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Small aperture + NV vs. Large Aperture

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#26 revans

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Posted 15 May 2024 - 10:10 AM

Kind of weird...I can see the Horse Head Nebula as a distinct notch at 3x (with an f/1.6 objective) using an NVD in my Bortle 8 sky and a 6nm H-alpha filter. It's one of the easier objects for an NVD. Of course aperture really helps to see it as more than a notch, and it looks a lot better in my 20" than in my 6" (and better stil at darker sites).

Not so weird, because what I'm wanting to see with NV is the horsehead shape I see using AP, not just a notch or a sort of shadowy area without form.  That I can see anytime with almost anything I put NV on.

 

Rick



#27 sixela

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Posted 15 May 2024 - 10:15 AM

Ah yes, to see a horsehead shape you need angular size, and to get angular size at a good f/ratio you need aperture.
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#28 bobhen

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Posted 15 May 2024 - 10:53 AM

Not so weird, because what I'm wanting to see with NV is the horsehead shape I see using AP, not just a notch or a sort of shadowy area without form.  That I can see anytime with almost anything I put NV on.

 

Rick

With NV, my C8 shows the Horse Head shape directly without averted imagination. Part of that is some aperture. But the other part is more image scale with the longer FL C8. Of course, it's not like even a 60- short camera exposure, but the "head" shape is rather easily discernible.

 

Bob 


Edited by bobhen, 15 May 2024 - 02:11 PM.

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#29 Mauro Da Lio

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Posted 15 May 2024 - 02:19 PM

Given the highly nonuniform spectral shapes of the NV tubes and the light pollution and sky glow I’m not sure such a simple calculation is entirely valid. Best to understand what wavelengths the majority of the photons you don’t want are and then filter accordingly so they don’t get amplified. 

Yes, that was an extremely simplified explanation. If you want information about the skyglow at different wavelengths here is a topic https://www.cloudyni...under-dark-sky/

 

If you want lots lots lots of descriptions of how objects compare under dark and semi-dark and polluted sky with large and small apertures and different tubes here they are: https://www.cloudyni...bserving-needs/

 

Finally, the spectral response of NVD varies between tubes. As you can see in the first link the longer the wavelength the more intrusive the airglow is.

 

So, if you want to observe from light polluted sky you should stay above the artificial light spectrum (for example above ~650 nm https://lightbulbexp...o-leds-produce/ and below the strongest airglow Meinel bands, e.g. https://www.cloudynights.com/topic/881355-considering-an-ovni-device-but-does-it-fit-my-observing-needs/?p=12966281

 

You must be aware, though, that even with the aid of an IR-pass filter the sky glow will be 1-2 magnitudes stronger than under a dark sky with/without an IR-cut filter. There are things visible in B3 that are not visible in B6 with NV. Most things under B6+NV are pale ghosts of their aspect under B2-3. Not to speak of the aesthetic aspect: for example, M81 from home SQM 18.8 with NV showed its halo with a lot and lot of "averted imagination". Under Dark sky and glass I can see shape and the two spiral arms. I could continue forever... that is why I still travel to dark sky even if I have a 60 cm Dobson at home and NV.


Edited by Mauro Da Lio, 15 May 2024 - 02:23 PM.


#30 PEterW

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Posted 16 May 2024 - 01:49 AM

Thanks, I’d missed that post.

Peter

#31 25585

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Posted 17 May 2024 - 11:07 AM

Would a larger aperture make a more expensive, higher-spec NVD less necessary. Say compare 5" F6 refractor with 10" F5 Newtonian.



#32 sixela

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Posted 17 May 2024 - 03:09 PM

That's just as with eyepieces. A larger telescope doesn't make you want lesser eyepieces.

An NVD with a better MTF (sharper views) and a better SNR ratio will require less magnification and a slower f/ratio (since you'll require less gain reduction to get the noise level down) to see some finer details, so you might see detail on a smaller scope that might require a larger telescope with a lesser NVD. So yes, I suppose in a way you can compensate for the lack of quality in the NVD with more aperture (but at the expense of the FOV in which you'll see a given detail). But frankly, you really want a good NVD in the larger scope to see even more.

Edited by sixela, 17 May 2024 - 05:53 PM.


#33 Mazerski

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Posted 17 May 2024 - 04:59 PM

Would a larger aperture make a more expensive, higher-spec NVD less necessary. Say compare 5" F6 refractor with 10" F5 Newtonian.

No. go bold and get the best tube you can as it will perform well in various modes with various scopes. 



#34 Joko

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Posted 19 May 2024 - 03:32 AM

Would a larger aperture make a more expensive, higher-spec NVD less necessary. Say compare 5" F6 refractor with 10" F5 Newtonian.

One example : witout NVD you won't see the horshead nebula in the 5" F6 refractor nor with 10" F5 Newtonian.

With NVD you'll see it in both telescopes. Better is the NVD better is what you see.


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#35 revans

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Posted 19 May 2024 - 05:15 AM

One example : witout NVD you won't see the horshead nebula in the 5" F6 refractor nor with 10" F5 Newtonian.

With NVD you'll see it in both telescopes. Better is the NVD better is what you see.

I think it depends on the Bortle level sky that you are in.  I can't see the definitive shape of the Horsehead nebula in Bortle 6 using a 5 inch binocular telescope with NV eyepieces. But it will show up in a 30 sec subexposure without NV. 

 

Rick



#36 a__l

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Posted 19 May 2024 - 05:25 AM

Would a larger aperture make a more expensive, higher-spec NVD less necessary. Say compare 5" F6 refractor with 10" F5 Newtonian.

There is slyness in the answers above.
A larger aperture collects more photons. But there will also be more of them from the background.
The horse's head has nothing to do with this. You transfer the hydrogen spectrum to an area that is clearly visible to you and increase the contrast (actually cut off the background) using a narrow-band filter.


Edited by a__l, 19 May 2024 - 05:34 AM.


#37 Speedy1985

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Posted 19 May 2024 - 07:06 AM

I think it depends on the Bortle level sky that you are in.  I can't see the definitive shape of the Horsehead nebula in Bortle 6 using a 5 inch binocular telescope with NV eyepieces. But it will show up in a 30 sec subexposure without NV. 

 

Rick

I’ve seen the HH in my 4” refractor using NV from my Bortle 8 yard, so I’m perplexed as to why a larger binoscope in slightly better skies wouldn’t. Maybe I hit the sky conditions lucky the night I did, or maybe yours were not good?


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#38 PEterW

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Posted 19 May 2024 - 10:48 AM

I find seeing conditions can vary a lot, so I have some “checking” objects to see if it’s worth looking for the fainter stuff. North America, rosette, Pac-Man, heart and soul… if they’re not looking good then do so thing else that evening, if they’re really obvious then go after the fainter stuff.

Peter
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#39 Bearcub

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Posted 19 May 2024 - 11:09 AM

What about the main mirror coatings.. shouldnt they be different for night vision devices?



#40 chemisted

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Posted 19 May 2024 - 03:26 PM

I’ve seen the HH in my 4” refractor using NV from my Bortle 8 yard, so I’m perplexed as to why a larger binoscope in slightly better skies wouldn’t. Maybe I hit the sky conditions lucky the night I did, or maybe yours were not good?

Don,

 

I quized him when he first reported that and it turned out he was not using proper narrow band filters. He had a plain red filter one side and an undefined 'hydrogen' on the other.

 

Ed 


Edited by chemisted, 19 May 2024 - 03:26 PM.

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#41 Speedy1985

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Posted 19 May 2024 - 03:47 PM

Don,

 

I quized him when he first reported that and it turned out he was not using proper narrow band filters. He had a plain red filter one side and an undefined 'hydrogen' on the other.

 

Ed 

Ah OK Ed, that explains it!



#42 revans

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Posted 19 May 2024 - 04:02 PM

Ah OK Ed, that explains it!

My H alpha filter is pretty old.  It is a Lumicon 12nm bandwidth and was one of the first models that became available years ago.  But that is all I have in a 1.25" size. My 3nm Halpha filters are 2 inch size and I can't use them with my binocular telescopes.  Just for enlightenment though, I will put one on my Esprit 120 and see if it makes a difference.  But the horsehead nebula isn't going to visible for many months frown.gif

 

But, I thought H beta was recommended for visual on the horsehead, or it used to be the recommended filter years ago.

 

Rick


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#43 Mauro Da Lio

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Posted 19 May 2024 - 04:15 PM

I have seen the HorseHead from home ay SQM 18.8 with NV and a UHC-S filter (dual 50 nm band) with a 10" dobsonian. I cannot see it without NV even if I use a 24" dobsonian.

The horse head is visible in the 24" under SQM 21.5 with a H-beta filter and is comparable.

 

However however...however....  All objects are better (definitely better) in the 24" with NV than in in the 10" with NV. The difference is large and I cannot find any way in which a superb NV with the 10" can outperform a good NV with 24" (whichever location and target).

 

For example, the ink spot (H-alpha is not the only thing), i.e., NGC 6520 and Barnard 86 show a lot more details in the 24". You get the same level of clarity, but in one case at 42X and in the other at 115x (so one see three times finer structures).

 

There obviously is a sweet spot between high NV specs and greater diameter. One can spend 10k in NV and 2k in the telescope, or 4k in NV and 8k in the telescope or some other combination. The two extremes are not going to be the best.

 

Djorgovski 2 and NGC 6520 (ink spot).

Sumerian 10".
- With the intensifier in afocal, with the Paragon 30 mm (42x), the globular is glimpsed in the center of a quadrilateral of stars. The ink spot (NGC 6520/CB 107) is seen with a fairly well-defined outline. Switching to Delos 17.3 (72x), the globular cluster is seen a little better, but the ink spot is a little worse.
- Without the intensifier, with the Delos 17.3, the ink spot can be seen but more blurred. The globular cluster, on the other hand, is at the limits of perception. With the Ethos 13 (96x), the cluster is seen slightly better but still at the limits of perception. The ink spot is large but still not well-defined in shape and edges.

Zen 24".
- Without the intensifier, the ink spot can be seen very well with the 60 cm aperture, and the globular is seen well with some background glow, better than with the intensifier in the 10".
- With the intensifier at prime focus (115x) one can see the ink spot very well and with sharp edges—the globular resolves into stars.



#44 chemisted

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Posted 19 May 2024 - 04:28 PM

 

But, I thought H beta was recommended for visual on the horsehead, or it used to be the recommended filter years ago.

 

Rick

The human eye sees H-beta (and OIII) just fine but is quite insensitive to H-alpha.  Thus, with glass, you use an H-beta filter.  The NVDs are the opposite.  They are tremendously sensitive in the red where H-alpha is.  So visual observing with NV means the old H-beta filter stays in the drawer.


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#45 Bearcub

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Posted 20 May 2024 - 08:07 AM

I have seen the HorseHead from home ay SQM 18.8 with NV and a UHC-S filter (dual 50 nm band) with a 10" dobsonian. I cannot see it without NV even if I use a 24" dobsonian.

The horse head is visible in the 24" under SQM 21.5 with a H-beta filter and is comparable.

 

However however...however....  All objects are better (definitely better) in the 24" with NV than in in the 10" with NV. The difference is large and I cannot find any way in which a superb NV with the 10" can outperform a good NV with 24" (whichever location and target).

 

For example, the ink spot (H-alpha is not the only thing), i.e., NGC 6520 and Barnard 86 show a lot more details in the 24". You get the same level of clarity, but in one case at 42X and in the other at 115x (so one see three times finer structures).

 

There obviously is a sweet spot between high NV specs and greater diameter. One can spend 10k in NV and 2k in the telescope, or 4k in NV and 8k in the telescope or some other combination. The two extremes are not going to be the best.

 

Djorgovski 2 and NGC 6520 (ink spot).

Sumerian 10".
- With the intensifier in afocal, with the Paragon 30 mm (42x), the globular is glimpsed in the center of a quadrilateral of stars. The ink spot (NGC 6520/CB 107) is seen with a fairly well-defined outline. Switching to Delos 17.3 (72x), the globular cluster is seen a little better, but the ink spot is a little worse.
- Without the intensifier, with the Delos 17.3, the ink spot can be seen but more blurred. The globular cluster, on the other hand, is at the limits of perception. With the Ethos 13 (96x), the cluster is seen slightly better but still at the limits of perception. The ink spot is large but still not well-defined in shape and edges.

Zen 24".
- Without the intensifier, the ink spot can be seen very well with the 60 cm aperture, and the globular is seen well with some background glow, better than with the intensifier in the 10".
- With the intensifier at prime focus (115x) one can see the ink spot very well and with sharp edges—the globular resolves into stars.

So you tested both scopes with nv at different magnifications, why not at same?




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