Some time ago, I got my first decent monocular. A PVS-5c might not be everyone's idea of state-of-the-art but I ended up with two tubes for less than $1000 - an exceptional price in Australia. 

However, worse luck, one had shockingly bad MTF. Some basic tests put the figure at around 8 to 12 lp/mm at best - well beneath the other which was approaching 40 lp/mm. 

Although the tube was still useful, despite the early signs of shading, I decided to take it apart and have a look. Why? During some trawling of the internet, I found information about the lack of Photocathode/MCP voltage leading to focusing issues between the two - leading to loss of resolution.

Some time ago, I got my first decent monocular. A PVS-5c might not be everyone's idea of state-of-the-art but I ended up with two tubes for less than $1000 - an exceptional price in Australia. 

However, worse luck, one had shockingly bad MTF. Some basic tests put the figure at around 8 to 12 lp/mm at best - well beneath the other which was approaching 40 lp/mm. 

Although the tube was still useful, despite the early signs of shading, I decided to take it apart and have a look. Why? During some trawling of the internet, I found information about the lack of Photocathode/MCP voltage leading to focusing issues between the two - leading to loss of resolution.

Additionally, I could see scintillations from the MCP ( thermal noise ) quite clearly and sharp to my eyes, as well as fixed patter noise. Clearly then the issue was somewhere between the photocathode and the microchannel plate.

A few emails had already passed back and forth between myself and Ed Wilcox who help me diagnose the shading ( it wasn't all that apparent at the time ). He suggested that this, my tube going to air, may have been the cause of the loss of MTF.

The other problem experienced looked like "ripples" moving outwards from bright points such as lights, to describe it. This was only present on the faulty tube and was not exhibited on the good tube.

So the tube sat on the shelf for some time and I thought little of it. Then some time later, I got hold of a mx10160 that had been depotted. Needing a power supply and knowing the relative similarity between Gen2 and Gen3 voltage levels, I grabbed the power supply.

Imagine my surprise to find that the mx10160 exhibited extremely low gain, barely visible in normal light, and also failed to provide any serious resolution. It was so bad that I could barely make out the drainage plug in my bathroom with the lights off, when the door was open with a CFL 20w light in the next room and the door wide open. To describe the image that I could see, it was incredibly noisy - like a PVS-4 on full gain on an overcast moonless night.

At this point, I began to suspect that there was more to this issue than met the eye. I managed to later obtain another PVS-5 power supply and tried it on the mx10160 - Although the PC voltage was very low, leading to a lot of noise in the image due to the MCP gain being up too high ( it was a filmed tube ) I found that  I could get an image now that was at least comparable to my good PVS-5C with perhaps some better contrast.

I then took the new PSU and connected it to the faulty tube - and was surprised to find that the resolution had returned to > 32lp/mm. However, now the shading had set in.

The next step was to try the old power supply again. Once more, the image appeared as though smothered in vaseline. The shading, of course, was still as bad and getting worse but it otherwise looked just as I remembered when it was still very bad.

As a final test, I took the new PSU and managed to find another tube - This new PVS-5 ( a green photocathode ) was able to work almost as well as my good PVS-5c -just slightly less gain and sensitivity to starlight and  no issues.

So what do I make of this?

Clearly, the MTF can be affected by the power supply. Additionally, since the gain component ( MCP ) seemed to work OK, it looks like it was the photocathode voltage that was the issue. This seems to be supported by the results with the mx10160 tube which had insufficient voltage for most electrons to make it through the film ( Typical films will stop all electrons with less than 150ev from penetrating through to the MCP ).

Additionally, through a little more correspondence with Ed Wilcox (Thankyou Ed, for your neverending assistance ) I was able to determine that the related issue I saw when looking at bright areas was an "inside" trick to spot tubes with low MTF.

I have also seen this result, more recently, on a video clip in which a Gen2 tube is exposed to light. This would also have the effect of lowering the MCP voltage due to powersupply overdrain. 

As a result, there appears to be a direct correlation between screen-MCP voltage and MTF - as such, it may be that underperforming tubes aren't faulty because of manufacturing issues, but are faulty because of an underperforming power supply.

If this is the case, then many tubes that fail to meet spec could be brought up to spec by replacing the power supply.

Additionally, greater yields during manufacture might also be achieved by testing power supplies for Photocathode voltage and stability prior to encapsulation.

Finally, having some means to "tweak" the photocathode voltage might allow civilian tubes to be improved with respect to MTF under dark conditions by adjusting the photocathode voltage. This would reduce the lifespan of the tube, of course, but that's not really an issue for civilian tubes.

In concluding matters, there is also one other relationship here that appears to exist - relevant to the EBI of thin-filmed Gen3 tubes and the photocathode voltage - however that's presently under research with another researched and is not considered within the scope of this discussion.

Please feel free to add comments - I will do my best to answer them ( or will appreciate any technical input ).

Still to do:

Get a high voltage, very high impedance multimeter probe so I can measure the PC voltage as a component of this research. 

Look at relationship to possible "directional" field across PC that seems to be apparent in some newer research. 

Thankyou: Edward R Wilcox Jr of Wilcox Engineering and Research. This research would not have been possible without his assistance and information.

Thankyou: Stanley from the US forums - His research into related issues and assistance has been immensely helpful in identifying that the power supply was at fault.