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Bortle 1 vs. 2.

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#26 ghayduke

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Posted 22 February 2024 - 11:36 AM

In 2003 I wrote John Bortle and asked what exactly the difference was between class 1 and 2 in terms of artificial sky glow, he did not reply. I suggested it was the seeing and transparency that his qualitative descriptions refer to, specifically the naked eye limiting magnitude. NELM is very seeing dependent. The best I've ever done is 7.7 at only 1500 feet above sea level but under exceptionally steady and transparent skies and after 3 hours of dark adaptation.

As others have stated the natural airglow varies a lot, especially near solar maximum, at its brightest even under the steadiest and clearest skies and far from artificial sky glow, one is lucky to glimpse 6.0-6.5 magnitude stars since the sky is so bright. I have often been disappointed traveling to a very remote area only to have a bright airglow night. If you seek out the darkest skies, do so near solar minimum. I remember in 2007 and 2008 in Death Valley and Grand Canyon National Parks experiencing magnificent naked eye views of the night sky.

If you take Bortle literally and insist that all of the characteristics he describes be met for Class 1 and 2, there is nowhere left in the lower 48 that meets them if you have clear level horizons (on a mountain top), as he states "some indication of light pollution is evident along the horizon" does not appear until Class 3. My experience these days is that this is true everywhere, even in Big Bend or southern Utah. 


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#27 Tony Flanders

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Posted 22 February 2024 - 03:00 PM

If you take Bortle literally and insist that all of the characteristics he describes be met for Class 1 and 2, there is nowhere left in the lower 48 that meets them if you have clear level horizons (on a mountain top), as he states "some indication of light pollution is evident along the horizon" does not appear until Class 3. My experience these days is that this is true everywhere, even in Big Bend or southern Utah.


No doubt. I think the spot I've been to in the U.S. that has the least artificial light pollution is likely Steens Mountain, but several towns are clearly visible from the top. On the other hand, I see no reason to think that they contribute enough light to increase the skyglow significantly. And they're not bright enough to affect dark adaptation either.

 

I think Bortle's descriptions of light domes are arguably the single weakest aspect of his article.



#28 KBHornblower

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Posted 24 February 2024 - 09:28 PM

I agree that Bortle's presentation of his scale is ambiguous in the dark end.  He included his personal naked eye limiting magnitude (NELM) for stars in each level, despite having said in the preceding article that it is not reliable.  In addition, an obvious light dome over a distant city is not the only thing that can brighten the zenith, and for a remote, desolate area I can imagine that the zenith could be virtually unaffected.  Scattered local lights could significantly brighten the zenith without making any visible domes.

 

I am in the process of making a simulator for testing my own response to various amounts of skyglow.  I have a good legacy cathode ray tube (CRT) monitor which can get darker than 23 on my SQM-L meter, and I have used the meter to calibrate the full range of brightness in my Corel PaintShop Pro software.  I can create a background of any desired brightness, and add 1-pixel spots for stars.  For large faint fuzzies such as the gegenschein, I can calculate the reduction in contrast with increasing skyglow and make test patterns accordingly.  I have the CRT set up in my old photo darkroom so I can get my eyes fully dark adapted.  More on this later.


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#29 KBHornblower

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Posted 25 February 2024 - 11:35 AM

I found that making faint fuzzy test patterns is going to be more complicated than I anticipated.  The gradations of brightness in PaintShop Pro are too coarse at these low light levels.  I will try using something resembling a dot matrix to feather the center into the background and defocus it with close-up glasses while viewing from a distance.



#30 TayM57

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Posted 25 February 2024 - 11:56 AM

No doubt. I think the spot I've been to in the U.S. that has the least artificial light pollution is likely Steens Mountain, but several towns are clearly visible from the top. On the other hand, I see no reason to think that they contribute enough light to increase the skyglow significantly. And they're not bright enough to affect dark adaptation either.

 

I think Bortle's descriptions of light domes are arguably the single weakest aspect of his article.

I was at Steens two years ago, near summer solstice and the night sky was never quite dark. And indeed, there was several sky domes visible from the top at about 9,500 feet.

 

Down at Fish Lake, it's a bit darker because the sky domes are blocked from view.
 



#31 Redbetter

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Posted 25 February 2024 - 07:11 PM

I was at Steens two years ago, near summer solstice and the night sky was never quite dark. And indeed, there was several sky domes visible from the top at about 9,500 feet.

 

Down at Fish Lake, it's a bit darker because the sky domes are blocked from view.
 

Summer two years ago is when I started noticing how bright the sky was frequently becoming at dark sites.  Whether this was solar activity related or the volcanic eruption's impact on the upper atmosphere or a combination, I don't know.  I do know that in the past two years it has been difficult to assess a site's inherent quality with respect to light pollution based on visual appearance or a meter.  There are periodic darker nights interspersed but they have become the exception rather than the rule on the clearest nights.  When sites that have run 21.6 to 21.7 on good nights in recent years are running 21.0 several clear nights in a row, something major has changed.

 

The problem with Bortle scale reliance on light domes and horizon effects is that the horizon is not fixed.  It is dependent on elevation and any obstructions.  If one is on a barren peak with no trees and without substantial extinction, then direct light sources (never mind the accompanying light domes) can be visible for many miles around.  Intervening ridges often block this, as will taller peaks, or nearby structures or trees.  So to make sense of any light dome's impact one would need to be able to judge how high it reaches into the sky, rather than simply counting light domes.

 

Without a meter, you can't truly compare how dark the sky is overhead in a valley vs. on a ridge/peak, because of the differences in horizon.  What actually matters in my experience is how dark the sky is directly overhead.  I am not going to preferentially direct my observing toward the brighter quadrants, and most of my observing will be 45+ degrees regardless, because that is where the darkest and lowest extinction areas are. 

 

Probably one of the best indications for the level of light pollution is how mid-altitude to high altitude clouds appear overhead and somewhat toward the brighter portion of sky vs darker.  Do they create black spaces with sky glow or Milky Way glow between?  Are they similar in brightness to the back ground sky?  Or are they brighter?  The paradox is that we are trying to observe without clouds, so evaluating things with scattered cloud pushing through is a less desirable condition.



#32 vsteblina

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Posted 25 February 2024 - 08:06 PM

No doubt. I think the spot I've been to in the U.S. that has the least artificial light pollution is likely Steens Mountain, but several towns are clearly visible from the top. On the other hand, I see no reason to think that they contribute enough light to increase the skyglow significantly. And they're not bright enough to affect dark adaptation either.

 

I think Bortle's descriptions of light domes are arguably the single weakest aspect of his article.

The darkest night I ever had was on the Beaverhead National Forest in Montana.  Nothing has ever even come close.

 

I winter in Arizona, but it is pretty light polluted these days compared to my memories of 1977.  These days, the darkest skies I see are in central Nevada along the Highway 93 and Highway 318 corridors.

 

Great kudos, the Bortle for coming up with the concept. I do have issues with M33 and its visibility.

 

It might be a worthwhile exercise to redo "Bortle" to make it more consistent and accurate with actual measurements.

 

Can anybody convince Bortle to do a Bortle 2 update??
 



#33 KBHornblower

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Posted 29 February 2024 - 03:38 PM

I agree that Bortle's presentation of his scale is ambiguous in the dark end.  He included his personal naked eye limiting magnitude (NELM) for stars in each level, despite having said in the preceding article that it is not reliable.  In addition, an obvious light dome over a distant city is not the only thing that can brighten the zenith, and for a remote, desolate area I can imagine that the zenith could be virtually unaffected.  Scattered local lights could significantly brighten the zenith without making any visible domes.

 

I am in the process of making a simulator for testing my own response to various amounts of skyglow.  I have a good legacy cathode ray tube (CRT) monitor which can get darker than 23 on my SQM-L meter, and I have used the meter to calibrate the full range of brightness in my Corel PaintShop Pro software.  I can create a background of any desired brightness, and add 1-pixel spots for stars.  For large faint fuzzies such as the gegenschein, I can calculate the reduction in contrast with increasing skyglow and make test patterns accordingly.  I have the CRT set up in my old photo darkroom so I can get my eyes fully dark adapted.  More on this later.

 

I found that making faint fuzzy test patterns is going to be more complicated than I anticipated.  The gradations of brightness in PaintShop Pro are too coarse at these low light levels.  I will try using something resembling a dot matrix to feather the center into the background and defocus it with close-up glasses while viewing from a distance.

I made a test pattern of stars which appears to be a good match for the real stars when I sit 24" from the screen.  As I darkened the background from 18.0 to 20.0 on the SQM-L meter, my naked eye limiting magnitude (NELM) went from about 4.2 to about 5.2.  With further darkening the rate of changed started diminishing and reached about 5.8 at skyglow 22, typical of a pristine sky.  The darkest I could make the background was close to 25, and the NELM was about 6.3.  That was beyond the range of the meter, but I estimated it by covering a swatch of 22 with a neutral density filter.

 

My NELM was about equal to what John Bortle reported for a bright city sky in his article, but far short of his 7.5 or better in a pristine sky.  That is simply the difference between his eyes in his visual prime and my eyes at age 76.

 

My next test will be looking at dim swatches at the minimum difference from the background which I can make.



#34 KBHornblower

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Posted 29 February 2024 - 09:15 PM

I just now made a test pattern with faint extended objects, with the sky set about 22.  I made disks that subtended 6 and 0.6 degrees at my standard viewing distance.  The least brightening the computer would display at those low levels was about 25% brighter than the background.  I could see the large spots but not the small ones.  I brightened the small ones to about double the background and I still could not see them.  They became visible when I leaned in close as if using binoculars.  I think this explains why I cannot see M33 with naked eye, even at Spruce Knob in excellent transparency.  At my age I just am not wired like John Bortle in his prime, when he seemed to have an owl's night vision.  If I tried to use his system I might rate a pristine sky as 3 at best.



#35 ghayduke

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Posted 01 March 2024 - 02:11 PM

Remember Bortle Class is not just about the best views for astronomy, but the entire night-time scene or "lightscape" around you.

This might be of interest, presented in 2015 at the Artificial Light at Night conference in Sherbrooke, Canada. An attempt at Bortle v.2.0

https://artificialli...2015_bortle.pdf




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