28 replies to this topic

[Eddgie]

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. 

 

[MartinMeredith]

You might find some helpful charts with stellar magnitudes in this thread. I'd be interested to see how deep you can go with NV.

 

Martin

[GlennLeDrew]

It's fairly straightforward to derive the aperture equivalent for stars.

 

For extended sources, such a notion is dependent on too many variables. For starters, an excessively dim nebula that could never be seen visually at *any* aperture with the ideal filter, but which yields to the NB device (with filter, too, if necessary) implies an *infinite* aperture gain, which of course is nonsensical.

 

Even for stars, one variable which might prove to be of some import is the system f/ratio, which controls noise (scintillation), the level of sky brightness and the gain applied (automatically or manually.)

[Eddgie]

You might find some helpful charts with stellar magnitudes in this thread. I'd be interested to see how deep you can go with NV.

 

Martin

Thanks for that chart, but sadly I think M57 is behind the trees in my back yard by the time it gets super-dark.

 

I will see if I can hit it tonight or tomorrow night though a gap between trees. 

[MartinMeredith]

I didn't mean to link only to M57. Did you check out the ones earlier in the thread too? NGC 6910 in Cygnus is still well placed.

 

Martin

[outofsight]

Eddgie, we could give it a go to a certain extent, but there are some (maybe big) problems. As GlennLeDrew said, "For extended sources, such a notion is dependent on too many variables". Maybe even for all sources besides the Moon.

 

Biggest problem (variable) I see, or don't, is, as usual for all of us earth bound observers, the sky quality. As we all are constantly reminded, whether (weather) we even know it or not, it all starts with the "seeing". So you'd need a "sky reference point". While the light pollution in any area may be relatively constant, what it reflects, or doesn't reflect, off of in the sky changes, to one degree or another, as part of the overall seeing, nightly. The seeing where you live might be different by a couple magnitudes or more, just to start with, and then you throw in light pollution where you live, won't be the same as someone else's. You get the idea.

 

So first, trying to figure out magnitudes between glass and night vision, you'd need the magnitude of the object, probably apparent and absolute, to begin comparison. Then, and this might be very hard to narrow down, you would have to have a sky, or seeing, reference point for each comparison session.

 

Having said that, and that's just the tip (of the tip, maybe) of that iceberg, I might be willing to give it a go with you. At the great expense of $0.99, we could each get one of these, or we could try those sky meters for a $100 bucks or so.

 

https://itunes.apple...d602989060?mt=8

 

Without at least an attempt at sky reference, what you can see to start with, I wouldn't see much point in doing it. But if you want to try it, general idea, we both take out a Comet Catcher, a PVS-7, and matching EPs and filters, etc. We both get a sky reference, as best we can, we agree on objects to view and try to do it on the same night (may not matter if we could somehow agree on a sky reference), and we start comparing notes. Just a basic idea.

 

I've often wondered what the real magnitude difference is, but then stopped wondering because I was too lazy or unconcerned to try to figure it out, and I know that for where I live, the difference simply falls under the term "a lot".

 

Let me know if you want to try to figure something out. I never saw the point in doing it just for where I live and am not sure I'd even give it a go, but for $0.99 I might.

Edited by outofsight, 03 October 2016 - 02:13 PM.

[PEterW]

NV see star brightnessea differently.. red ones appear much brighter... will skew this... I know to my comments st as it makes star hopping a real pain!!

Interesting conclusions.

Peter

[GlennLeDrew]

Peter makes a good point. NV devices have extended red response into the near IR, which if not suitably filtered significantly biases to the stars of cooler surface temperature, or suffering significant interstellar extinction/reddening.

 

Perhaps the better stars to use would have a B-V color index range of 0.0 to 0.6, or the color of between Vega and the Sun, respectively.

 

This implies that the instrument used should not surpass the magnitude limit for the source of B-V color index data. The Hubble Guide Star Catalog might suffice for now. The GAIA data becoming available is a significant improvement.

Edited by GlennLeDrew, 03 October 2016 - 02:30 PM.

[PEterW]

... however I can see more stars naked eye than with an unfiltered NV. Add the 680nm pass filter and then the little twinkles come out to play. ( I also use my black flock lined lens shield "drainpipe" to reduce the local pollution sources. Looks like Eddgie has his work cut out working out which stars we are allowed to compare with and how to find them.....

Peter

[Jeff Morgan]

The NV tube brings in so many stars that counting becomes difficult.

 

It seems to me the best way to proceed is to locate a field that is sparse under NV and do the comparison there. Somewhere near the galactic pole perhaps.

[Rickster]

Even though there are too many variables to make this an authoritative (verifiable) study, I think that this is a worthwhile effort.  We are bound to advance our understanding in one or more ways.

 

I have been thinking about comparing a Pronto/ PVS-7 combination, to a AT6IN/ 26mm eyepiece combination.  Both have roughly the same focal length, so that would eliminate one variable.  I could extend the test to a 10" dob coupled to a focal reducer that would put its focal length in the same ballpark  as the Pronto and the AT6IN.

 

My gut feeling is that I can see more with the Pronto /PVS-7 combination than I can see with either of the other two using standard eyepieces.

Edited by Rickster, 03 October 2016 - 10:49 PM.

[Eddgie]

My goal was more to be able to better promote NV by giving an aproximation on how much gain they could expect to see by using a NV device in an existing scope.

 

I read threads all the time where people are looking at buying a much larger scope than they already own but are concerned about the cost or the logistics.

 

I have on many occasions suggested that an alternative that they might not be famaliar with would be to use an image intensifier with their existing scope and that this would give a result that would be similar to increasing aperture, but I was hoping to have a consensus opinion on how much that increase would be eqivilent to.

 

It sounds though that this is simply to impractical to do, and I am not going to kill myself to collect data that is unreliable or open to a variety of valid arguments to dispute it.  I would rather be observing. 

 

Still, last night I did another field where I counted 30 stars using averted vision and a 20mm eyepiece, and while still an estimate, I did attempt to count the stars in the field using NV and got to 170 in the same field.   It is just to hard to count stars in a field with any great accuracy, but if anything I think I understated for fear of counting the same stars twice.

 

While it may not be valid to suggest that this is like improving aperture, what I do know is that I can see far more stars with far more ease using NV, and in the end, that is good enough for me.

 

As a side note, I have become a big fan of Mel Bertals and his approach to Richest Field Telescopes.   The more I use NV, the more addicted to hyper-rich fields I have become. If I were ever to go back to conventional astronomy, I think it would be to scope in the pattern of Mel's RFT scopes, with very fast focal ratios, coma correctors, and wide field eyepieces.  My bet is that Mel is getting higher star counts than we do, at least when I am viewing from my back yard).  When I use a glass eyepiece, the field just looks empty. 

Edited by Eddgie, 04 October 2016 - 09:22 AM.

[MartinMeredith]

I for one would be interested in the outcome of such a study, even if done informally. And if a few NV users were to do something similar the results could be compared and perhaps an approximate statement of aperture gain could be made. 

 

Maybe rather than star counting you could sketch a small part of the field all the way down to the dimmest stars? e.g. some part of the double cluster?

 

Martin

[GlennLeDrew]

To obtain the largest number of stars in the FOV, whichever equipment is used, involves one critical stratagem. And that is that the limiting magnitude correspond to the inflection in the function where the rate of increase in star numbers with decreasing brightness decreases.

 

If we lived in a Universe in which the star number density was everywhere uniform, and there was no interstellar extinction, every instrument would qualify as 'richest field.' But we live inside a very polluted galaxy. Between the light-blocking haze concentrated toward the mid-plane and the rapid decrease in star numbers perpendicular to the disk, a limit is reached at some center magnitude where the numbers of stars per unit brightness interval ceases to increase at the same rate as for brighter intervals.

 

The magnitude at which this inflection occurs varies by galactic latitude. Nearer to the mid-plane, where the number density is largely constant to considerable distance, our horizon is extinction-limited. Toward the galactic poles extinction plays no role, and the actual fall-off in the star number density is the arbiter. If memory serves, for milky way fields the magnitude at which star counts fall off is 11-11.5. Toward the galactic poles this occurs at about 9m.

 

To reach the required magnitude limit and at the same time encompass the largest chunk of sky, the following attributes for a visual instrument are desirable, in order of importance/consideration:

 

- The eyepiece has the largest possible AFoV.

- The exit pupil matches the iris, so that magnification is minimal and hence TFoV is maximal.

- The objective aperture provides the light grasp to reach the required magnitude limit.

- The objective f/ratio delivers the iris-matching exit pupil for the eyepiece chosen.

 

Note that the objective f/ratio is the least important consideration, it being settled upon after other criteria are met.

 

Now, for an NV-based system, which has its sensor located at the principal focus, f/ratio is a first (or at least early) consideration from the standpoint of noise (scintillation) and FOV.

[Eddgie]

To obtain the largest number of stars in the FOV, whichever equipment is used, involves one critical stratagem. And that is that the limiting magnitude correspond to the inflection in the function where the rate of increase in star numbers with decreasing brightness decreases.

 

If we lived in a Universe in which the star number density was everywhere uniform, and there was no interstellar extinction, every instrument would qualify as 'richest field.' But we live inside a very polluted galaxy. Between the light-blocking haze concentrated toward the mid-plane and the rapid decrease in star numbers perpendicular to the disk, a limit is reached at some center magnitude where the numbers of stars per unit brightness interval ceases to increase at the same rate as for brighter intervals.

 

The magnitude at which this inflection occurs varies by galactic latitude. Nearer to the mid-plane, where the number density is largely constant to considerable distance, our horizon is extinction-limited. Toward the galactic poles extinction plays no role, and the actual fall-off in the star number density is the arbiter. If memory serves, for milky way fields the magnitude at which star counts fall off is 11-11.5. Toward the galactic poles this occurs at about 9m.

 

To reach the required magnitude limit and at the same time encompass the largest chunk of sky, the following attributes for a visual instrument are desirable, in order of importance/consideration:

 

- The eyepiece has the largest possible AFoV.

- The exit pupil matches the iris, so that magnification is minimal and hence TFoV is maximal.

- The objective aperture provides the light grasp to reach the required magnitude limit.

- The objective f/ratio delivers the iris-matching exit pupil for the eyepiece chosen.

 

Note that the objective f/ratio is the least important consideration, it being settled upon after other criteria are met.

 

Now, for an NV-based system, which has its sensor located at the principal focus, f/ratio is a first (or at least early) consideration from the standpoint of noise (scintillation) and FOV.

While I realize that you don't need a fast focal ratio to get a big exit pupil, Mel Bertals is using very fast systems because he can exploit the modern coma corrector and the modern 2" 100 degree field eyepieces to get the biggest possible field as well as the richest possible field.

 

Mel goes into great detail on this topic on his web page and I am a big fan of his work and his methods.

 

http://www.bbastrode...Telescopes.html

 

 

If I were ever to go back to traditional astronomy, I would do so with the same approach that Mel is using. 

Edited by Eddgie, 04 October 2016 - 02:20 PM.

[PEterW]

Impressed you got to 170 without getting lost or the doubt creeping in. Seeing a "sea of stars" down the eyepiece shows how many stars (and possible solar systems) there out there that naked eye and other observing can't (easily) show. Mel also has access to some very good skies to help him to see the really challenging stuff. The quantum efficiency of NV isn't actually very good, CCD and the eye are better, but the eye has limited spectral bandwidth and CCD don't deliver moving pictures (prefering longer exposures).

Buying fast mirrors is nearly as tricky as finding good NV systems... need to know where and have a good wallet.. unless you fancy grinding your own!

Cheers

Peter

[GlennLeDrew]

If one is restricting to the RFT mode at a maximal or near maximal exit pupil, the primary mirror might exhibit as much as a 1 wave error to no real detriment. (To be sure, a 1/2 wave error definitely will be just fine.) Even if the eye's own aberrations were to be minimal, it might not be possible to discern the spherical aberration.

 

And I'm reasonably certain a 1 wave error will not impair the image for an NV device.

 

In such an operating regime there is no requirement for the deft and finessing touch of a Zambuto or a Lockwood here.

[shams42]

Some mirror problems would be an issue for NV. I previously had a 16" mirror (made by a custom optician not listed in my sig) that had a very severe edge problem, which manifested as a bright and ugly halo of light around bright objects. This was easily observable around bright stars with standard eyepieces, and I have no doubts that it would have been basically unusable with night vision.

 

So I think we need a fast mirror with a good edge but not a super-accurate figure. 

 

The mirror in my 6" f/4 has a pretty gnarly star test but puts up a nice image with NV.

[View2]

Limiting magnitude you should be able to see is
3.7 x 2.5 logD^2. Log is base 10 and D is the diameter in millimeters of your objective lens.

Edited by View2, 04 October 2016 - 06:57 PM.

[Kstevens]

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. 

Cataracts and floaters are a visual or EAA astronomer's worst problem. Cataracts are insidious because they gradually take your vision acuity and color perception as they advance.  I almost lost interest in visual and EAA astronomy until cataract lens replacement surgery restored my vision which was the best investment I ever made. 

[Jeff Morgan]

My goal was more to be able to better promote NV by giving an aproximation on how much gain they could expect to see by using a NV device in an existing scope.

Could be tricky due to varying spectral characteristics of the target.

 

But if you are talking just stars ... continuous spectrum - it should be fairly easily derived from the system gain of the NV system, making an allowance for the filter used. For general viewing, something like a 650nm longpass.

[Jeff Morgan]

To obtain the largest number of stars in the FOV, whichever equipment is used, involves one critical stratagem. And that is that the limiting magnitude correspond to the inflection in the function where the rate of increase in star numbers with decreasing brightness decreases.

 

If we lived in a Universe in which the star number density was everywhere uniform, and there was no interstellar extinction, every instrument would qualify as 'richest field.' But we live inside a very polluted galaxy. Between the light-blocking haze concentrated toward the mid-plane and the rapid decrease in star numbers perpendicular to the disk, a limit is reached at some center magnitude where the numbers of stars per unit brightness interval ceases to increase at the same rate as for brighter intervals.

 

The magnitude at which this inflection occurs varies by galactic latitude. Nearer to the mid-plane, where the number density is largely constant to considerable distance, our horizon is extinction-limited. Toward the galactic poles extinction plays no role, and the actual fall-off in the star number density is the arbiter. If memory serves, for milky way fields the magnitude at which star counts fall off is 11-11.5. Toward the galactic poles this occurs at about 9m.

 

To reach the required magnitude limit and at the same time encompass the largest chunk of sky, the following attributes for a visual instrument are desirable, in order of importance/consideration:

 

- The eyepiece has the largest possible AFoV.

- The exit pupil matches the iris, so that magnification is minimal and hence TFoV is maximal.

- The objective aperture provides the light grasp to reach the required magnitude limit.

- The objective f/ratio delivers the iris-matching exit pupil for the eyepiece chosen.

 

Note that the objective f/ratio is the least important consideration, it being settled upon after other criteria are met.

 

Now, for an NV-based system, which has its sensor located at the principal focus, f/ratio is a first (or at least early) consideration from the standpoint of noise (scintillation) and FOV.

There was an analysis published some years ago on this in Sky & Tel. The author concluded that optimum instrument would reach the magnitude you mentioned (11.5) and no more was required.

 

IIRC, his solution was a 5" f/5 refractor. I'll have to look that article up for review.

[GlennLeDrew]

Limiting magnitude you should be able to see is
3.7 x 2.5 logD^2. Log is base 10 and D is the diameter in millimeters of your objective lens.

The problem with a 'one size fits all' limiting magnitude formula is that it doesn't take into account the actual sky brightness and the observer's own 'baseline', which includes also the iris diameter.

 

A far better approach:

- Start with one's own NELM for the particular conditions.

- Calculate the light grasp afforded by the area ratio of instrument aperture to iris.

- Convert the light grasp to magnitudes.

- That determine the 'basic' instrumental limiting magnitude for the maximum exit pupil, which equals the iris diameter. This is all one needs for an EFT.

 

For example: We have a 150mm aperture objective, a 6.5mm iris diameter, and a NELM of 5.8.

 

Light grasp = (150 / 6.5)^2 = 532

 

Magnitude equivalent = LOG(532) * 2.5 = 6.81

 

Instrumental magnitude limit for a 6.5mm exit pupil = 6.81 + 5.8 = 12.61.

 

If one wishes to account for transmission efficiency and/or loss to secondary obstruction, for a 90% efficient system subtract 0.1m, and for an 80% efficient system subtract 0.2m. (For 85%, subtract 0.15m, etc.)

 

The instrumental magnitude limit improves *roughly* in step with the sky darkening as the exit pupil shrinks/magnification increases. For instance, going from a 6.5mm exit pupil to 3mm darkens the sky by a factor of (6.5 / 3)^2 = 4.69. In magnitudes this is LOG(4.69) * 2.5 = 1.68. And so our 150mm aperture working at a 3mm exit pupil might have a limiting magnitude of about 12.61 + 1.68 = 14.29. In actuality, I doubt the gain is this good, for the dimmed sky causes the visual system noise to be relatively worse, which should impact the faintness limit reachable. But the main point is that the instrumental limiting magnitude improves--atmospheric seeing permitting--until the exit pupil gets to as small as about 1mm or perhaps a bit less.

[outofsight]

Once again, I'll use what GlennLeDrew said, makes it easier, "The problem with a 'one size fits all' limiting magnitude formula is that it doesn't take into account the actual sky brightness and the observer's own 'baseline', which includes also the iris diameter."

 

Bottom line, some kind of baseline would be required or the information your gathering is only good for your own viewing, at that time, and location. That information would still probably be better than nothing, but it might be considerably better if there was at least an attempt at a baseline and some coordination. If the margin of error is one magnitude, just thinking off the top of my head (why use the whole thing), that information would probably be important and significant. If the margin of error is two magnitudes, that might be OK, all depending. Margin of error three magnitudes (actually, probably anything more than two), then it would probably be a waste of time as far as the information being useful to anyone but the original observer.

 

But if anyone wants to develop some kind of basic protocol, I might be willing to help gather some info. When I first got an NV device I thought about trying to measure the magnitude difference, but that would only quantify one aspect of using a truly portable device to get such a massive increase in viewing quality in my light polluted space.

Edited by outofsight, 05 October 2016 - 02:10 PM.

[Rickster]

REVENGE OF THE PRONTO

 

Last night I compared a 70mm Televue Pronto equipped with a PVS-7 NVD as an eyepiece, to an Orion 250mm dob equipped with a 27mm Televue Panoptic eyepiece.  To make the focal lengths of the two scopes similar, I inserted a 2X Barlow between the the Pronto and the PVS-7.  This setup provided similar fields of view for the two telescopes.  It also handicapped the Pronto with a focal ratio of f6.8 x 2 = f13.6, compared to the native f4.7 of the dob.

 

Viewing was done from my front yard in a small rural town.  Skies were clear and the Milkyway was faint from this location.  "Thanks" to street lights and house lights, I did not have to use a flashlight during setup and teardown.

 

Observations of M31 and nearby stars/galaxies were compared between the two scopes.   M31 was almost directly overhead.

 

The Pronto was the obvious winner, showing much higher brightness on M31.  At first look it appeared that the star fields were identical.  However, without much trouble, I was able to locate a nearby star that was barely (but clearly) visible in the Pronto that could not be seen at all in the dob.

 

Given that the clear aperture of the dob is roughly 3 times larger (considering the secondary mirror obstruction) than the Pronto, I think it is safe to say that my PVS-7 provides well over a factor of 3x in "aperture gain."

 

Now, taking into account that the tube in my PVS-7 is a C version, and is degraded, I would expect that those of you with newer high grade tubes might see "aperture gains" in the 10x ballpark.

 

Finally, this observation was performed from a "green zone."  When I get a chance I plan to run this drill again from a dark site.