Last night I was out with the Comet Catcher and PVS-7 and while I got sidetracked troubleshooting what I thought was an optical problem (turned out to be the advancing cataract development in my right eye) I just started by playing around.
I used a 20mm Plossl as a plug in the Comet Catcher. This eyepiece actually produces a true field that is very near the true field of the PVS-7 so that seems to have worked out. I have of course compared the NV to glass before, but not using the same size true field so as to equalize the area of view. I have of course compared the NV to glass before, but not using the same size true field so as to equalize the area of view.
I used a random field and put a bright-ish star at the center and I counted six moderately bright stars and using averted vision and with very careful study, I think maybe another 20 or so stars came into view, and maybe if my eyes could have fully dark adapted (which may not even be possible under my less than dark environment, though my back yard is reasonably dark given that I live 3.5 miles from down town) I could perhaps have increased that count to 40 or maybe more, but as it was, I think my total count was about 25 to 30 stars.
Again, I have done this kind of comparison before, but never with a true field of about the same area, and because most eyepeices I used thended to be the same power, you got a much wider true field with glass than with the PVS-7 and that actually works to the advantage of glass eyepieces, though I could almost always in any given field (except Globulars) see more stars using NV.
Well, when I swapped out the 20mm Plossl for the PVS-7, the star count jump was huge. It was not doubled or even tripled. From where I could barely see 30 stars, the field went to having to many stars to easily count (and I did not even try to use averted vision). With glass, there was kind of an unequal distribution of stars across the field, but I was viewing near zenith to minimze sky brightness and near the Milky Way is near zenith for me, and suddenly I could see that the entire field had very faint stars scattered across it. Where there was a big void around the brighter star in the center of the field using glass, there were not a dozen faint stars peaking out of the background, and as I moved further around the field, these faint stars were everywhere.
While there were to many to easily count, I did a rough area count and multipled my sample count over the field size and estimated that there were maybe 150 stars visible to the PVS-7,
I would guess that I was picking up at least a full doubling of apertures worth of stars, and perhaps closer to 2.5 apertures.
In the past I have compared the Comet Catcher to the 12" dob, and even here, I think I was able to see stars that the dob could not quite eek out but I know that I could see all stars in the comet catcher that were visible in the dob with conventional eyepieces.
Now given that the Comet Catcher is maybe 15 or 20 years old and likely does not have the benefit of the enhanced aluminum optics on the Dob, and has a much larger (by percentage) light loss do to secondary shading, the dob was at an advantage of not only being over two times larger, but very likely more efficient as well.
I intend to do a more formal study in the coming nights using very detailed star charts to try to get a more specific figure on the limiting magnitude increase offered by NV and I encourage anyone else that wants to try to pin down a figure to join me in this effort.
I have in the past said that I thought that NV could at lest give the same result as doubling the aperture of an existing scope and I fell confident in this estimate, but I had read that some believe the figure is closer to tripling the aperture, so if we could kind of find some consensus as to the actual gain, I think this would be useful in general discussions with people not using NV as to be able to represent the actual gain with more supporting data.
Supposed to be clear for the next couple of nights and I am dusting off my reference books. Will try again and follow up with hopefully more accurate results using plotted limiting magnitudes.
If you already have your own estimates, I would love to hear them. I just want to feel confident that if I tell someone that they can get the same result as doubling the aperture of an existing scope that what I am saying is a reasonable statement.