I have considerably more limited exposure to different tubes that you guys, especially having them at the same time in housings to test side by side. So I want to ask you guys that have seen plenty of tubes, does EBI and Halo contribute to perceived contrast and resolution?

In the beginning I did not notice much difference in tubes except the obvious, low light performance and SNR if far enough from each other. Another thing is that a spec sheet is something quite unheard of on this side of the pond if speaking of Gen3 tubes.

Now I have only two tubes, and between them there is some SNR difference, enough to be noticeable, and the one with more noise has very small halo and EBI, and even though it is supposed to have a lot lower photocathode response, in ways it actually outperforms the "better" tube in many situations, even in low light it's hard to say which is better. Go low enough and it seems the very low EBI saves it while having much more noise than the other one.

With the lower SNR tube, it has a seemingly better resolution, while in reality it only looks like that. You do actually see more detail with the better SNR tube, but it's a lot more fuzzy.

The question is, can I make any conclusions from this? Is generally a low EBI paired with a low Halo (or only one of them possibly) linked to good contrast and crisp image? Even in higher light where you'd think EBI doesn't matter much?

Thanks in advance!

High EBI is linked to a brighter image, with something like a milky glow in the darkest parts of the image on the darkest nights.

Large halo is linked to lower resolution, but has so little impact overall that it's not worth considering. 

Neither predicts, with any degree of certainty, whether a tube will have good contrast or resolution.

David. 

Thanks David.

It really is hard to grasp how different specs relate to each other. The tube with higher EBI does seem to have a slight "glow", best noticed in very dark or quite dark with daycaps on, then the one with lower EBI does seem to resolve more while being much noisier. Also has deeper blacks overall no matter the light level.

Hi Sam,

There is a reason for this - Tubes are "Proximity Focus" - that is to say, electrons come out of the photocathode in a spray, much like a spray can. If the MCP is too far away, even if only one point of the photocathode is illuminated, the spray of electrons would cover the entire MCP. By bringing the MCP closer to the photocathode, this spray is reduced, much like bringing a spray can close to a wall makes very fine lines ( Just in case anyone is wondering, no, I am not a graffiti artist, nor have ever made graffiti... But I have messed up painting stuff many times by getting too close. ). 

This is what is known as "proximity focus" - Just get the MCP and Photocathode so close that they almost touch, and there's no need for electrostatic lenses. 

So a tighter focus also means a smaller halo.

Now if you followed me this far, you're probably saying, quite correctly, that the light intensity won't change the angle of the spray - Well, "close" is a relative term, and two factors affect "close" here -

One is the physical distance between them, and the other is the electrostatic field intensity - Much like 6 meters is very high here, but not so high on the moon, a high electrostatic field intensity also improves focus - so more PC voltage, more focus.

Now if we look at other mechanisms inside the tube,  then we get PC voltage being fairly constant - excepting autogating, which we can ignore in this context - and the response to high light, to protect the internal environment, is to reduce the photocathode voltage, hence the electrostatic field changes in intensity, and the focus goes out a little bit.

The end result is that US tubes lose resolution quickly in high light, as they respond to bright light by reducing the field strength at the PC, and the bigger the halo, the further the PC and hence the greater the loss of resolution. It also is very likely one of the reasons that photonis tubes don't lose any resolution in high light, since they use a different method to deal with the light and are probably less prone to high light damage being multialkali. 

At this point, I also have to mention that I'm absolutely amazed that anyone actually noticed this, let alone measured it and was aware of it. This is a pretty obscure bit of knowledge here. 

David

Thanks you all to my untrained eye most of this is undetectable.