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Optimal exit pupil size for city light pollution

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#51 Rich V.

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Posted 19 July 2021 - 10:24 PM

MT4, you can go a long way with mounted 18x70s.  My 16x70 FMTs worked for me for years before I moved up to a bino telescope.  You just might have "gotten your cart before your horse", in a sense.   wink.gif    No problems with the 3.8mm exit pupil, really, you just won't have a "black velvet " sky background under your conditions but they'll still show plenty of stars.  More than you'll see stopping them down to darken the sky.  A 4° FOV gives you a good point of view to learn your way around the sky.  Use your smaller binos as "finders" to work out the larger patterns in the stars.

 

Both the 18x70s and the 82mm BT will amaze you under dark skies when you get the chance.  Then you can see the "diamonds on black velvet" you hear about when people under dark skies try to rub in in... laugh.gif

 

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#52 MT4

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Posted 19 July 2021 - 10:44 PM

Rich - You're absolutely right about my getting my cart before my horse.  I was reading about how people described seeing Messier objects and galaxies and thought that I needed higher powers and bigger apertures to achieve the same in my heavily light polluted skies.  Then I got really frustrated that I could never achieve the same no matter how high the mag and how big the aperture.

 

Having taken a step back and re-evaluated my approach, I now think it's much better to learn the sky first.  For the next couple of months, my Nikon 18x70 on a portable tripod/502 video head will be my workhorse setup, with my wide-field 7x and 8x as finders.  When the time comes for my trip up north, I think I'll be ready to be amazed with views of "diamonds on black velvet" through my Kowa Highlander smile.gif


Edited by MT4, 19 July 2021 - 11:35 PM.

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#53 Fiske

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Posted 19 July 2021 - 11:47 PM

MT4, 

 

After years of doing telescopic observing, and learning the constellations really as their bright asterism shapes to star hop to DSOs, I'm finding that exploring the entire constellation and figuring out where the borders are is remarkably enjoyable. And that can be done under light polluted skies. Plenty to see, colorful stars, lesser known asterisms, double stars. Just becoming familiar with star fields is fun. 

 

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#54 MT4

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Posted 21 July 2021 - 08:29 PM

Thanks Fiske for the recommendation to explore the constellations and figure out where their borders are.  I did exactly that last night on Serpens and Ophiuchus constellations.  I started from 1 Oph and 2 Oph, now an easily recognizable pair of jumping-off stars, and was able to follow the shape of the snake to its head and all the way back.  Then I walked up the right side of Ophiuchus to Rasalhague and Rasalgethi (Hercules).   I did this using three pairs of Nikon binoculars, Nikon Ell 8x30 (8.8 degs), NIkon EDG 7x42 (8 degs) and Nikon 18x70 (4 degs), in decreasing order of TFOV.

 

With the Nikon 8x30's 8.8-deg FOV, following the Serpens constellation was almost too easy a task, even for a newbie like me smile.gif   It was decidedly harder to do so with the 4-deg Nikon 18x70, but since I used the biggest Nikon last I was able to manage it just fine.  As a bonus, I found a region near Rasalgethi with many stars in the 18x Nikon it immediately gave me the impression of a bouquet of flowers.  Checking Stellarium and my Jumbo Pocket Sky Atlas, there's no name for this starry region so from now on I'll remember it as the bouquet of flowers right under Rasalgethi smile.gif

 

Lunar viewing was an essential part of my routine nowadays.  The moon was pretty low in the sky last night and it took a while for it to come into view near the top of a tree.   I was free standing with the Nikon 8x30 in my left hand and the Nikon 7x42 in my right hand.  Alternating between the two binoculars, it was hard to decide which one gave the better view.  The view through the Nikon 7x42 was decidedly brighter and flatter, but the Nikon 8x30 had the twin advantages of slightly bigger image scale and bigger FOV giving more context.  I gave up trying to compare the two instruments and instead just focused on enjoying the magnificent views of the moon hanging between the branches.

 

I spotted Vega high overhead and proceeded to test the Nikon 8x30's FOV.  No problems fitting the Lyra constellation in the 8x Nikon's FOV for a great view.  The Nikon 7x42's 8-deg FOV wasn't enough to frame the whole Lyra constellation but nevertheless gave a nice view, if only the background could have been a little darker as it should under darker skies.  The best instrument for viewing Lyra was without a doubt the 18x Nikon.  The Double-Double and another double star near Vega were easy to resolve in the 18x Nikon.  I learned later that each star in the Double Double was a double star but the 18x Nikon didn't have enough resolving power.  (It'll be a task for my Kowa Highlander Prominar.)

 

During my observing session, I spotted many helicopters and airplanes flying overhead.  It was really nice tracking those through my Nikon binoculars as they flew across the night sky and disappeared into the distance.  I suppose some of those helicopters might have been connected with the ill-fated Tokyo Olympic Games.

 

Due to my brightly-lit skies, the 6mm exit pupil of the Nikon EDG 7x42 caused the background to be a bit too bright for my observing taste.  As such, I found that I reached for the Nikon 8x30 more often than the Nikon 7x42 despite the former not having a flat field.  It didn't hurt that the Nikon 8x30 had a larger image scale and a slight larger FOV capable of framing the whole Lyra constellation.

 

I'll try masking down the Nikon 7x42 into a 7x35 and compare with the Nikon 8x30 again.


Edited by MT4, 21 July 2021 - 08:43 PM.

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#55 MT4

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Posted 22 July 2021 - 12:21 AM

There's a Nikon 7x50 SP being offered on a Japanese auction site.  The current price is about 100 USD and there's about 9 hours left to go.  Final price may reach 250 USD or so.  There's another one with a buy-now option for a little over 300 USD.

 

I've been very tempted to put in a bid for the chance to own a sample of the legendary instrument that is still considered by many to be the reference standard for edge-to-edge sharpness.  However, the experience with my Nikon EDG 7x42, an excellent instrument by the way, under my brightly-lit skies, has left me very reluctant.  I guess no 7mm exit pupils for my heavily light-polluted skies.  That'd rule out all the 8x56 Dialyts and Victory FLs that I've seen popping up on the same auction site.

 

Oh well, I've promised myself to not buy any more non-IS binoculars in the near term and instead focus on learning the sky.  Maybe it'd be wise to stick with that promise smile.gif


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#56 Rich V.

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Posted 22 July 2021 - 10:05 AM

I guess no 7mm exit pupils for my heavily light-polluted skies.  That'd rule out all the 8x56 Dialyts and Victory FLs that I've seen popping up on the same auction site.

 

For most anybody, they'll show the brightest image possible but that also means the sky will appear just as bright as it does by the naked eye, for better or worse.  Even under dark skies, a 7mm exit pupil still looks pretty bright to me (as there is no really dark sky) and I still prefer a smaller exit pupil that provides an aesthetically more pleasing view for many objects, even if it's not the brightest view. 

 

Rich.


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#57 MT4

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Posted 22 July 2021 - 11:05 AM

My Nikon EDG 7x42 has been masked down to 37.5mm for an exit pupil of 5.3mm, down from the original 6mm.  Similarly, my Nikon 18x70 has been masked down to 62mm for an exit pupil of 3.4mm.

 

In both cases, I am giving up about 20% light gathering for the sake of getting more aesthetically pleasing views of celestial objects.  I consider this light gathering loss, and the resultant loss of a few stars, to be the cost of dealing with severe light pollution where I am.  When I get to darker sites, the aperture masks / step-down rings can be easily taken off.



#58 pat in los angeles basin

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Posted 22 July 2021 - 11:19 AM

MT4; Can you actually perceive the darkening of the night skies with such a modest mask? I will have to try to mask down the  sears 8x50 I have to 30 and see if I can perceive the differences- clouds gods permitting (fat chance!).    Regards, Pat



#59 MT4

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Posted 22 July 2021 - 11:26 AM

MT4; Can you actually perceive the darkening of the night skies with such a modest mask? I will have to try to mask down the  sears 8x50 I have to 30 and see if I can perceive the differences- clouds gods permitting (fat chance!).    Regards, Pat

 

Yes I can.  After masking my Nikon EDG 7x42 down to 34mm, the view felt a bit dim.  Took off the aperture masks and the view felt a bit too bright for my taste.  So 37.5mm was my half-way compromise.

 

As noted earlier, for me the "ideal" exit pupil size that give aesthetically pleasing views depends on the mag.   The higher the mag, the lower the "ideal" exit pupil size.   This oddity might just be my eyes or might only apply under severely light polluted skies.



#60 TheUser

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Posted 22 July 2021 - 11:33 AM

when it comes to light pollution it's not about exit pupil size so much

 

everything should be simple: bigger the aperture higher the brightness, smaller the aperture higher the contrast; so it's up to you where you find the balance.

 

some people really say that image of DSO with small exit pupil is better, but I'm afraid there's many of personal here


Edited by TheUser, 22 July 2021 - 11:34 AM.


#61 MT4

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Posted 22 July 2021 - 11:42 AM

when it comes to light pollution it's not about exit pupil size so much

 

everything should be simple: bigger the aperture higher the brightness, smaller the aperture higher the contrast; so it's up to you where you find the balance.

 

some people really say that image of DSO with small exit pupil is better, but I'm afraid there's many of personal here

 

That's all true.

 

Except that in my case, my average SQM reading is under 17.  My local skies are always well lit, regardless of the hour of the day.   Therefore, too much brightness in the view is not a good thing for my local viewing conditions.  Hence, the masking experiments to figure out the "ideal" exit pupil sizes that would give me aesthetically pleasing views without losing too much light gathering.

 

I should add that just as Tony said near the top of this thread, in terms of sheer reach i.e. how many stars one can see, the higher the mag and the bigger the aperture the better.  



#62 ECP M42

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Posted 22 July 2021 - 03:51 PM

I find this set of statements rather paradoxical.

 

when it comes to light pollution it's not about exit pupil size so much

 

everything should be simple: bigger the aperture higher the brightness, smaller the aperture higher the contrast; so it's up to you where you find the balance.

 

some people really say that image of DSO with small exit pupil is better, but I'm afraid there's many of personal here

Visual observation is always a personal matter!
And it is therefore wrong to think exclusively of the instrument, without taking into account the individual who will use it.
The same tool placed in the eyes of various individuals will give different results due to the individuality of the eyes and perception. 

 

Contrast is the ratio of two levels of brightness. And contrary to what you say, the only characteristic that can regulate (or that can modulate) light pollution, by increasing or decreasing the apparent contrast, is precisely the exit pupil. Aperture alone says nothing about visible brightness, without first being related to magnification. Therefore, what really matters with binoculars is the exit pupil (but also with the telescope). 


Edited by ECP M42, 22 July 2021 - 03:53 PM.


#63 ECP M42

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Posted 22 July 2021 - 04:04 PM

 

for me the "ideal" exit pupil size that give aesthetically pleasing views depends on the mag.   The higher the mag, the lower the "ideal" exit pupil size.   This oddity might just be my eyes or might only apply under severely light polluted skies.

This oddity it is also visible in other observation conditions, for example during twilight. 

I've been studying it for a few years now and it seems to indicate that magnification has an influence on the brighter apparent vision. This is obviously a photometric inconsistency, but in fact, our perception does not follow the photometric rules to the letter, especially those that describe only the instrument. 

 

Complex speech that will encounter strong divergences, caused both by the narrow mindset and by differences in personal perception. 


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#64 Rich V.

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Posted 22 July 2021 - 11:08 PM

when it comes to light pollution it's not about exit pupil size so much

 

everything should be simple: bigger the aperture higher the brightness, smaller the aperture higher the contrast; so it's up to you where you find the balance.

 

some people really say that image of DSO with small exit pupil is better, but I'm afraid there's many of personal here

 

Well, it would be nice if everything were simple but, still, exit pupil determines overall brightness of the image.

 

Larger apertures show brighter stars but raising the magnification, thus reducing exit pupil size, at any given aperture increases the apparent contrast between sky background and stars. The smaller exit pupil darkens the background sky relative to the stars.  Smaller apertures just show dimmer stars.  A 10x25 bino won't show anywhere near as many stars per unit area as a 40x100 even though they have identical 2.5mm exit pupils.  DSO will be the same surface brightness in either but the image scale will be four times greater in the 100mm bino, illuminating 16x more area of your retinas.

 

On extended objects, for any given aperture, reducing the exit pupil dims the background sky and extended objects proportionally so contrast doesn't change.  Everything is just dimmer.  

 

Rich


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#65 MT4

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Posted 23 July 2021 - 01:36 AM

I just took my Nikon EDG 7x42 and EII 8x30 on a short hike along a tiny river in the south-western side of Tokyo.  It's called a river but it's barely 6-7 meters across, with a narrow walkway on just one side.  It's covered by exceedingly tall trees all around and there's a nice temple at the end of the walk.  Walking along the river felt like stepping into an alternate universe that is quite different from regular Tokyo city life.

 

What I discovered was that I enjoyed the view through the EDG 7x42 quite a bit more than the EII 8x30, an excellent instrument in its own right.  This preference had nothing to do with weight, size, FOV, or presence/lack of a field flattener.  Instead, it all came down to the EDG 7x42 having a 6mm exit pupil vs. the Ell 8x30's 3.8mm exit pupil.  I put my new 37.5mm aperture masks on the EDG 7x42 but found that I still preferred the 6mm exit pupil.

 

In short, I like the 6mm exit pupil of the EDG 7x42 for daytime use where the view is breathtakingly sharp, bright, and so very easy to take in.  For nighttime use, I'll put on my 37.5mm aperture masks to dim the view including the background sky for a more aesthetically pleasing view.

 

The aperture masking experiments that I've done recently have turned out to be quite a money saver for me.  I now no longer consider 7x50 / 8x56 / 8.5x42 / 8.5x44 / 10x70 instruments for nighttime use, nor 7x25 / 8x25 / 10x25 instruments for daytime use.

 

To paraphrase something I've learned on CN and add a bit of a qualifier on exit pupil:  "Every body needs a good 7x with an exit pupil of 6mm or more." smile.gif



#66 ECP M42

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Posted 23 July 2021 - 03:02 AM

"Every body needs a good 7x with an exit pupil of 6mm or more." smile.gif

Sin! I just opened a theme on 7x35 with 5mm pupil bawling.gif

 

 

 

PS: What is it that changes you during the day to use binoculars with 6mm or 3.75mm pupils?


Edited by ECP M42, 23 July 2021 - 03:04 AM.

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#67 MT4

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Posted 23 July 2021 - 03:36 AM

Sin! I just opened a theme on 7x35 with 5mm pupil bawling.gif

 

 

 

PS: What is it that changes you during the day to use binoculars with 6mm or 3.75mm pupils?

 

It's the ease of taking in the view, thanks to the large exit pupil and the low mag.   It was so easy I could use my 780g Nikon EDG 7x42 one-handed.



#68 TheUser

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Posted 23 July 2021 - 04:21 AM

sharp and bright views depends also on optics coating, not only the exit pupils size



#69 MT4

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Posted 23 July 2021 - 05:00 AM

sharp and bright views depends also on optics coating, not only the exit pupils size

That's true enough.

 

In this case, the comparison was between one of Nikon's best-ever porro binoculars, the modern classic that is the Ell 8x30 and Nikon's current top of the line roof binocular, the EDG 7x42.  Both were bought new around the same time late last year.  One would hope that coatings would be more or less comparable.

 

In any event, the deciding factor for me, for daytime use, wasn't brightness or sharpness.  It was all about the ease of taking in the view when you have a large exit pupil.  This is in stark contrast to night time use for which I have a clear preference for smaller exit pupils, due to the off-the-charts level of light pollution where I am.


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#70 TheUser

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Posted 23 July 2021 - 06:15 AM

...

It was all about the ease of taking in the view when you have a large exit pupil

...

 

would you please tell how exit pupil size help on taking the view? isn't it a FOV which is matter here?
 



#71 MT4

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Posted 23 July 2021 - 07:35 AM

would you please tell how exit pupil size help on taking the view? isn't it a FOV which is matter here?
 

 

It's all about ease of eye positioning.

 

Let's take an extreme case of a small exit pupil, say 1mm.   If your eyes are positioned to be off by 1mm, you see nothing.   It should be obvious that a large exit pupil of 6mm is much more forgiving as far as eye positioning is concerned.


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#72 Northern

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Posted 23 July 2021 - 08:01 AM

The larger the exit pupil, the easier it is to center your own eyes within the "sweet spot"

 

A larger exit pupil then 7mm isn't needed, but it would make the binocular very comfortable to look through.


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#73 MartinPond

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Posted 23 July 2021 - 08:31 AM

For me, under messy skies,

more power cannot increase contrast that doesn't exist.

The assumption that the grey level is almost black is invalid.

   At least,,,,,here.   It just isn't true.

 

I find the best brightness, around 5mm exit pupil,

   helps my optic cortex do the signal processing to see 

   more subtle colors and details.



#74 pat in los angeles basin

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Posted 23 July 2021 - 10:19 AM

Strangely, I've found 5 and 6 mm exit  pupil binos that still have fussy eye placement. A shrine manon  and a universe brand (both 7x35 10 DEG) come to mind- unless you have the IPD set right and position the eye just so- meh , pretty crappy view. Take the time and get everything just right and quite rewarding. I put those anomalies down to eyepiece design but can't prove it. .  Pat


Edited by pat in los angeles basin, 23 July 2021 - 10:20 AM.

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#75 MT4

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Posted 23 July 2021 - 11:42 AM

Strangely, I've found 5 and 6 mm exit  pupil binos that still have fussy eye placement. A shrine manon  and a universe brand (both 7x35 10 DEG) come to mind- unless you have the IPD set right and position the eye just so- meh , pretty crappy view. Take the time and get everything just right and quite rewarding. I put those anomalies down to eyepiece design but can't prove it. .  Pat

 

I think I know what you're talking about.   My Fujinon FMT-SX 10x50 is quite fussy with eye placement, despite having a 5mm exit pupil.   I think it's definitely something to do with eyepiece design.

 

Fortunately, my Nikon EDG 7x42 has no such fussiness with eye placement.  Its 6mm exit pupil is so easy to handle that it's a pure joy to use when standing up and walking around pointing at targets at a moment's notice.  It doesn't hurt that the 8-deg FOV is stunningly sharp right to the edge.  I've heard the Nikon EDG 7x42 described as Nikon 7x50 SP's equivalent in roof prism format.




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