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Optimal exit pupil size for city light pollution

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#1 MT4

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Posted 15 July 2021 - 07:24 AM

I observe mostly from my mid Tokyo location where light pollution is literally off the charts.  (I recently bought myself a Sky Quality Meter and much to my dismay its readings have been consistently around 16 or thereabout, which seems much worse than is typical for Bortle-9 skies.)

 

I've leaned that an effective workaround for city light pollution is to increase the power and reduce the exit pupil.  With the rainy season finally starting to come to an end at long last, I just had a chance to conduct a simple experiment to see how brightness changes with exit pupil size while keeping the mag constant.  In this experiment, I used my Nikon 18x70 as the test instrument and put on step-down rings of varying sizes to change the exit pupil.  (There's a key reason why I used my Nikon 18x70 as the test instrument:  I've been seriously thinking about buying a Canon 18x50 IS.)

 

    Step-down ring size          Exit pupil size

    62                                      3.4mm

    58                                      3.2mm

    52                                      2.9mm

    48                                      2.7mm

 

What I found was that at exit pupil size of 2.9mm I started to notice that the view was really dim and that I was having difficulties spotting some of the stars.  At 2.7mm, it was really too dim for my taste.  (It occurred to me then that 2.7mm is about the exit pupil size of the Canon 18x50 IS.  This experiment caused me to re-think my plan to get a Canon 18x50 IS.  I'll repeat the experiment another night, and if I continue to get the same results then I'd have no choice but to change my plan and get a Canon 15x50 IS instead.)

 

While it should be no surprise that reducing the exit pupil size would lead to dimmer views of the sky, what's surprising (to me at least) is that I have had no issues at all with the views through my Kowa  Highlander Prominar at 32x (exit pupil = 2.6mm) and 50x (exit pupil = 1.6mm).  Never once while looking through my Kowa have I had the thought that the view is too dim for me to make out the stars.  In fact, it has been the exact opposite:   I can see a lot more stars at 50x compared to 32x.

 

So what is going on here?  Why would I have no trouble whatsoever with an exit pupil of just 1.6mm in my 50x82 Kowa but find the views through my simulated Nikon 18x48mm too dim?   Is it true that small exit pupil sizes need to be compensated with much larger objective sizes to prevent the views from being too dim?

 

Thank you in advance for your explanations.

 


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#2 Fiske

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Posted 15 July 2021 - 07:29 AM

Because with the larger aperture instrument, the field is still brighter despite the smaller exit pupil. (Queue Jon, Rich, and others with helpful formulae to provide the math for this. wink.gif )

 

Fiske


Edited by Fiske, 15 July 2021 - 07:31 AM.

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#3 ihf

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Posted 15 July 2021 - 07:41 AM

Sounds fishy. Area light sources should be identical for same exit pupils. Only point ligh sources go up with magnification.

As for step down rings they are imperfect simulations of reduced aperture. Blocking out a ring only works perfectly at the field stop. Putting a step down at the lens creates a falloff to the edges.

Try a 10x25 for comparison.
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#4 Upstate New Yorker

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Posted 15 July 2021 - 08:14 AM

Your post is interesting.  I think that the main effect you are picking up is darkening of the background sky as magnification goes up so that you can see more.  I have what I think is a similar situation.  I live in light-polluted Orlando, Florida USA, where the seeing or steadiness of the air is good, but the sky transparency is low because of the large number of water droplets suspended in the air.  The light pollution interacts with the water droplets, apparently through internal reflections off the inside of the droplets back to the observer.  When the night sky is at its worst, you observe a gray shimmering mass of sky all around you that is almost greasy in its appearance.  It doesn't help that Disney World is only four miles away, although the lights do go off in the early hours of the morning...

 

With this as background I should report that I own two Canon IS binoculars, a 10x42L model and a 18x50 model.  I use the 18x50s much more even though image stabilization works better for the 10x42Ls.  Your post suggests one reason why: the darkening of the background sky is greater with the 18x50s.

 

Thanks for your report and you deserve a medal in my opinion, for trying to observe given your local seeing conditions.


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#5 Fiske

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Posted 15 July 2021 - 08:25 AM

Thanks for your report and you deserve a medal in my opinion, for trying to observe given your local seeing conditions.

waytogo.gif


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#6 vtornado

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Posted 15 July 2021 - 08:49 AM

I can't say for downtown Tokyo,  I can tell you for my yard in a Red/Grey zone outside of Chicago.  

 

Have you looked at a full moon at night through  your binos?

When I take the binos away and look at the sky and there is a big "black" hole in my vision.  I assume this is from my eyes loosing all dark adaptation, except for

the edges.

 

Well that same thing happens when I use 7x50 binos on the empty night sky,

because the sky is so bright.  I assume any night vision I had is gone.

 

10x50 and the effect is much less pronounced or maybe not there.

 

I am thinking I should get a pair of 8x40's since 10x50 is a bit too much for

me to hand hold.

 

Huh ... and after reading your post maybe I just stop down my 7x50's with an aperture mask.



#7 MT4

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Posted 15 July 2021 - 08:55 AM

On some Canon IS related threads, I've seen some observers report/opine that the relatively small exit pupil of the Canon 18x50 IS model would be too small for them.

 

On the other hand, I would imagine that for a typical telescope user, operating at mags of 100 or above, the Canon 18x50's exit pupil would be plenty big.  I've heard of 100mm/120mm APM BT users using their BTs at mags well above 100.

 

Can exit pupil size be directly compared between instruments of very different powers?  In other words, is it the main factor determining brightness for an observer, assuming that optics quality is comparable?

 

Or is it true that a small exit pupil really needs the objective to be large enough to prevent the view from going too dim ?

 

Thank you.


Edited by MT4, 15 July 2021 - 09:21 AM.


#8 MT4

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Posted 15 July 2021 - 09:07 AM

I can't say for downtown Tokyo,  I can tell you for my yard in a Red/Grey zone outside of Chicago.  

 

Have you looked at a full moon at night through  your binos?

When I take the binos away and look at the sky and there is a big "black" hole in my vision.  I assume this is from my eyes loosing all dark adaptation, except for

the edges.

 

Well that same thing happens when I use 7x50 binos on the empty night sky,

because the sky is so bright.  I assume any night vision I had is gone.

 

10x50 and the effect is much less pronounced or maybe not there.

 

I am thinking I should get a pair of 8x40's since 10x50 is a bit too much for

me to hand hold.

 

Huh ... and after reading your post maybe I just stop down my 7x50's with an aperture mask.

 

City light pollution is why I don't have a Nikon 7x50 SP or Nikon 10x70 SP in my lineup.   I have a Nikon EDG 7x42 and a Fujinon FMT-SX 10x50 and the sky background is quite bright in both instruments.

 

I've found that my little Nikon EII 8x30 with its 3.75mm exit pupil gives very pleasant views of the night sky.  It doesn't show a lot of stars where I typically observe but those stars visible in the central 70% of the FOV are all pinpoint tight and intensely bright against a darkened background.  I don't know if this is all the magic of the small exit pupil or the little Nikon EII 8x30 truly has excellent contrast.  Perhaps it's a combination of both factors at work here.

 

I was disappointed to find out that masking down my Nikon 18x70 to an exit pupil size of 2.7mm caused the view to be too dim for my taste.  Will try again tomorrow night with my new observing hood to get rid of those pesky street lights and park lights.


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#9 Tony Flanders

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Posted 15 July 2021 - 10:02 AM

I observe mostly from my mid Tokyo location where light pollution is literally off the charts.  (I recently bought myself a Sky Quality Meter and much to my dismay its readings have been consistently around 16 or thereabout, which seems much worse than is typical for Bortle-9 skies.)


Yes, Tokyo is supposed to be just about as anywhere in the world. Much brighter than midtown Manhattan, for instance.
 

I've leaned that an effective workaround for city light pollution is to increase the power and reduce the exit pupil.  With the rainy season finally starting to come to an end at long last, I just had a chance to conduct a simple experiment to see how brightness changes with exit pupil size while keeping the mag constant.


I have never found that to be the case. The one and only one situation where I find that stopping down binoculars' aperture can be helpful is when viewing tiny, ultrabright targets at magnifications far below optimal. In particular, masking down binoculars can definitely make Jupiter's moons easier to see. But that's a case where Jupiter is far brighter than you would like it to be, and even the moons are so bright that they will be obvious no matter how much you stop down the aperture.

When viewing anything dimmer than magnitude 2 or 3, stopping down binoculars is always a clear loser, in my experience. A huge loser for viewing deep-sky objects. And that's every bit as true under bright skies as under dark skies.

Now if you decrease the exit pupil as in a telescope or binocular telescope -- by leaving the aperture the same and increasing the magnification -- then the view almost always improves, at least up to an exit pupil of 1 or 2 mm, depending on the target. But that's a totally different experiment.

 

To a first approximation, leaving magnification the same and increasing aperture almost always improves the view. And leaving the aperture the same and increasing magnification almost always improves the view as long as you don't go too far.

 

In even simpler terms: aperture good, magnification good.


Edited by Tony Flanders, 15 July 2021 - 10:14 AM.

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#10 Tony Flanders

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Posted 15 July 2021 - 10:07 AM

Well that same thing happens when I use 7x50 binos on the empty night sky,
because the sky is so bright.  I assume any night vision I had is gone.


I doubt it. First of all, binoculars are redundant in this case -- the empty night sky is still brighter to the unaided eye than it is through any binoculars. So unless you have spent your whole session avoiding looking up, your night vision is already pretty bad.

Second, in my experience doing urban observing at SQM levels between 17 and 18, I actually do maintain some modicum of dark adaptation. That's obvious because things get instantly worse if (for instance) I inadvertently look directly at a streetlight.

In any case, an urban night sky is still not bright enough to cause my pupils to contract. The loss of dark adaptation in this case has to do with the retina, not the size of my pupil.


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#11 Fiske

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Posted 15 July 2021 - 10:45 AM

In even simpler terms: aperture good, magnification good.

Nice! waytogo.gif

 

I recall seeing a formula from Jon Isaacs, to do with binocular selection, which says something like: "aperture good, magnification better." grin.gif

 

And here is an interesting thing I learned from my friends in the Double Star forum. To resolve closer doubles, DS observes will deliberately look at a white light source to reduce pupil dilation which actually increases visual acuity, the theory being that the outer areas of our pupils have various lens defects so masking them out increases resolution. There is a trade off here based on stellar magnitude -- some point at which the practice stops being beneficial as double star component magnitude decreases.

 

Fiske


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#12 KennyJ

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Posted 15 July 2021 - 11:01 AM

It may be that using two eyes instead of one tends to make the difference more noticeable.


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#13 MT4

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Posted 15 July 2021 - 06:33 PM

The one and only one situation where I find that stopping down binoculars' aperture can be helpful is when viewing tiny, ultrabright targets at magnifications far below optimal. In particular, masking down binoculars can definitely make Jupiter's moons easier to see. But that's a case where Jupiter is far brighter than you would like it to be, and even the moons are so bright that they will be obvious no matter how much you stop down the aperture.

When viewing anything dimmer than magnitude 2 or 3, stopping down binoculars is always a clear loser, in my experience. A huge loser for viewing deep-sky objects. And that's every bit as true under bright skies as under dark skies.

Now if you decrease the exit pupil as in a telescope or binocular telescope -- by leaving the aperture the same and increasing the magnification -- then the view almost always improves, at least up to an exit pupil of 1 or 2 mm, depending on the target. But that's a totally different experiment.

 

To a first approximation, leaving magnification the same and increasing aperture almost always improves the view. And leaving the aperture the same and increasing magnification almost always improves the view as long as you don't go too far.

 

In even simpler terms: aperture good, magnification good.

 

Thank you so much Tony for your very clear explanation.  Apparently I had somehow developed the misguided notion that having a smaller exit pupil is an advantage for spotting DSOs under bright skies.  I now understand that the small exit pupil is not the primary factor, but is instead a by-product of increasing the mag.

 

Your simpler-term summary of "aperture good, magnification good" is a great summary and I'll be sure to keep that in mind.  Thanks again.



#14 MT4

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Posted 15 July 2021 - 06:45 PM

Nice! waytogo.gif

 

I recall seeing a formula from Jon Isaacs, to do with binocular selection, which says something like: "aperture good, magnification better." grin.gif

 

And here is an interesting thing I learned from my friends in the Double Star forum. To resolve closer doubles, DS observes will deliberately look at a white light source to reduce pupil dilation which actually increases visual acuity, the theory being that the outer areas of our pupils have various lens defects so masking them out increases resolution. There is a trade off here based on stellar magnitude -- some point at which the practice stops being beneficial as double star component magnitude decreases.

 

Fiske

 

That's a great tip.  Thank you for sharing it Fiske.  (And I love that quote from Jon Isaacs:  "aperture good, magnification better."  This and Tony's explanation have cleared up many things for me and for that I am very grateful.)

 

I think I may have accidentally stumbled upon the idea of looking at a bright light source as a temporary visual acuity "booster".  (There are so many street lights / park lights everywhere in Tokyo and so it's not hard to inadvertently look at such light sources.)  The first couple of times it happened I didn't quite understand what was going on since I'd read that you need to keep your eyes dark adapted for the best resolution of DSOs.  Now I understand why:  it's all to do with controlling the imperfections in one's eye lenses.



#15 MT4

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Posted 15 July 2021 - 07:41 PM

Having slept on my little experiment and findings, I think I may have some sort of an explanation.  With all the ambient light around me, perhaps my pupils only open to around 3.5mm (I wasn't using any observing hood and didn't do anything to help get my eyes dark adapted) and so stopping the Nikon 18x70 down to 62mm/58mm didn't really cause the view to be that much dimmer.  On the other hand, when stopping the Nikon down to 48mm, for an exit pupil of 2.7mm, the change in the brightness of the view was quite drastic and I lost quite a few stars.

 

As to why I don't have any issue with the brightness of the view in my 82mm Kowa, I could only guess that at 82mm the objectives collect enough light that 32x / 50x is not a problem.

 

As an FYI, I measured the brightness of my skies again at around mid-night and the readings went up to be almost 17.  Still dismal by many people's standards but I'd take any little improvement I can get smile.gif



#16 gwlee

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Posted 15 July 2021 - 09:19 PM

I have found that the simplified “Bishop Index” (magnification x aperture) does as well as any I’ve tried for ranking how my binoculars compare for astronomy.

The higher the number the better. Example: 7x50=350; 10x50=500.
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#17 Rich V.

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Posted 15 July 2021 - 09:48 PM

Though having the same 2.5mm exit pupils, I'd take a 20x50 over a 10x25 anytime if I want to go deeper to see fainter stars.  I'd take your 32x82 over either, though. 

 

Pick either index you like; Bishop or Adler, to use for a comparison.  They all say more mag along with more aperture wins, despite the same overall "brightness" of equal exit pupils.  If you want a hand held bino, IS or not, you're going to have to make a compromise about how deep you can see.

 

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#18 Rich V.

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Posted 15 July 2021 - 10:28 PM

BTW, MT4, how do you like the night performance of your 10x35 EII compared to the 8x30 or your 7x35s?  I see quite a positive difference in the night time with the 10x EIIs myself vs the others, and even despite the 5mm EP of the 7x35s. Try comparing the differences between them while you test your new chair.  You'll see everything better comfortably seated, I'm sure.  It could be informative.

 

I'm not experienced with heavily LP skies, fortunately for me, but I think you'll still see the differences between them even in Tokyo.

 

Rich


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#19 MT4

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Posted 15 July 2021 - 11:56 PM

BTW, MT4, how do you like the night performance of your 10x35 EII compared to the 8x30 or your 7x35s?  I see quite a positive difference in the night time with the 10x EIIs myself vs the others, and even despite the 5mm EP of the 7x35s. Try comparing the differences between them while you test your new chair.  You'll see everything better comfortably seated, I'm sure.  It could be informative.

 

I'm not experienced with heavily LP skies, fortunately for me, but I think you'll still see the differences between them even in Tokyo.

 

Rich

 

The incessant rains have gone away, hopefully for good this year, but the clouds are still there in the sky.  The next time the sky is clear, which I am hoping will happen by this Sun or early next week, I'll take some of my binoculars including both Nikon EII's out for a spin.  They've been locked up inside my moisture-proof cabinet for far too long already and I assume that they'd want a breath of fresh air.  And you're right it'd be a good way to break in the new stargazing chair smile.gif

 

Thank you very much for your many informative posts, Rich.  I have learned a lot from them.



#20 MT4

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Posted 16 July 2021 - 08:15 AM

The sky was partially clear today and so I thought I would repeat my little aperture masking experiment.  This time I brought my Maven B5 15x56 with me for a 4-way comparison.

 

Viewing conditions:

Transparency:  Mediocre at best, partially cloudy and humidity of 81%.

Seeing:            (I don't know how to judge the seeing / steadiness of the air.  But perhaps doesn't matter for the mag range in question.)

SQM reading:  Just a tad under 16.  (Would improve to about 17 at mid-night.  I didn't want to wait until mid-night for fear that the clouds might come back in full force.)

 

Test subjects:

Stars centered on 1 Oph.  See attached star chart, courtesy of the Stellarium app on my iMac.

 

Test instruments / configurations:

Maven B5 15x56 stopped down to 52mm for an exit pupil of 3.5mm

Nikon 18x70 stopped down to 48mm, 52mm, and 58mm for exit pupils of 2.7mm, 2.9mm, and 3.2mm respectively.

 

Note that I deliberately gave the stopped down Maven 15x a small aperture advantage over the smallest aperture of the stopped down Nikon 18x.  One of the objectives of the experiment was to see which instrument would have better reach.

 

 

Test results:

 

I started out with the Maven 15x52mm. 

I could see the three main stars without any problem. 

Of the four stars circled in blue, I could see the two brightest ones.

Of the two stars circled in orange, I thought I could see just the brighter one.

The star circled in red was invisible.  There was not even a hint of it.

  => Lowest mag visible in this round:  6.95

 

Next up was the Nikon 18x48mm.

I could see the three main stars without any problem.

Of the four stars circled in blue, I could see the three brightest ones.

Of the two stars circled in orange, I could see both.

The star circled in red was invisible.  There was not even a hint of it.

    => Lowest mag visible in this round:  7.40

    => So the Nikon 18x48mm had better reach than the Maven 15x52mm.  A whopping improvement of 2 extra stars !!

 

Next up was the Nikon 18x52mm.

I could see the three main stars without any problem.

Of the four stars circled in blue, I could see the three brightest ones. (A little easier to see these stars.)

Of the two stars circled in orange, I could see both.  (A little easier to see these stars.)

The star circled in red was invisible.  There was not even a hint of it.

    => Lowest mag visible in this round:  7.40

    => A little better than the Nikon 18x48mm, though no new stars gained.

 

Next up was the Nikon 18x58mm.

I could see the three main stars without any problem.

Of the four stars circled in blue, I could see the three brightest ones. (Much easier to see these stars compared to Nikon 18x48mm.)

Of the two stars circled in orange, I could see both.  (Much easier to see these stars compared to Nikon 18x48mm.)

There was a clear hint of the star circled in red.

    => Lowest mag visible in this round:  7.40

    => 1 extra star visible compared to the Nikon 18x48mm / 18x52mm.

    => View much brighter and preferable to those in Nikon 18x48mm / 18x52mm

 

 

Conclusions:

(1) First and foremost, it is hard to come up with good conclusions when viewing conditions are poor (that's my local observing site smile.gif) and when the tester is not very experienced (that's me smile.gif

(2) Tony and Jon were exactly right:  aperture good, mag good/better.

(3) Under my challenging viewing conditions, the Nikon 18x48mm had better reach than the Maven 15x52mm.  This finding is consistent with both Bishop index and Adler index.

           Bishop index:

                Nikon 18x48mm:   864

                Maven 15x52mm:  780

           Adler index:

                Nikon 18x48mm:    125

                Maven 15x52mm:   108

 

(4) I deliberately set up the test such that I would use the Nikon 18x48mm before the Nikon 18x58mm.   While the views through the Nikon 18x58mm were brighter and preferable, the views through the Nikon 18x48mm weren't objectionable at all.   Had I switched the order, I might have found the reduced brightness in the Nikon 18x48mm jarring and hence more of a problem.  In other words, it's all relative.

 

(5)  I prefer the image scale in the Nikon 18x far more than the Maven 15x.   In theory it should only be 20% bigger but somehow it felt way bigger and preferable.  I am not sure if this has something to do with some sort of optical illusion coming from the fact that the Nikon 18x has a wider AFOV compared to the Maven 15x.

 

(6) The Nikon 18x70 is a marvelous instrument.  I would never let it go, even if I ended up buying the Canon 18x50 IS.  To my eyes, there was no vignetting whatsoever even when I stopped the Nikon 18x70 down to 48mm.  How could that have been possible?   Something to do with good illumination across the FOV?

 

Based on today's findings, the Canon 18x50mm is looking mightily attractive to me.

Attached Thumbnails

  • 1 Oph.jpg

Edited by MT4, 16 July 2021 - 08:33 AM.

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#21 MT4

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Posted 16 July 2021 - 11:10 AM

The night sky was mostly clear and so I thought I'd redo my little aperture masking experiment one more time, looking at the starfield around 67 Oph, 62 Oph, 60 Oph and the Summer Beehive Cluster.  I was able to star hop to the Summer Beehive cluster and it was there that the difference between the Nikon 18x48mm and the Maven 15x52mm was clear.  The Summer Beehive cluster was quite faint in the latter instrument and I might have overlooked it had I not checked the Stellarium app earlier and learned that it was there.   In the Nikon, the Summer Beehive Cluster was much more defined and couldn't really be missed.

 

This second aperture masking test of the night has left me without a doubt that the Nikon 18x48mm wins against the Maven 15x52mm in terms of reach, despite the latter having a 17% advantage in light gathering.  ("Aperture good, mag good/better")

 

Despite the loss in terms of reach, it's not all bad for the Maven B5 15x56.  Its 4.5-deg TFOV was wonderful for star hopping and for providing context.  Stars were intensely bright and sharp to about 85% of the FOV.  I've heard the Maven described as the poor man's Swarovski SLC 15x56, and I am very happy that I didn't shell out over 3k USD for the Swaro.

 

On a side note, it was really my first time enjoying the summer night sky and the view was breathtaking with so many bright stars everywhere, even in my dismal local viewing conditions.  I can't imagine what the view under darker skies would be like.  I'll find out for myself early in the fall when I take a trip up north.


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#22 ECP M42

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Posted 16 July 2021 - 11:45 AM

If I can make two points, the main one is that the retina needs also 30-40 minutes before adapting to the dark. It is therefore very important to keep them adapted when we change the various binoculars on testing.

The second point is that the eyepiece hoods are necessary and mandatory, during these tests (but also during normal observations), especially under starry skies heavily polluted by artificial light that is around us (very illuminated city areas). 

 

Only after at least ten minutes of observation inside the eyepieces with lens hoods, can I see what the binoculars show me from my polluted site.

 

I am glad that you have reflected during the night and that you have go to redo the test with a little more consistency. Your point 4 is describing it and got you right to step 5.

 

(4) I deliberately set up the test such that I would use the Nikon 18x48mm before the Nikon 18x58mm.   While the views through the Nikon 18x58mm were brighter and preferable, the views through the Nikon 18x48mm weren't objectionable at all.   Had I switched the order, I might have found the reduced brightness in the Nikon 18x48mm jarring and hence more of a problem. 

 

(5)  I prefer the image scale in the Nikon 18x far more than the Maven 15x. 

 

 

(6) ... To my eyes, there was no vignetting whatsoever even when I stopped the Nikon 18x70 down to 48mm.  How could that have been possible? 

The vignetting of the pupill it occurs mostly in the path of the lens rays near the focal point, so "undersized" prisms are the primary cause. But the size and optical pattern of the eyepieces are also another cause. 

Any baffles close to the optical center of the lenses (masks) instead, do not create vignetting, but only reduction of the diaphragm. 

 



#23 MT4

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Posted 16 July 2021 - 05:41 PM

If I can make two points, the main one is that the retina needs also 30-40 minutes before adapting to the dark. It is therefore very important to keep them adapted when we change the various binoculars on testing.

The second point is that the eyepiece hoods are necessary and mandatory, during these tests (but also during normal observations), especially under starry skies heavily polluted by artificial light that is around us (very illuminated city areas). 

 

 

With the sky and ground so brightly lit in mid Tokyo, regardless of the hour of the day, I don't think there is such a thing as dark adaptation.  Even if you stay in a dark room for an hour, the moment you step outside all that dark adaptation is instantly gone.

 

I have an observing hood and a few bino bandits and some home-made light blocking sheets, but all I use them for is to block out the pesky street lights and park lights around me.


Edited by MT4, 16 July 2021 - 08:55 PM.


#24 MT4

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Posted 16 July 2021 - 06:22 PM

Back to the main topic of discussion, through all these aperture masking experiments, I've discovered that the "optimal" exit pupil size that gives me tight and bright stars against a properly darkened background is not a single number but instead is a range.   (Note that since a typical SQM reading for my night skies is 16, you're all forewarned that the below findings may not apply to the vast majority of observers smile.gif)

 

    Nikon EII 8x30:        3.8mm

    Maven B5 15x52*:   3.5mm

    Nikon 18x58*:          3.2mm

    Kowa 32x82:           2.6mm

 

What's interesting is that my perceived "optimal" exit pupil size decreases as the mag goes up.  I don't know what theory may explain this.  Perhaps theories don't really apply when the level of light pollution is literally off the charts smile.gif

 

I've grown very fond of the views through my stopped down Nikon 18x58*.  I am sure there's a significant light loss and a loss of a few stars, but those pinpoint tight, intensely bright stars shining against a darkened background are very addicting to look at.  The thing about step-down rings is that it's easy to take them off so if I feel that I may be losing too many stars it's easy to get back to using the original Nikon 18x70.



#25 MT4

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Posted 16 July 2021 - 07:59 PM

BTW, MT4, how do you like the night performance of your 10x35 EII compared to the 8x30 or your 7x35s?  I see quite a positive difference in the night time with the 10x EIIs myself vs the others, and even despite the 5mm EP of the 7x35s. Try comparing the differences between them while you test your new chair.  You'll see everything better comfortably seated, I'm sure.  It could be informative.

 

I'm not experienced with heavily LP skies, fortunately for me, but I think you'll still see the differences between them even in Tokyo.

 

Rich

 

Hi Rich,

 

I've compared my Nikon EII 10x35 against my EII 8x30 on a number of occasions.  These are all quick-look type of comparisons and not scientific nor rigorous by any stretch, mostly because I am not experienced enough to be able to discern the minute details so I would typically just go with what my eyes see and my brain perceives. 

 

In a nutshell, for my severely light-polluted skies, both Nikons provide a nicely darkened background against which stars shine brightly and are pinpoint in the central 70% of the FOV.  Both have the same AFOV, which is very wide at 70 degrees.  The EII 10x35 has better reach for sure, but at the expense of a not-insignificant loss of TFOV.

 

Given my dismal local viewing conditions, I tend to use my 7x, 8x and 10x binoculars as companions to my bigger-aperture-higher-mag binoculars and not the main instruments for a typical observing session.  I am afraid that between the two instruments I prefer the Nikon EII 8x30 for its wide TFOV which provides wonderful context on any target I look at.  The EII 10x35's 7-deg TFOV is nothing to laugh at, but it still doesn't compare to the EII 8x30's 8.8-deg TFOV. 

 

My Kowa 7x35 with its 10-deg TFOV would provide even better context but the 5mm exit pupil might not be ideal for my brightly-lit skies as far as getting a darkened background is concerned.  That being said, I haven't had a chance to properly evaluate my Kowa 7x35 since I procured it a month ago.  The sky is just now starting to clear up.

 

Please note that the above is just for my skies where the average SQM reading is about 16.  I'd imagine that the 10x35's significant reach advantage would improve under darker skies where the balance may very well tip in its favor.


Edited by MT4, 16 July 2021 - 08:48 PM.



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