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Non H-α: Comparision NVD and glass only

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#1 Uwe Pilz

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Posted 18 September 2023 - 11:40 AM

Dear observers,

 

I read the very valuable thread of Mauro da Lio, where he compares night vision with glass only. I was slightly disappointed, that Mauro found little improvement when observing non-Hα objects.

 

Last weekend we had our telescope meeting HTT. I had enough time there for experimenting with my NVD. As always, I made sketches. I started with M31 and surroundings, because I have three different galaxies at once. The first sketch is about the smaller M110 an M31. I used my 12 inch Dobsonian for all the observations.

 

I found some improvement when using my NVD.

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#2 Uwe Pilz

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Posted 18 September 2023 - 12:37 PM

Next I tried observing detail in the dark lane of M31. Ususal, I see this clearly with glass only. But M31 was not very high in the sky and so I saw only a sharp edge, which is sometimes all we see at dark lanes. With the NVD I saw all much more clearly.

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#3 sixela

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Posted 18 September 2023 - 02:59 PM

SQM reading of the sky background, just to give us some context?

#4 WheezyGod

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Posted 18 September 2023 - 03:37 PM

I agree the SQM is important since Mauro’s findings were that NV didn’t provide a benefit to non-Ha targets in dark skies vs. glass only.

Aside from SQM I think there’s the importance of focal ratio for this comparison that continues to get neglected.
Mauro was almost always going for a decent amount of magnification on a lot of targets; more than most of us tend to use with our NVDs. As we know, that means a much higher f ratio which means less of an incremental performance from NV vs. glass only.

The reason that’s important here is because Mauro likely used a lot more magnification if he looked at these targets than you did when you created your sketches with M31 probably being an exception.

I do wonder for non-Ha objects that are hard to detect in dark skies if NV wins at low magnification. For objects that aren’t seen with glass only, perhaps NV can detect certain ones at a low magnification. I felt like this was missing from the assessment, that the amount of magnification used across the different objects made the assessment more biased towards glass for an average person thinking of getting into NV and using it at a dark site.

#5 sixela

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Posted 18 September 2023 - 04:10 PM

Mauro was almost always going for a decent amount of magnification on a lot of targets; more than most of us tend to use with our NVDs.

For non-narrowband objects it makes a lot of sense. I never go under 2x less magnification than I tend to use with glass. With glass I tend to use an 8mm Delos for galaxies, so on these I need a 16mm equivalent eyepiece. That's a TMB-barlowed OVNI-M. For larger galaxies overviews (for which 16-26mm eyepieces are used when using glass just to get enough field of view) I tend to use either the plain Paracorred OVNI-M or even swap the Paracorr for a Nexus reducer.

If you go well below that magnification then the fact you can see sharper with mesopic vision does not compensate the smaller image scale.

That's the beauty of scotopic vision: it's extremely fuzzy, but very, very, very sensitive, allowing you quite some image scale to compensate for its fuzzy nature, and for apparent surface brightnesses brighter than roughly 26 mag/arcsec² is fairly noise free (an NVD could only dream of the SNR).

Of course you do need to be perfectly dark adapted and need to hone your averted vision skills (and patience!)

An NVD in a dark sky is just another tool in the box (and needs another eye). Some things that are really hard with glass (like the really thin dust lane in NGC 7814) are easy-peasy in the NVD, most Arp features in Arp galaxies are also easier to pick out, some galaxies that are just blobs with glass turn into obvious spirals, and the cores of elliptical galaxies in rich galaxy clusters are just too many to count (seriously...you can start to barlow to get more magnification and some fuzzy stars at 'regular power' turn into galaxies, revealing that you were seeing too many to sketch or even count). And it's not always the bright stuff that's better with the NVD, I'd count the tighter but wider spiral arm of NGC7479 as faint but it was more apparent in the NVD and obviously split in two (with Astronomik L1 filter and OVNI-M in prime in a Paracorr).

But other things are easier with glass...we had stunning views of M33 this weekend where with glass you could see two faint but wide arms under the bright arm going to NGC 604, and as a result it was impossible to fit M33 in the FOV of a 26T5 in a 400mm Dobson. The NVD was most useful to pick out HII regions and smaller star clusters, but some of the larger star clusters in M33 were also easier with glass. And do what you want, the faint tidal tails thrown out of both components of M51 were just harder to see with the NVD; I can echo Mauro's experiences with that and not even the Astronomik L1 managed to change that (although it narrowed down the difference).
 

perhaps NV can detect certain ones at a low magnification

It's true that you can e.g. filter more aggressively with an NVD to get more contrast and compensate by using less magnification with NVD, but glass usually wins hands down for really large but faint contrast features in a dark sky. You'd be hard pressed to see Barnard's Galaxy with an NVD, even though it sticks out like a sore thumb in a 20x80 in a dark sky; in the NVD it's undoubtedly there at high gain but then it's buried in the noise, and if you reduce the gain it becomes too faint. On the other hand, use the NVD with some more image scale and the HII-regions in it are easy and even detailed in a large scope (as is M33's NGC 604), although it's fairly eerie to see extragalactic HII-regions float in apparent nothingness...

The most bizarre experience remains NGC 5053. It's a lot harder with glass but there is a definite glow behind the stars that you eke out of the background. With the NVD it looks like an open cluster, with all the bright stars immediately apparent but seemingly nothing else beyond them except a really faint but small glow in the middle that's really hard to see through the noise.

Edited by sixela, 18 September 2023 - 04:34 PM.


#6 WheezyGod

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Posted 18 September 2023 - 04:51 PM

For non-narrowband objects it makes a lot of sense. I never go under 2x less magnification than I tend to use with glass. With glass I tend to use an 8mm Delos for galaxies, so on these I need a 16mm equivalent eyepiece. That's a TMB-barlowed OVNI-M. For larger galaxies overviews (for which 16-26mm eyepieces are used when using glass just to get enough field of view) I tend to use either the plain Paracorred OVNI-M or even swap the Paracorr for a Nexus reducer.

If you go well below that magnification then the fact you can see sharper with mesopic vision does not compensate the smaller image scale.

That's the beauty of scotopic vision: it's extremely fuzzy, but very, very, very sensitive, allowing you quite some image scale to compensate for its fuzzy nature, and for apparent surface brightnesses brighter than roughly 26 mag/arcsec² is fairly noise free (an NVD could only dream of the SNR).

Of course you do need to be perfectly dark adapted and need to hone your averted vision skills (and patience!)

An NVD in a dark sky is just another tool in the box (and needs another eye). Some things that are really hard with glass (like the really thin dust lane in NGC 7814) are easy-peasy in the NVD, most Arp features in Arp galaxies are also easier to pick out, some galaxies that are just blobs with glass turn into obvious spirals, and the cores of elliptical galaxies in rich galaxy clusters are just too many to count (seriously...you can start to barlow to get more magnification and some fuzzy stars at 'regular power' turn into galaxies, revealing that you were seeing too many to sketch or even count). And it's not always the bright stuff that's better with the NVD, I'd count the tighter but wider spiral arm of NGC7479 as faint but it was more apparent in the NVD and obviously split in two (with Astronomik L1 filter and OVNI-M in prime in a Paracorr).

But other things are easier with glass...we had stunning views of M33 this weekend where with glass you could see two faint but wide arms under the bright arm going to NGC 604, and as a result it was impossible to fit M33 in the FOV of a 26T5 in a 400mm Dobson. The NVD was most useful to pick out HII regions and smaller star clusters, but some of the larger star clusters in M33 were also easier with glass. And do what you want, the faint tidal tails thrown out of both components of M51 were just harder to see with the NVD; I can echo Mauro's experiences with that and not even the Astronomik L1 managed to change that (although it narrowed down the difference).

It's true that you can e.g. filter more aggressively with an NVD to get more contrast and compensate by using less magnification with NVD, but glass usually wins hands down for really large but faint contrast features in a dark sky. You'd be hard pressed to see Barnard's Galaxy with an NVD, even though it sticks out like a sore thumb in a 20x80 in a dark sky; in the NVD it's undoubtedly there at high gain but then it's buried in the noise, and if you reduce the gain it becomes too faint. On the other hand, use the NVD with some more image scale and the HII-regions in it are easy and even detailed in a large scope (as is M33's NGC 604), although it's fairly eerie to see extragalactic HII-regions float in apparent nothingness...

The most bizarre experience remains NGC 5053. It's a lot harder with glass but there is a definite glow behind the stars that you eke out of the background. With the NVD it looks like an open cluster, with all the bright stars immediately apparent but seemingly nothing else beyond them except a really faint but small glow in the middle that's really hard to see through the noise.


Yup well said! I think it’s interesting that after the many years of NV discussions, there’s now a detailed discussion on the impact of NV at a dark site.

At the same time, it’s also not the most meaningful argument against NV since most of us have NV because we don’t live and can’t frequently get to dark sites. Plus it’s so much easier not having to worry about dark adaptation or using averted vision.

Plus with this dark site discussion we’ve always been focused on NV use with a telescope and haven’t compared it’s use handheld vs. naked eye/binoculars which I would expect NV to win at more often given it’s low f ratio (unless someone is using a cheap lens).
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#7 Uwe Pilz

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Posted 18 September 2023 - 11:01 PM

> SQM reading of the sky background, just to give us some context?

 

We have a decent sky at the meeting. I got 21,3 ... 21.4 in zenith.



#8 jesse 3

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Posted 18 September 2023 - 11:20 PM

Last Friday, I observed under 21.4 sky with 12” dob and PVS14. Your drawings match my experience. I also gained a little more contrast with 642nm pass filter.

#9 Uwe Pilz

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Posted 19 September 2023 - 12:08 AM

Next object: Dumbbell nebula. 

 

The experience with glass only and with NVD was very different. With glass and , the outer lobes are very distinct. But all of the image is very fuzzy.

 

With NVD and [OIII] the outline is very sharp, but the lobes are slightly vague. Of course, there are many stars in the nebula, among them the central star. I reduced the focal length with a reducer lens to f/=3.1. This is my usual setup for Hα work.

 

I like the NVD view very much. I had a similar experience with M42, but i found not the time for a sketch: The dawn was faster.

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#10 Mauro Da Lio

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Posted 19 September 2023 - 11:03 AM

@Uwe.

 

Concerning M110 and M32 I did not report them because, usually I do not consider elliptical galaxies worth of observations (no details). However, the cores (as well as the cores of the others) are very bright in the infrared. 

 

Concerning M31, it is quite strange you could not see the dark bands. I used Ethos 21, 59.5x with 1.7° true filed of view. You need a large field to appreciate them, but they stands out very well in dark sky. With the NVD they were visible in a similar way (but with no wow effect) with the IR cut filter and invisible without. Overall I would say no gain with NV. 

 

In general, the observing technique for glass and scotopic vision requires using as much magnification as possible. In scotopic vision rods are "binned", hence one has very high sensitivity (26 mpsas, if not more) but low resolution. As long as one can see the field stop, one is above the threshold of the rod system. Typical exit pupils of 2 mm are often the best choice.

 

I did not report the views of NGC 6804. With NV and no filter you see only (many) stars, including the central one. With the IR cut, the nebula is visible as a faint glow at 48x, very similar to the view at 42x and 59x with glass. However you can increase the magnification and, at 186x I could glimpse the two brighter arcs on the edge of the disk.

 

Concerning M27. Your sketches are fine with one caveat. With NV and IR cut the hourglass strands out but the reflection lobes fade. With glass I prefer no filter. If the sky is dark enough I see the same nebula but with more stars. As a matter of facts, I use OIII and H-beta only to reveal nebulosity that is not visible without. Otherwise, if the nebulosity is already visible without filters, it may improve slightly with Baader CLS filter, while maintaining the stars.

NV without filters shows more stars, but if I put the IR cut on I see the same stars that I see with glass. That was also the case of NGC 6804.

 

Concerning the spiral arms of NGC 7479, or M51, I would say that the improvement with NV and IR cut is minor (if not zero). I used an Optlong IR cut filter. Alexis said two theoretically identical IR cut filter are in practice different. May I did not have the best performing IR cut filter.

 

However, we are debating about incremental improvements. Greater improvements can be obtained with increased aperture. I would say a 40 cm with glass outperforms any object in 25 cm with NV and IR cut.


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#11 chemisted

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Posted 19 September 2023 - 11:53 AM

 I would say a 40 cm with glass outperforms any object in 25 cm with NV and IR cut.

I have posted more than once that I have resolved NGC 6749 into many dozens of stars with 25 cm and NV.  What do you see with 40 cm (or larger) and glass?


Edited by chemisted, 19 September 2023 - 12:02 PM.


#12 Joko

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Posted 19 September 2023 - 12:33 PM

With the NVD they were visible in a similar way (but with no wow effect) with the IR cut filter and invisible without. Overall I would say no gain with NV.

....

Concerning M27. Your sketches are fine with one caveat. With NV and IR cut the hourglass strands out but the reflection lobes fade. With glass I prefer no filter. If the sky is dark enough I see the same nebula but with more stars. As a matter of facts, I use OIII and H-beta only to reveal nebulosity that is not visible without. Otherwise, if the nebulosity is already visible without filters, it may improve slightly with Baader CLS filter, while maintaining the stars.
NV without filters shows more stars, but if I put the IR cut on I see the same stars that I see with glass. That was also the case of NGC 6804.

....

However, we are debating about incremental improvements. Greater improvements can be obtained with increased aperture. I would say a 40 cm with glass outperforms any object in 25 cm with NV and IR cut.

Few tips :

About M31 the dark bands are better seen with NV without filters. Don't use any filter.

About M27 you should use an Halpha filter (or dual band filters). Don't use other filters.

There is 3 to 4 magnitudes gain on stars with NV.
Any globular or open clusters will be much better visible than with glass eyepiece if you don't use an IR cut filter and most targets will be better in the 25cm + NV than in 40" with glass. Don't use IR cut filter.

Edited by Joko, 19 September 2023 - 01:23 PM.

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#13 Uwe Pilz

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Posted 19 September 2023 - 01:44 PM

Last sketch, M33. I was never very lucky with the Triangulum galaxy. And so it was at our meeting. With glass only, I saw less detail. I used in this case only one magnification. I saw Hα regions and star clouds with higher magnification.

 

With the NVD I used an IR block filter at 40x. The view at the eyepiece was very disappointing: I saw only the inner part of it. But when I transferred my eyepiece sketch in my observation log book I found that I saw a lot of star clouds and Hα regions. All in all, I saw a lot with the NVD

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#14 sixela

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Posted 19 September 2023 - 01:54 PM

That’s bizarre, and only the central portion of M33 is on the sketches (you need more than double the field stop drawn to see most of it including the fainter arms.) On my f/3.73 plus Paracorr I usually use a 17mm to 31mm eyepiece for that, and down to 8mm for details.

The start of the two brightest arms, one to NGC604 and one opposite, should be obvious with glass at a site with an SQM reading of 21.3.

I find fluffy spirals quite tough for an NVD (but that does not hold for small details within).

Edited by sixela, 19 September 2023 - 02:00 PM.


#15 sixela

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Posted 19 September 2023 - 02:05 PM

About M31 the dark bands are better seen with NV without filters. Don't use any filter.

Not my experience last weekend In Grandpré (in a 150mm f/3.45 and TV67 afocal setup, to get enough field). The bands were better defined with an Astronomik L1 UV/IR cut. Granted, there was very little man made light pollution high up in the sky over there.

As for M27, I like a dual band better than a H-alpha at a site this good (you can see concentric shells in the faint lobes that are harder in H-alpha alone). It’s different from my Bortle 5 site, where a dual band picks up more light pollution.

But if you like to see more stars, who am I to tell anyone not to enjoy the view with an IDAS LPS-D3? I certainly did. If you have a filter slide it’s easy to try all filters, although obviously I agree you should not use *only* that filter.

Edited by sixela, 19 September 2023 - 02:08 PM.

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#16 Mauro Da Lio

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Posted 19 September 2023 - 03:59 PM

I think there is some misunderstanding about sky conditions (and perhaps user background). I do not speak for polluted sky.

 

Under a dark sky, the sky brightness is dominated by airglow. The sky shines at about 20 in the IR, whereas it is about 21.5 in the visible band. In this condition one has two options:

 

a) using the NV without filters. In that case the NVD will show a contrast similar to what can be seen with glass under a 20 sky. One can definitely see more stars, but the faint nebulosity of unresolved stars (be them in globs or spiral arms) is not visible. For example NGC 6217 in 25+NV shows the same stars seen with 60 cm, but in the latter the stars float over a glow. Also, in the latter the stars have colors, flicker and the fainter ones pop in and out of view.

 

b) using an IR block filter. In that case the NVD is affected by the skyglow in the visible band only (21.5) and shows the same contrast seen with gas. However, blocking IR reduces the limiting magnitudes. In fact, with the IR block, the stars are the same (even fewer) than with glass, but the glow of unresolved stars (globs and arms) is seen.

 

In light polluted sky things may be different. I understand people may use NV for broadband observations in polluted sky because they have non access to dark sky. In that case my considerations do not hold. However, I like being under a sky full of stars.

 

As for what concerns the magnitude gain, one can use the charts around M57 to evaluate it. For example here: https://www.cloudyni...mag-around-m57/ . But there are better charts.

My (quick) evaluation was 2 magnitudes. In fact the 25 cm with NV (and without filters) showed the same stars shown by 60 cm. However the 25 without filters is working like in a SQM 20 sky whereas the 60 had the 21.5 sky, resulting in a much better contrast.

 

Finally, a consideration about the observer. Observing with glass is a completely different technique than with NV. With glass we use the rod system and high magnification. Seeing things in this way requires training. With NV we use the cones, look at much smaller images in higher resolution. It may be that people expert in using the rod system see more in that way and people with less experience under dark sky see more with NV.

I definitely see the same things (more or less) in the two ways, and see enormously more things with 60 cm. Next new moon I will be in a B2, 21.9 sky. I bet I will see more there with 25 cm than with NV in my usual 21.5 sky.

 

What an expert observer can see with glass is well shown by Bertrand Laville's drawings.

Here is M27 with 25 and 63.5 cm: http://www.deepsky-d...t254/dsdlang/fr


Edited by Mauro Da Lio, 19 September 2023 - 04:23 PM.

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#17 sixela

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Posted 19 September 2023 - 04:51 PM

Mhh -- what I saw last weekend (with NVD and dual-band filter) on M27 was better than the best T635 sketch, and I have only a T508 ;-). Pretty much the same extent (I did mention the concentric shell structures in the outer lobes in an earlier post), only with far more detail in the brighter parts (which included narrow brightenings in parts sketched as uniform). Incidentally, you do really need an NVD with fairly decent quantum efficiency in OIII (mind you, that will still not give you the same radiant sensitivity as in H-alpha) and an objective that does not kill blue if you use an afocal configuration. The dual band filter I use is pretty much worthless when I use my Carson/Fujinon PVS14 objective, in the sense that it then just behaves like a 5nm H-alpha filter.

Mind you, I find that with 10-20 minutes of patience I see finer details with glass too than on the sketches. It just takes a lot more time, something that someone used to NVD observing may fail to appreciate (observing with an NVD also reveals more detail with careful observing, but there are more immediate results with NVD).

I think a 2 magnitudes gain on stellar objects is a fairly low estimate. For me 3 is more typical (obviously using averted vision techniques, it's easy to get "lazy" with an NVD and not use them. Interestingly, I require different averted vision on the NVD for optimal results, with the gaze directed away less far than with glass).

But it requires enough magnification, and the optimum may not lie at the same magnification as with glass (it's tricky, because magnifiying more makes the background darker but also more noisy). If I go for maximum stellar magnitude I'm usually well into "barlowed prime" territory, with temperature and seeing also playing a role in what I end up using.

But a lot depends on the conditions, and not even two dark sites are exactly alike (not to mention transparency differs from night to night). So I think your assessment is entirely accurate, and I'm certainly in general agreement with most things noted (including your observations about the contrast and usefulness of filters).

Also no argument about techniques being different with glass and NVD. That's why I am now glad I have a monocular NVD, since it allows me to keep an eye dark adapted (although I sometimes switch to my dark adapted eye for aggressively filtered H-alpha views, but not without having adjusted the gain down to make the sky background as dark as the naked eye one; I find only a 5nm or more narrow filter allows me to do that well).

Edited by sixela, 19 September 2023 - 05:18 PM.


#18 chemisted

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Posted 19 September 2023 - 05:34 PM

Mauro,

You didn't answer my question. What do you see when you observe NGC 6749 with glass?

#19 sixela

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Posted 19 September 2023 - 05:47 PM

It's pointless to argue about that: Mauro knows darn well that some objects are better with NVD, and if he didn't observe the globular you mentioned in the observing sessions he's just discussed, he's not going to comment. Plus I know darn well that you can't resolve even the edge stars with glass eyepieces unless you have roughly 24" of aperture (at least that's what I needed). Even if you take only a 2 mag gain on stellar magnitudes (the ballpark figure Mauro himself put forward), you should be able to do the same with a 10" if you add an NVD.

Might as well ask about the fainter Palomars or Tarzans, while we're at it.

His point is that there are classes of objects where glass eyepieces are competitive, even without changing the scope or conditions. But apparently that assertion is enough to provoke almost hostile questioning about completely different objects which he did not bring up.

And while we're on the subject of globulars, I can sure resolve more stars in NGC 5053 with the NVD, but do I like that view over the glass eyepiece one (in a good sky, and with lots of patience)? Not really, because it's missing the faint underglow almost completely.

Edited by sixela, 19 September 2023 - 05:59 PM.


#20 WheezyGod

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Posted 19 September 2023 - 06:18 PM

I think there is some misunderstanding about sky conditions (and perhaps user background). I do not speak for polluted sky.

Under a dark sky, the sky brightness is dominated by airglow. The sky shines at about 20 in the IR, whereas it is about 21.5 in the visible band. In this condition one has two options:

a) using the NV without filters. In that case the NVD will show a contrast similar to what can be seen with glass under a 20 sky. One can definitely see more stars, but the faint nebulosity of unresolved stars (be them in globs or spiral arms) is not visible. For example NGC 6217 in 25+NV shows the same stars seen with 60 cm, but in the latter the stars float over a glow. Also, in the latter the stars have colors, flicker and the fainter ones pop in and out of view.

b) using an IR block filter. In that case the NVD is affected by the skyglow in the visible band only (21.5) and shows the same contrast seen with gas. However, blocking IR reduces the limiting magnitudes. In fact, with the IR block, the stars are the same (even fewer) than with glass, but the glow of unresolved stars (globs and arms) is seen.

In light polluted sky things may be different. I understand people may use NV for broadband observations in polluted sky because they have non access to dark sky. In that case my considerations do not hold. However, I like being under a sky full of stars.

As for what concerns the magnitude gain, one can use the charts around M57 to evaluate it. For example here: https://www.cloudyni...mag-around-m57/ . But there are better charts.
My (quick) evaluation was 2 magnitudes. In fact the 25 cm with NV (and without filters) showed the same stars shown by 60 cm. However the 25 without filters is working like in a SQM 20 sky whereas the 60 had the 21.5 sky, resulting in a much better contrast.

Finally, a consideration about the observer. Observing with glass is a completely different technique than with NV. With glass we use the rod system and high magnification. Seeing things in this way requires training. With NV we use the cones, look at much smaller images in higher resolution. It may be that people expert in using the rod system see more in that way and people with less experience under dark sky see more with NV.
I definitely see the same things (more or less) in the two ways, and see enormously more things with 60 cm. Next new moon I will be in a B2, 21.9 sky. I bet I will see more there with 25 cm than with NV in my usual 21.5 sky.

What an expert observer can see with glass is well shown by Bertrand Laville's drawings.
Here is M27 with 25 and 63.5 cm: http://www.deepsky-d...t254/dsdlang/fr


Got it so the 20SQM “limit” or “ceiling” applies just in the IR. There’s a lot of objects where this limit wouldn’t apply though that are at the upper end of the visual spectrum. What are some objects that you felt were most impacted by this? It would be objects that NV clearly wins on in suburban skies but loses on at a dark site.

I would definitely be in that less experienced bucket and that would certainly explain why my club’s 21.0ish SQM site showed a lot more detail on all objects than my backyard site which is probably a little worse than 20.0.

What’s interesting as well is the finding that at a dark site an IR pass filter should provide a benefit on most non-Ha objects because it blocks the airglow. In contrast, a lot of other observers with NV found using an IR filter has no benefit on most stellar objects in darker skies. Unless, most stellar objects don’t emit enough light in the IR end of the spectrum for this to have an impact on most objects.

#21 chemisted

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Posted 19 September 2023 - 07:58 PM

I know darn well that you can't resolve even the edge stars with glass eyepieces unless you have roughly 24" of aperture (at least that's what I needed). 

This is the point I was bringing up.  I resolve NGC 6749 to the core, not just the edge stars, with my RC-10 and NVD Micro.  Medium aperture (40-60 cm) and glass cannot compete when it comes to these globular clusters that are heavily obscured by interstellar dust.



#22 sixela

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Posted 20 September 2023 - 02:12 AM

at a dark site an IR pass filter should provide a benefit on most non-Ha objects because it blocks the airglow.

An IR cut filter (shortpass) does. An IR pass is the opposite and is a longpass filter, and is useful in light pollution where the IR airglow is less bad than the manmade light pollution in visible light.

Mauro was only talking about extended objects with a broad spectrum there (think “galaxies” or the unresolved glow of globulars).
Stellar objects (including stars resolved in globs) are better without filter, since they remain high contrast features even with a slightly brighter background.

Edited by sixela, 20 September 2023 - 02:24 AM.


#23 sixela

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Posted 20 September 2023 - 02:26 AM

This is the point I was bringing up. I resolve NGC 6749 to the core, not just the edge stars, with my RC-10 and NVD Micro. Medium aperture (40-60 cm) and glass cannot compete when it comes to these globular clusters that are heavily obscured by interstellar dust.


No argument from anyone, but Mauro clearly announced the targets of interest to him in this thread.

Already on page 1 did we hint at the fact they’re tougher than others if you want the NVD to compensate for aperture.

#24 Uwe Pilz

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Posted 20 September 2023 - 04:06 AM

> The start of the two brightest arms, one to NGC604 and one opposite, should be obvious with glass at a site with an SQM reading of 21.3.

 

Dear sixela and all others,

 

as you know, we have less space for using different magnification with NVD, because we need an optimal illumination of the photo cathode. This is the reason why I used only one magnification with glass only. I thought that would be a fair comparison.

 

Most probably, I would have seen the dark band in M31 with lower magnification. And most probably I would have seen remarkably more on M33 if I had played with magnification.

 

Instead, I used only one magnification and sketched what I saw. Then I switched to the NVD, looked there for an optimal f/ ration and sketched what I saw. In both cases, no composition of different views at different magnifications.



#25 sixela

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Posted 20 September 2023 - 08:32 AM

Quite, but as I indicated in another thread, the optimal magnifications can differ quite a bit between NVD and glass. Glass usually wants more magnification, but you can still see the same field by using 68°, 82° or 100° eyepieces. What magnification and FoV did you have for your eyepiece sketch (we do have the information for the NVD)?

So I'm not questioning the validity of the comparison; it's just that you often need to use many different magnifications both with glass and the NVD to appreciate all that each can eke out of the background, often at very different magnifications.

Also, if you switch between glass and (unfiltered) NVD, then glass will always lose unless you either use another eye (that you keep dark adapted for glass) or wait for 15 minutes after you've used the NVD. There's a lot to pick out of an NVD screen with mesopic vision, but with glass it's going to give you only slim pickings. Even with the naked eye my "NVD eye" has a drastically lower limiting magnitude than my "glass only" eye when I dedicate one eye to each.

Edited by sixela, 20 September 2023 - 08:34 AM.



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