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NV Compared to Increased Aperture on M13

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#1 MattJ

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Posted 25 May 2017 - 08:04 AM

Obsession Telescopes, on their website, has a comparison of M13, first the "typical" 8 inch view, and then what it looks like through their increasing aperture scopes.  I was just curious to get some input on:

 

Aperture and speed of your scope

What color zone you observe from

When you observe with a normal glass eyepiece, where does your view of M13 fall on the Obsession scale?

When you use NV as an eyepiece, where does your view fall on the scale?

What filter and magnification do you prefer when viewing M13 with NV?  

 

Thanks!



#2 Eddgie

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Posted 25 May 2017 - 09:01 AM

First, it is very hard to make this comparison I think because stars are just a lot brighter in NV and comparing a smaller aperture to a larger aperture is problematic because to get the same image scale you have to greatly barlow the NV device and while you can Barlow and you can do so without a lot of loss (on stars), there is still some decrease in performance.

 

As and example, if you use something like a C14 with a conventional eyepiece at 325x (12mm Nagler T4) the core of M13 is highly resolved and in fact, M13 barely fits into the eyepiece. 

 

If you use something like a 6" f/4 scope, M13 is much brighter, but the image scale is so small that you can't really resolve deep into the core, and getting a night vision device to 325x with a 6" f/4 scope would require the equivalent of a 2mm eyepeice, or in other words, I would have to have a 10 power Barlow!!!!

 

So, I would say that it is difficult to make this kind of comparison. 

 

What I can say though is this.   Using my 12" dob, I can often partially resolve globulars that were hard to even see as more than a pale glow in the C14 using conventional eyepieces.

 

Even with very small scopes, I can at least partially resolve the bigger Globulars right from my red bordering on white zone skies.   Heck, even a 200mm SLR lens will partially resolve M22.

 

So, while I cannot really answer your question, if you are attempting to understand the increase in limiting magnitude, I would say that it is like doubling aperture.  I can probably resolve globulars from my yard using a 6" scope that would take a 12" scope to do using glass eyepieces, and I would probably be able to resolve globulars in teh 12" that would take a 24" to do



#3 pwang99

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Posted 25 May 2017 - 09:06 AM

As Eddgie says, there is resolution and there is brightness.

In my C11, with non-dark-adapted eyes, I feel like my view is closer to what they're showing as the "8" view". I feel like my Mod3 bumps that up to looking rather like the 18" or 20" view, as portrayed on the Obsession website.  That's also non-dark-adapted, but with NV gear it's moot.



#4 Eddgie

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Posted 25 May 2017 - 09:08 AM

As to the other questions, for Globulars in my red/white zone skies, I use a Baader 610nm long pass filter when using telescopes, and a 650nm when using 3x (58mm aperture).

 

I can easily see a great number of Globulars from my light polluted skies at 3x.  M13 is easy and I even think I can see some granularity.  M5, M22, M92, M10, and several others are pretty easy to see at 3x.  (By the way, I can see the Whirlpool Galaxy and its companion at 3x from my yard. Tiny, but not hard to spot).

  

But once again, even though they are easy picking using a 58mm aperture, the image scale is to small to resolve.   The big ones are pretty bright in the 6" f/4 and you can resolve stars across the core, but the core is so bright that the fainter stars simply merge together.  You can pick out the brighter stars across the core though.

 

You really need about the equivalent of 3000mm of focal lenght to get good results on most nebula though.  Barlowing the 12" works well for the bigger ones, but some of the smaller ones could probably use 4000mm.

 

Bright though.   I have never resolved as many globulars using conventional eyepieces as I can now, but for Globulars, you really need a bigger scope to get the image scale required to space the stars in the core far enough apart to seperate them or their are just so many that they merge together.   You would still be able to see some of the brighter stars across the area of the core, but when the scale is small, the dimmer stars are so close together that the main part of the core is just a granular glow.  


Edited by Eddgie, 25 May 2017 - 09:10 AM.


#5 bobito

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Posted 25 May 2017 - 09:33 AM

Their picture for the 12.5" looks exactly like what I see through my f/10 12" SCT in a yellow zone.

 

Bob



#6 bobhen

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Posted 25 May 2017 - 10:55 AM

Scope: One of the scopes I use is a Mewlon 210 F11.5 around 2,400mm FL

 

Zone: Gray/White zone Philadelphia suburbs, very heavy light pollution

 

Normal glass view: About what the 8” Obsession image shows give or take depending on magnification

 

With NV: The image can look like ALL of the brighter obsession images give or take depending on the magnification or barlow that I use. Brightness or dimming is not as impacted with magnification using NV as it is with unassisted viewing. The other difference is that the cluster will generally be more resolved across the core than the Obsession images show. 

 

Filter: I use a Baader 610 or 685 longpass filter

 

Magnification: no barlow around 90x with barlows around 180, 225, and 260x

 

NOTE: If you have not read the Night Vision article, Seeing Through The Dust in the July 2017 issue of Sky and Telescope, the author uses a NV eyepiece (a 20-year old Collins I-3) to observe and RESOLVE distant globular clusters that are just barely seen as faint smudges when using regular eyepieces. That article might clarify or put things in a better perspective.

 

Bob



#7 MattJ

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Posted 25 May 2017 - 01:20 PM

Thanks everyone.  All very interesting.  I did read the Sky and Telescope article.  I knew that NV was more sensitive to red stars, but the part about it being able to "cut through" interstellar dust was new to me.  And he says that, with his 10 inch scope and NV, he can see stars down to almost 17th magnitude.  Wow!  



#8 Eddgie

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Posted 25 May 2017 - 04:00 PM

  And he says that, with his 10 inch scope and NV, he can see stars down to almost 17th magnitude.  Wow!  

I have kind of estimated that I see about 3 magnitudes deeper with NV in some cases (because different stars have different emissions and red stars are far easier in NV). 

 

I know that I could see stars using the 5.5" Comet Catcher that were sometimes difficult in my 12" dob, and the Comet Catcher I came to find out when I sold it had mirrors that had started to deteriorate.  This is constant across observing conditions ( the increased limiting magnitude).  If I take the scope to darker skies and use glass, I go deeper than I can in the city, and if I put the NV eyepiece in, I get about the same gain as I do in the city.

 

Everything is better under darker skies with NV in the exact same way that it is better using glass.  


Edited by Eddgie, 25 May 2017 - 04:01 PM.


#9 The Ardent

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Posted 25 May 2017 - 04:13 PM

For M13 and other bright globulars I prefer normal eyepiece to see the orange stars in 18". The subtle colors are lost with NV. One thing I've always pushed the envelope is seeing color in faint stars. Below a certain magnitude, stars are colorless. Add dark sky, aperture, experience, and practice, this can vary.

Don't have good vision or good color vision, use NV.

For viewing globs in the city where LP prevents observing, use NV

For dim globs that are not detectable with your aperture, use NV.

I believe that no matter what the target, always compare visual with NV to be a better observer.

#10 outofsight

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Posted 25 May 2017 - 04:21 PM

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.


#11 GlennLeDrew

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Posted 25 May 2017 - 05:28 PM

Using a globular cluster for such a comparison is problematic due to the star crowding. An NV device has not the resolving power of the eye where stars are concerned, and so the depth of penetration can be compromised where stars are getting to the point of blending together.

 

Ed suggests an effective two-fold boost in aperture when using NV, which is equivalent to 1.5 magnitudes, whereas outofsight finds 3 to as much as 5 magnitudes in improvement (the latter figure representing a 10-fold aperture increase.)

 

Device capability and gain setting, filtering employed, and the optical eyepiece exit pupil in effect when determining the non-NV limiting magnitude are worth knowing. Not to mention whether isolated stars in a dark field vs crowded star fields and/or nebulous 'backgrounds' were the test targets.



#12 outofsight

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Posted 25 May 2017 - 05:50 PM

Yeah MattJ, GlennLeDrew's first paragraph above sounds like a valid point. That kind of goes with what I said about mine seems like it can look like any of the images on the Obsession page.

 

http://www.obsession...scopes.com/m13/

 

When you go from no NV and then plug the NV in, in my case, you go from not much of an image to a very solid image, like the Obsession images, 15" and above. But I'll still try to check that out tonight and try to provide an objective report tomorrow, or so.

 

By the way, NV is rentable if you have a great need to know what it would do in your equipment.

 

Edit: I will go back out on that limb here and say, with the reasonably best tubes of today, a good SN and low EBI, etc., 4 magnitudes, at least. With a typical PVS-7, a solid 3 magnitudes, but with two eyes and other factors, it may seem like more, who knows. All I know for sure, no going out on a limb, for the light pollution where I live it is easily the best for my astronomy dollar. I would prefer dark skies, but short of that, NV fits the bill like nothing else. Imaging is OK and I might get into it one day, but this is the best I can do for what I like to do. I like all this stuff, have fun.


Edited by outofsight, 25 May 2017 - 06:02 PM.


#13 Rickster

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Posted 25 May 2017 - 06:21 PM

Last October I performed an experiment along this line and decided that NV gives roughly a 3x to 5x aperture gain.  As noted above, there are many factors involved, so it isn't always that simple.  But I still think it is a good rule of thumb.

 

The experiment  pitted a 70mm Tele Vue Pronto w/ NV against a 250mm Dob w/o NV.  The Pronto easily won.  I called the post:  "Revenge of the Pronto."

 

https://www.cloudyni...to#entry7470035



#14 Eddgie

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Posted 25 May 2017 - 07:40 PM

It is very difficult to make these comparisons.

On M29, in my Comet Catcher, I could see stars that were difficult to see with the same focal lenght eyepiece in the 12" dob, but if I really cranked the power in the dob and used averted vision, I could often just pick out most of those stars.

 

When I put the NV device into the 12", I could see that some of the principle stars in M29 are very close doubles or even triples and these do not show in the 12" with glass eyepieces (under my conditions).

 

I am though purposefully conservative when it comes to things like this. I am very comfortable saying that for stellar targets, the benefit is like doubling the aperture because I have done enough comparisons with scopes having this differential that I feel confident that most people would agree that this is a minimum rough comparison.

 

The problem though is that there are a lot of variables with exit pupil and magnification in general that come into play. 

 

And I am conservative because I have a serious doubt that my 12" with NV would show as many stars as a 36" scope (triple the aperture) using glass eyepieces. I will never know though, and until I do, I stick to my "Double the Aperture" so that I don't over-state the gain from night vision.   Maybe it is higher than this, but again, using high powers and averted vision, I can often find pretty much all of the stars I can see in my small scopes.    I don't like the view as much because struggling to see things with averted vision is  not nearly as enjoyable as seeing them easily in the NV device.  If you look really hard though, you often find many of those same stars.    I have over and over, but it was pushing myself.

 

 

But who wouldn't be interested even if it is only "Double" the aperture.   If you can get the same performance with an $800 10" Dob as you could with a very expensive and hard to manage 20" dob, well, the case cost issue of NV start to look like less of an objection. 



#15 chemisted

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Posted 26 May 2017 - 07:58 AM

I am the author of the S&T article.  In it, I state that the 40" at Yerkes has the same limiting magnitude as my view of NGC 6749. Thus, it is possible to conclude that, in this comparison of my 10" reflector/I3 combination to a 40" refractor, I achieved a 3 magnitude boost in performance.  Dennis di Cicco has the same vintage Collins I3 that he used for the images he generated for this article.  He indicates a 2 magnitude boost for NGC 6760, another heavily obscured cluster.  As others have pointed out, there are caveats that accompany all comparisons.  Mine, for example, is a bit of apples to oranges when comparing a modern reflector to an old large refractor that will have significant light loss to absorption.  Some time ago in a club newsletter I stated that my view of NGC 6934 was identical to that published by Steven Coe in his book Deep Sky Observing using a 36" reflector. After years of studying this topic I am comfortable with saying a minimum of two to two and a half magnitude gain is to be expected when using NV to enjoy globular clusters.

 

Interestingly, I have just acquired a top-of-the-line NVD Micro.  I haven't had it in the ten inch yet, but with my TV-140 I am definitely going deeper than the Collins I3 (on one of my all-time favorite clusters, M3).

 

Clear skies!

Ed


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#16 chemisted

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Posted 26 May 2017 - 08:09 AM

Oops, I just checked and that last comparison to Coe was with NGC 6144 (not 6934).



#17 Doug Culbertson

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Posted 26 May 2017 - 08:16 AM

Eddgie, on 25 May 2017 - 8:40 PM, said:

 

But who wouldn't be interested even if it is only "Double" the aperture.   If you can get the same performance with an $800 10" Dob as you could with a very expensive and hard to manage 20" dob, well, the case cost issue of NV start to look like less of an objection. 

 

Not to mention not having to bust your agates setting up and tearing down a big dob. Before NV I had planned to dump a great deal of money into a 20" or so dob when I retire, but NV has eliminated aperture fever for me. Now I think more in terms of moderate sized fast optics that are easy to take out and set up and will excel with my NV devices.


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#18 Eddgie

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Posted 26 May 2017 - 09:48 AM

 

Eddgie, on 25 May 2017 - 8:40 PM, said:

 

But who wouldn't be interested even if it is only "Double" the aperture.   If you can get the same performance with an $800 10" Dob as you could with a very expensive and hard to manage 20" dob, well, the case cost issue of NV start to look like less of an objection. 

 

Not to mention not having to bust your agates setting up and tearing down a big dob. Before NV I had planned to dump a great deal of money into a 20" or so dob when I retire, but NV has eliminated aperture fever for me. Now I think more in terms of moderate sized fast optics that are easy to take out and set up and will excel with my NV devices.

 

 

 

Your point about the bigger telescope is of course not lost one bit on me.  As I recently detailed in my decision process for buying the Boren Simon 6" f/2.8, when the 8" and 10" versions are really not that much more expensive and are  in my reach financially, I kept coming back to the same hard truths.  The first is that many of what are now my favorite targets, nebula, are very large and a 2+ degree true field seems well suited to study of these objects.

 

But larger and more dominant truth was that if it was bigger than 6", I would be less inclined to use it because of the mounting issues.

 

I leave an Astro-Tech Voyager out on the patio 24/7, and this as it turns out (and it was bought specifically with the 6" Boren Simon in mind) is a fantastic mount for the 6".   It is light enough that I can move around the yard to get better sight lines past obstructions.  

 

I do though have the 12" and I do use it a lot as well, but once again, this is because I store it on the covered patio and can move it out quickly with a hand truck.

 

But the 6" f/2.8 was a hard acknowledgement to the fact that these days, simpler and lighter is better for me.

 

And the NV made this not only practical, but super-enjoyable, and in a very real sense, for many of the best targets for night vision, which are of course nebula, a small and fast aperture is almost essential!



#19 Jeff Morgan

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Posted 26 May 2017 - 11:12 AM

Not to mention not having to bust your agates setting up and tearing down a big dob. Before NV I had planned to dump a great deal of money into a 20" or so dob when I retire, but NV has eliminated aperture fever for me. Now I think more in terms of moderate sized fast optics that are easy to take out and set up and will excel with my NV devices.

 

 

I think it is only human nature (and especially the nature of us OCD amateur astronomers) to try to quantify things like "how much more" or "how much deeper". And as more people start checking out NV options, these questions will continue.

 

As we all know, the answer varies by object class, and often by object within a class. The answer is usually "a lot more/a lot deeper". Safe to stick with doubling the aperture, realizing the gross approximation that is.

 

But more importantly is what Doug brings out. Beyond trying to quantify the performance advantage is the impact it has on current equipment and observing styles:

 

1) Your current scope can realistically be your last telescope. Even if it is smaller.

 

2) Being able to effectively use your scope from the light polluted backyard opens up more opportunity to observe.

 

Getting to dark skies is still desirable, but lets face it - putting together a dark sky trip requires work, planning, and great big chunks of time. It's an expedition. You never get back that time spent driving to and from the site.

 

I'm sure some of us will eventually get 20" f/3 scopes - I may be one of them. But the increased opportunities to me with easily manageable (and affordable) scopes - priceless.


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#20 Eddgie

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Posted 26 May 2017 - 11:54 AM

 

I'm sure some of us will eventually get 20" f/3 scopes - I may be one of them. But the increased opportunities to me with easily manageable (and affordable) scopes - priceless.

 

This is why I do quite often respond to posts in other forums about the use of night vision gear. 

 

Many people for a variety of reasons, won't commit to a much larger scope because they know that the level of effort involved rises quickly.   We all know the multitude of concerns about owning very large instruments.

 

I think that of course the OP is trying to get a more precise quantification of the performance increase, and I understand that, but I think that for globulars, it is a very hard class of object to make this kind of aperture comparison. 

 

Omega Centauri was magnificent in the 6" f/2.8 and  even at f/4 is was fabulous, but I have seen it with a C11 and a 35mm Panoptic, and I would say that while the view was very different, it was just as amazing in the C11 as it was in the 6" even  f/4.



#21 Starman81

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Posted 26 May 2017 - 12:53 PM

 

Not to mention not having to bust your agates setting up and tearing down a big dob. Before NV I had planned to dump a great deal of money into a 20" or so dob when I retire, but NV has eliminated aperture fever for me. Now I think more in terms of moderate sized fast optics that are easy to take out and set up and will excel with my NV devices.

 

 

I think it is only human nature (and especially the nature of us OCD amateur astronomers) to try to quantify things like "how much more" or "how much deeper". And as more people start checking out NV options, these questions will continue.

 

As we all know, the answer varies by object class, and often by object within a class. The answer is usually "a lot more/a lot deeper". Safe to stick with doubling the aperture, realizing the gross approximation that is.

 

But more importantly is what Doug brings out. Beyond trying to quantify the performance advantage is the impact it has on current equipment and observing styles:

 

1) Your current scope can realistically be your last telescope. Even if it is smaller.

 

2) Being able to effectively use your scope from the light polluted backyard opens up more opportunity to observe.

 

Getting to dark skies is still desirable, but lets face it - putting together a dark sky trip requires work, planning, and great big chunks of time. It's an expedition. You never get back that time spent driving to and from the site.

 

I'm sure some of us will eventually get 20" f/3 scopes - I may be one of them. But the increased opportunities to me with easily manageable (and affordable) scopes - priceless.

 

I wonder when will the others realize this? 



#22 Vondragonnoggin

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Posted 26 May 2017 - 01:09 PM

It's not a simple answer though. Per object, per type of scope used and speed of scope, per filter selection. There are a number of variables to qualify perceived aperture increase equivalents.

 

I think it's safe to post twice aperture with caveats. Sometimes it will be a 5x perceived increase and other times none.

 

I also think its important to gently give a push to the EAA forum for more specific examples.



#23 GlennLeDrew

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Posted 26 May 2017 - 01:48 PM

When it comes to emission nebulae, in some cases the relative aperture gain via NV can be considerable indeed, even 'infinite', if one wants to engage in superlatives. Really subtle Sharpless objects are invisible through an eyepiece using *any* aperture with the best filtering. But a small camera lens attached to an NV device with an H-alpha filter will handily bring out the larger examples. Invisible vs visible; 0 vs 1; an infinite improvement! ;)

 

Furthermore. Low surface brightness, low contrast nebulae and galaxies through the eyepiece are practically featureless outside of the grosser structure, even for quite large apertures. But a small scope with an NV device reveals subtler brightness differences than can a visual scope having, say, 10X the aperture, if not more.

 

Finally, objects which are too dim to elicit a color response, or those for which only a small portion is sufficiently bright to do so, might as well be examined with a monochromatic NV device.


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#24 bobhen

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Posted 26 May 2017 - 03:00 PM

In the S & T July 2017 NV article the author uses a 10-inch Newtonian and a 20-year-old Collins I-3 eyepiece to observe distant globulars.

 

Here is a quote from that article…

 

“In an article published in 1900, George W. Ritchey wrote, “The visual limit . . . of the 40-inch [great refractor at Yerkes] is at about 16.5 to 17 magnitude.” He was talking about globular clusters. This comparison provides a measure of aperture increase that I’m achieving with the electronic eyepiece coupled to my 10-inch reflector.

 

And of course Glen is right as well. On some objects the increase using NV is infinite. I can’t see the Horsehead nebula from my location with any size amateur telescope, but I can see it easily with a 4-inch refractor and NV. So what is the aperture increase when using night vision and seeing an object compared to using regular eyepieces and not seeing anything?

 

Bob


Edited by bobhen, 26 May 2017 - 03:01 PM.


#25 The Ardent

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Posted 26 May 2017 - 03:44 PM

that article was relevant 20 years ago.


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