When you look at the Obsession website, which is cool, thanks for pointing it out, you see that it goes from an 8" with a limiting stellar magnitude of 14, to a 25" with a limiting stellar magnitude of 17. I'm going to go out on a limb and say, with a typical PVS-7, you pick up 4 to 5 magnitudes. I could easily be wrong, it could be only 3, but I'll go with 4 to 5 for my viewing circumstances. And the author of what is mentioned in posts 6 and 7 seems to corroborate an estimate of 3 to 4, and I'm trying to be conservative or reasonable in my estimate because I really haven't tried to check the magnitude difference very carefully, and it depends on the NV device.
I was thinking about this a couple of weeks ago when I was viewing, and I immediately thought 4 to 5 magnitudes, but then I got to thinking is it really that much, and then the question became compounded when I started thinking about what is an acceptable image. For me, I had to quit thinking of the faintest thing I could see, and start thinking of the difference in magnitudes for an acceptable image. Here's an example, I'm looking at the Ring Nebula, I can see it in a regular eyepiece (EP), if I didn't already know what I was looking at then I might not necessarily know what it is. I put a PVS-7 into the focuser and then the Ring is well defined and I'd know what I was looking at no matter what. With the regular EP I might not know there's a hole in the middle of it, etc. Then I add a 7nm H-alpha filter to the PVS-7, and then I have my best view of the Ring ever because I've never been able to look at it through this new scope and PVS-7. The scope was a 12" F/4.9 newt.
That's a good example, but the "acceptable image" question is a big question for these comparisons.
I've asked this question about the magnitude difference before and never have seen it fully answered, and I've never tried to carefully answer, partly due to the fact that as soon as you plug the NV in the answer that comes back is A LOT, or just plenty of difference and you almost immediately forget about the question just as you immediately see things that you simply cannot normally see, without something like doubling or tripling your aperture.
So what I quickly concluded whenever I thought about it while using the equipment was that it was at least a solid 4 magnitudes, YMMV. And then you have to include the comparison caveats that Eddgie brought up in post 2, and any other comparison warnings you can think of, and there are plenty of them. I've found in the last year or so, that it's harder to do equipment comparisons than I had originally thought, starting with seeing.
Tell you what though, if I get the chance, tonight I'll try to do a direct comparison with M13 and those images. pwang99 immediately said 18" to 20", that's probably a very fair and accurate estimate. But there is one problem for me that I'll list right off the bat, I think it can look like any of those images except for the smallest one, the 12.5" Classic, or even the 15". Just like regular astronomy, it depends on what you plug the NV into, at the end of the day it's still just an EP if you're plugging it into a telescope. Maybe a very nice EP, but still an EP.
Edited by outofsight, 25 May 2017 - 04:32 PM.