with the LIF too...


Cheers.

G.

Thanks for the idea!  

I'll be trying this with my IR filters as soon as I get home today!

Good one. Thanks.
Reflections? I get reflections in my PVS-14 with no filter at all.

Thanks guys... yeah, hopefully the price for those cheap UV filters doesn't skyrocket due to the Night Vision tax... :)

G.

Correct. Thanks for the clarification. Some of the good UV filters have multicoated surfaces and those will alleviate, if not completely remove, the unwanted reflections...

G.

I already have the 30mm 720nm IR filters.  So, I can take one of them apart by unscrewing the locking ring.  Then, I add an o-ring to the 14's objective, and re-use the filter material and the filter's lock ring and toss the filter's outside frame.  Do I follow correctly?

Yes.

G.

I did something similar a few months ago, except mine was to protect the lens from a BB strike while using my 14's for Airsoft (say what you will).
I went to Home Depot and picked out a middle thickness lexan sheet and cut a circle that would fit inside of the IR filter I had kicking around. Same thing you did, Un do the locking ring and took out the glass and placed the lexan in. I didn't notice any optical degradation under normal usage and the lexan took a point blank shot no problem (not while it was installed on the 14's obviously).

Roger that TNVC Eric. That's what I get. Reflections from facing surfaces between elements in my PVS-14 from light sources outside.

That is not what I did. The sac window is part of the objective lens, not part of the external filter.

G.

I love threads like this. Great stuff.

Well I do believe I prefaced mine with "something similar".

And I do believe it Is not something similar; what you did is no different than buying the regular sac window. You still can't use the 3X magnifier, nor screw it into the objective lens, nor put the lens cap on.... That's the whole point of this post. A useful 5 dollar sac window that allows you to use the 3X magnifier and use the regular lens cap on.

G.

If the IR filters I currently have work fine and allow the 3x magnifier to be screwed in, would I benefit any from doing this mod?  Does the o-ring make it better?

watertight, allows you to use slip-on 3x, and standard sac window.

I see it mentioned in the original post, but has anyone actually tried transplanting an IR filter into a LIF?  If so, how did you do it?  I've long thought that an IR filter w/ the housing of an LIF would be just beyond perfect.