7 replies to this topic

[Lambda]

Hello.

 

I use regularly a NV scope equipped with a second-hand gen III tube: MX10160 tube.

 

The tube works well but has a permanent "shadow or ghost image" in negative (shape of a window or a door ???) due I guess to a miss-use from the previous owner.

 

Such defect is not dramatic and I am used to observe sky or terrestrial landscape while forgetting that...

In extreme dark condition, such ghost pattern is by the way not noticeable at all.

 

But, I just read that said defect may be partially removed by using the "dark shoebox trick", which would basically consist in running the tube on, in a fully dark enclosure, during several days.

 

So, fake or truth?

If truth, what would be the physical mechanism behind this trick which would allow to (at least partially) "regenerate" the tube (its photocathode? its screen? both?)?

 

Did some of you try this trick and be successful? without any secondary drawback (for instance: finding back an homogeneous image, but at the cost of a loss of sensitivity?)

 

I am tempted to try that, but I have only this one gen III tube and being in Europa, I would not like to destroy it... Too expensive and hard to find them here...

 

If this "dark shoebox" trick is not clearly indicated as valid by you, I will simply keep on using and enjoying my tube according its current state.

 

 

In advance thank you for your feedback/advice on that.

 

Clear sky.

 

Lambda

[Eddgie]

The tube in a night vision device is like the screen on an old time television and just like the old televisions that had burn-in from people playing "Pong", the phosphor can burn in.

 

The other thing that can happen is that a charge in the shape of the image can build up on the photocatode.   This condition will usually correct itself in time though normal use.  Putting the cap on the device and leaving it in a dark place would in essence be the same as normal use in that there is a charge on the photocatode and in time, the charged spots should equalize. 

 

The thing is that if you have used the tube regularly and the image is still there, I would think the chances are that it is not going to help.

 

On the other hand, if the tube is capped (tape over the inside of the Mil-spec lens) and put into a dark place and left to run overnight, that would be like many hours worth of use, and if it is due to a charge built up, then that should do it, and since there is almost zero risk in doing any harm, I would say it is worth a try.

 

I have also been told that putting the tube in the refrigerator or freezer overnight could help. 

 

(I did have a problem like this once.   I went in and left my monocular on a post-top (thin film tube) but accidentally left it on and it was pointed towards my house window.  Now almost all of the lights were off in the house, but there was a lamp on somewhere so there was a "bright" window frame in the view (bright to the image intensifer) against a dark background. I was only inside for 10 minutes and when I came out and started using the tube again, I was horrified to see the ghost of the window.   This image faded pretty much while I was using the tube.  Within about 30 minutes, it was gone. I was lucky.)

Edited by Eddgie, 30 January 2019 - 08:24 AM.

[Rickster]

I have a similar ghost image in my tube. I tried the shoebox trick. It did't work.

[11769]

The tube in a night vision device is like the screen on an old time television and just like the old televisions that had burn-in from people playing "Pong", the phosphor can burn in.

 

The other thing that can happen is that a charge in the shape of the image can build up on the photocatode.   This condition will usually correct itself in time though normal use.  Putting the cap on the device and leaving it in a dark place would in essence be the same as normal use in that there is a charge on the photocatode and in time, the charged spots should equalize. 

While the phosphor can burn in, it's not a failure mode that will be noticed during the life time of the tube. Any damage to the photocathode happens way faster and that's what forms a latent image. 

 

The photocathode is very conductive and does not build up any localized charge in any appreciable way. Dark streaks left on the screen while panning across bright lights (or from a bright localized flash) are the result of temporary charge depletion of the MCP. Not exactly a failure mode or damage. Corrects itself quickly with no lasting damage. 

 

The latent image that everyone is so justly horrified of is the result of localized photocathode damage from excess positive ion bombardment. Gen 3 PCs are very susceptible to ion damage and positive ion density is a function of emitted electron density and thus incident light. Autogating, while not originally designed to address this damage mechanism, helps to "pull" out any ions that became implanted in the photocathode. 

 

The "dark shoe box" trick certainly does work and carries no risk. A Gen 3 tube is rated for 10k hours of operation at some nominal light level, a practical in-service nominal light level. In pure darkness, that 10k hour figure will be greatly exceeded. Have had this work on at least three tubes with tangible results. Two were autogated and had a dark latent image that persisted until the next day with the tube powered off. No change from one night to the next. Running the tubes overnight greatly reduced the dark latent image. Running them a few more nights resulted in the dark latent image slowly fading until no trace remained whatsoever. Performance did not suffer. 

 

Same trick has also worked with a nongated tube. Bought it used knowing that it had a black rectangular blem. No change in the first week when I tested it and then life got in the way and prevented me from playing with it some more. After using it a bit and letting it run over night, the black solid rectangular blem revealed itself to be "EXIT" burned so severely into the PC that the letters were not initially even noticeable. Over the course of using that tube, the latent image faded until one had to look for it. Never went away but was a vast improvement of the initial state and this was for very severe damage. 

 

I used that last tube to verify a couple of rumors regarding PC damage and how to fix it. A dark shoe box works for relatively minor damage and works relatively slowly. Repair occurs optimally at some very low light level. Low enough that there is no impact to the life of the tube. Can be accelerated with localized light on the PC as well. Works even with nongated tubes. The optimal route is still very much a science project and is not something to be duplicated, at least just yet. The dark shoe box? Knock yourself out. 

[Jeff Morgan]

The "dark shoe box" trick certainly does work and carries no risk. A Gen 3 tube is rated for 10k hours of operation at some nominal light level, a practical in-service nominal light level. In pure darkness, that 10k hour figure will be greatly exceeded.

 

I did the math once with length of average observing session of 3 hours with the conservative assumption the device is left ON the entire session (which of course I never do).

 

Over a 12,000 hour tube life, it worked out to 50 years of observing sessions. I will be thrilled if I need to be buying a new intensifier at the age of 105.

 

I would say a periodic Dark Shoe Box session would be called for once or twice a year "needed or not". Cap on, sealed in the Pelican case, case in the gun safe. Not much risk I am seeing.

Edited by Jeff Morgan, 30 January 2019 - 11:42 PM.

[Lambda]

Hello...

 

First of all, a very big thank to all of you for having shared your knowledge and experience on this thematic!

It is extremely useful to me... I am always amazed to see a such high concentration of knowledge and capabilities among this community!

 

So, I am convinced to give a try to my tube...

 

I could simply proceed as following:

 

- cap on my NV scope, and scope stored in a dark cupboard.

- NV scope switched on tonight (around 07H00 PM, dark here in Netherlands).

- NV running on at least for 12 hours in a row.

 

In the following morning (at 07H00 AM)

- switching off the NV tube.

- bringing it to a closed and dark room (no windows: bathroom in my case).

- testing my tube by switching it on and scanning the immediate environment in the dark room, by just benefiting from a touch of light coming from under the door of said dark room.

- slightly and very progressively opening the door for increasing slightly the light level and checking if I still notice the presence of this ghost image and potential improvement.

 

 

Repeating eventually the full cycle  for improvement...

 

Please do not hesitate to correct me, if I mistake myself here...

 

Again thank you for your help. It would be super nice, at least on a psychological point of view, to find back a relatively "clean image".

 

Clear sky!

 

Lambda

[highfnum]

i calculated 35 years at 10000 hrs 

it don't matter im dead anyway 

[gman1971]

Hey man, I just read this..., I own a bunch of NVDs and the shoebox trick does work, but to a degree. I've had a 5 minute still image "burn-in" on one of my nicest MX-10160 tubes and the "shoebox" trick took 12 hours to completely get rid of it. The tube is perfect now. Now, I am not certain that the burn in is caused by the phosphor as much as it is by the ion barrier film or photocathode depletion. My Gen2 doesn't seem to have this problem as much as any of my Gen3s do, regardless of Omni IV to Omni VII (don't own any early Gen3 so I can't compare)... Usually Gen3 seems the most affected by the burn-in IMO. I've also found out that turning ON the illuminatior and placing your hand over the objective lens for a couple of seconds gets rid of the streaks caused by street lights and the brightest celestial objects (not the Moon, never look at the Moon, right? haha)

 

If its a very bright light you might have to live with the damage, but for most burn in, like the one caused by leaving the NVD turned ON and pointed to a place with no bright lights (just ambient) you should be able to get it out this way. I've also removed the daylight pinhole of all my NVDs, it is a surefire way to create a nice circular burn-in on the tube if you accidentally leave it ON.

 

Just leave the tube with its cap ON, close the daylight hole, and in my experience it doesn't really matter if its on a drawer or 6 feet under, just ensure no light hits the PC and leave it be somewhere for a few hours (turned ON)

 

G.