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Bortle scale accuracy?

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#1 Darren Drake

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Posted 20 June 2013 - 05:52 PM

I don't normally come over to this forum but i am hoping to clear something up. I was told once that the clear sky clock Bortle scale has had some areas assigned their respective colors based on info taken when snow was on the ground and is therefore inaccurate. These areas would be asigned colors that suggest the sites are not as good as they should be. My primary summer site in lower Michigan is a yellow zone but it seems more like a green zone to me...

#2 mak17

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Posted 20 June 2013 - 06:16 PM

Ive found it to be pretty accurate. Im in a grren zone now after observing in nothing but blue zones for a year and it is not quite as dark. The yellow zone near home in florida is not as dark as here. Ill be goong to a grey zone and anticipate it being darker then blue zone. I think where the accuracy problems come in with cleardarksky is with everything else.

#3 Tony Flanders

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Posted 21 June 2013 - 04:41 AM

I don't normally come over to this forum but i am hoping to clear something up. I was told once that the clear sky clock Bortle scale has had some areas assigned their respective colors based on info taken when snow was on the ground and is therefore inaccurate. These areas would be asigned colors that suggest the sites are not as good as they should be. My primary summer site in lower Michigan is a yellow zone but it seems more like a green zone to me...


First of all, please don't call it the Bortle scale. BrooksObs, the expert on the subject, is welcome to correct me. But as far as I'm concerned, any connection between these colors and the Bortle scale is very nearly coincidental.

The color zones come from the World Atlas of Light Pollution, based on satellite measurements taken in the late 1990s. The Bortle Dark-Sky Scale is a set of criteria for evaluating the actual condition of your skies based on what you can see with your own eyes.

In any case, you are correct about snow biasing the Light Pollution Atlas. The original post in this forum is here. And here is my blog on the subject.

The fact that someone in Tennessee finds the color zones accurate confirms this; there's not much snow in Tennessee.

#4 BrooksObs

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Posted 21 June 2013 - 08:23 AM

I would add to Tony's remarks that the combining of the Bortle Dark Sky Scale ratings with zones on light pollution maps was done without any input from me, so I cannot verify any degree of accuracy in this sort of application. In some cases it may prove reasonably accurate, while in other situations the ratings may be off significantly.

I will also point out a factor that nearly always fails to be taken into consideration in regard to these light pollution maps. That is the actual sky illumination visible at a given location created by distant sources and its potential impact on observing.

Maps based on satellite images are from looking down upon the light sources and from that perspective infers their degree of impact. What is not acknowledged in any fashion is the visibility from a distance and degree of illumination imparted to the surrounding atmosphere by these sources. Such can be apparrent from far beyond the location of the sources themselves.

Even at great distances from intense light sources the sky can be seen noticeably illuminated in the source's direction. Thus, light pollution maps can suggest a site to be one, or two, levels darker than it actually may be. Then, too, this impact varies over the course of the seasons, in many instances becoming enhanced further when the trees have lost their foliage and sometimes greatly so in winter when the ground is snow-covered.

The Bortle Dark Sky Scale is intended for an evaluation of sky darkness and clarity by the observer at the specific time he is at a site, not truly any sort of method for predicting conditions to be anticipated when going there.

BrooksObs

#5 Tony Flanders

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Posted 21 June 2013 - 09:07 AM

I will also point out a factor that nearly always fails to be taken into consideration in regard to these light pollution maps. That is the actual sky illumination visible at a given location created by distant sources and its potential impact on observing.

Maps based on satellite images are from looking down upon the light sources and from that perspective infers their degree of impact. What is not acknowledged in any fashion is the visibility from a distance and degree of illumination imparted to the surrounding atmosphere by these sources. Such can be apparrent from far beyond the location of the sources themselves.


Actually, that is factored into the Light Pollution Atlas. It uses Roy Garstang's well-known formula for light dispersion and applies it to the direct measurements from the DoD satellites. That's why, for instance, only the center of Adirondack State Park is shown as really dark. The park has huge sections that generate no light at all (no electric wires), but it's surrounded by large and small cities on most sides.

#6 BrooksObs

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Posted 21 June 2013 - 11:13 AM

The applications of formulae to supposedly account for effects of scatter at a distance is all well and good, but in the real world just how accurately does it do so?. What I've experienced over many years as an observer is that such formulae do not unusually indicate the true situation.

For example, I can tell you folks that the last time I was down at TSP I was rather shocked to note that I could definitely detect the presence of cities like Stockton and even Midland-Odessa, at very great distances from the ranch. And this impact was quite distinct well up into the sky, too! Now I don't believe for a moment that any formula would actually fully predict that to be the case.

Likewise, a very experienced observer friend of mine has spent occasional summers in the Adironack Park over many years. Although the light pollution maps suggest this region is a dark as is possible(black on their scale)he indicates being able to detect the presence of a very subtle glow in the southern sky from the NY metropolitan area ~250 miles to the south. Be assured that the Adirondacks are no Bortle Dark Sky Scale class 1 site these days.

If you can detect any trace of sky-glow be assured that it's impacting your overall level of sky darkness, whether some plot says it is, or not.

BrooksObs

#7 Tony Flanders

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Posted 21 June 2013 - 11:46 AM

The applications of formulae to supposedly account for effects of scatter at a distance is all well and good, but in the real world just how accurately does it do so?


Modestly well -- as well as you could expect. You can read Garstang's papers yourself if you want to know more.

The actual scatter will depend on the conditions on any given night. For instance, high clouds above a distant city make its light travel much farther. Conversely, low fog can suppress the light almost completely.

Likewise, a very experienced observer friend of mine has spent occasional summers in the Adironack Park over many years. Although the light pollution maps suggest this region is a dark as is possible(black on their scale)he indicates being able to detect the presence of a very subtle glow in the southern sky from the NY metropolitan area ~250 miles to the south. Be assured that the Adirondacks are no Bortle Dark Sky Scale class 1 site these days.


Actually, the Light Pollution Atlas shows the darkest part of the Adirondacks as gray, the second-darkest zone. It shows no black zones at all east of the Mississippi.

Don't be too quick to assume that the glow south of the Adirondacks is primarily from New York City. There's a solid line of medium-sized cities just south of the park. Albany puts out a huge amount of light, as I know to my sorrow.

#8 BrooksObs

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Posted 24 June 2013 - 08:36 AM

I'm sorry, Tony, but I don't for a minute buy your explanation of the Adironack skyglow I cited, as originating locally. The individual I'm quoting has a level of experience likely far beyond just about anyone on this forum and certainly knows whether the source of illumination he indicates was relatively near, or very far off. Likewise, I've distinctly seen the glow of the New York metropolitan area from the region well west of Albany myself.

The sad fact is that intense sources of light pollution are subtly evident hundreds of miles away on really clear nights. Darkest local conditions may often coincide with periods of higher humidity that makes the air less transparent, blocking distance light sources. I've already cited that Midland-Odessa's sky illumination is visible in the clear air at TSP. And some years ago I was informed by a professional pilot/amateur astronomer that on the Hawaii-LA run, from 35,000 feet it was possible to home in on the glow of LA from 600 miles out at sea!

Amateur astronomers still don't appreciate half the impact light pollution actually has.

BrooksObs

#9 derangedhermit

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Posted 25 June 2013 - 03:05 AM

I would add to Tony's remarks that the combining of the Bortle Dark Sky Scale ratings with zones on light pollution maps was done without any input from me, so I cannot verify any degree of accuracy in this sort of application. In some cases it may prove reasonably accurate, while in other situations the ratings may be off significantly.

I will also point out a factor that nearly always fails to be taken into consideration in regard to these light pollution maps. That is the actual sky illumination visible at a given location created by distant sources and its potential impact on observing.

Maps based on satellite images are from looking down upon the light sources and from that perspective infers their degree of impact. What is not acknowledged in any fashion is the visibility from a distance and degree of illumination imparted to the surrounding atmosphere by these sources.

BrooksObs

It's unfortunate that the misunderstanding still exists, but those who are aware that there is no connection between the Bortle scale and the World Atlas of Artificial Night Sky Brightness try to remove the confusion when aware of it.

No one would expect you to vouch for the accuracy of the other work once they understand that you are neither associated with it nor familiar with it, as you both state and demonstrate above.

#10 vsteblina

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Posted 25 June 2013 - 11:50 AM

For example, I can tell you folks that the last time I was down at TSP I was rather shocked to note that I could definitely detect the presence of cities like Stockton and even Midland-Odessa, at very great distances from the ranch. And this impact was quite distinct well up into the sky, too! Now I don't believe for a moment that any formula would actually fully predict that to be the case.

Likewise, a very experienced observer friend of mine has spent occasional summers in the Adironack Park over many years. Although the light pollution maps suggest this region is a dark as is possible(black on their scale)he indicates being able to detect the presence of a very subtle glow in the southern sky from the NY metropolitan area ~250 miles to the south. Be assured that the Adirondacks are no Bortle Dark Sky Scale class 1 site these days.

If you can detect any trace of sky-glow be assured that it's impacting your overall level of sky darkness, whether some plot says it is, or not.

BrooksObs


I agree. There are VERY few truly dark spots left in the lower 48.

Here is photo of the sky glow from Seattle in Wenatchee. Downtown Seattle is about 125 air miles from Wenatchee. Those mountains in the lower part of the picture are 9,000 feet and Seattle is on the OTHER side of them.

Check out the aurora and compare that to the sky glow from over a hundred miles away.

Posted Image

The other point is truly great nights are fairly rare in even dark sky areas. The two best nights I had were on the Beaverhead National Forest outside of Yellowstone and up at my cabin in the Washington Cascades. In that case, heavy fog covered the entire state with the house and observatory ABOVE the fog layer which prevented any "light leakage".

My observation has been ANY moisture or aerosols in the atmosphere REALLY affect the sky IF ANY light is visible.

#11 Tony Flanders

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Posted 25 June 2013 - 02:06 PM

Here is photo of the sky glow from Seattle in Wenatchee. Downtown Seattle is about 125 air miles from Wenatchee. Those mountains in the lower part of the picture are 9,000 feet.


For the record, Wenatchee is 95 miles from the center of Seattle. It's pretty depressing that the mountains didn't block more of the light, but that's likely due to the fact that Seattle had clouds above it. As I said, high clouds above a city make its light spread much farther.

I have a rather similar picture taken 120 miles north of the center of New York City.

Since the mountains are quite close to Wenatchee and the clouds are directly visible in the photo, they must actually be quite high.

Was this actually taken inside Wenatchee? If so, I'm surprised that the local light pollution wasn't more prominent.

The good side of all of this is that the Seattle skyglow seems to be confined to a fairly modest band along the horizon, not spreading very high.

#12 Tony Flanders

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Posted 25 June 2013 - 02:15 PM

Here is photo of the sky glow from Seattle in Wenatchee. Downtown Seattle is about 125 air miles from Wenatchee. Those mountains in the lower part of the picture are 9,000 feet.


Wait a sec ... something's wrong here. The city of Wenatchee is almost due east of Seattle, but this photo was taken to the south-southeast of Seattle, possibly near Mt. Rainier. That's the Big Dipper in the upper right corner, and Polaris is just out of the frame to the right.

#13 vsteblina

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Posted 25 June 2013 - 05:04 PM

Just took a compass bearing and the left side of the picture is due WEST. So the sky glow is from Seattle up to Everett and a little beyond.

You might be getting thrown by the fact that Wenatchee is significantly farther north than Boston.

I measured the distance on an atlas from our vacation home to downtown Seattle at a hundred miles. Our house in Wenatchee is 12 miles east of our vacation home.

Wenatchee does have considerable sky glow. The electricity rate is TWO cents a Kilo-watt hour. Talking to people about turning out lights to save money is a non-starter.

We are 5 miles north of downtown and up in elevation. Most of the town is south due to the rivers. There is only sage north of us.

I really do notice the effect of moisture and aerosols in the air. It does make a significant difference.

I am about 5.7 to the north.....and considerably less due south when looking over town.

The good news is Seattle is the cloudiest part of the United States (outside of Alaska) and Wenatchee according to the Chamber of Commerce gets 300 days of sunshine. The clouds in Seattle are low in elevation so most of the time we do not get to see Seattle's sky glow.

That picture does represent the worst sky glow from Seattle that I have seen. You are probably right about the high clouds reflecting the light from Seattle.

#14 Starhawk

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Posted 26 June 2013 - 07:35 PM

Going from the maps, we should be starting up an astronomy village in southeastern Oregon.

-Rich

#15 mountain monk

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Posted 26 June 2013 - 08:59 PM

Yep. Somewhere between Adel and Denio along Route 140. One lonely road, though the banjoes can sometimes be distracting.

Dark skies.

Jack

#16 Ekyprotic

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Posted 27 June 2013 - 04:11 AM

Where is the most light pollution free zone in the lower 48? I thought it was somewhere in Nebraska based on some LP maps I've seen- but maybe they're wrong?

Anyhow since we have some experts here I wanted to ask a question about the Bortle Scale. Hypothetically speaking, if we were able to remove all light pollution (natural and artificial) and have perfect seeing what would be the magnitude of the dimmest stars we could see? Magnitude 8, like the lowest levels on the Bortle Scale talk about? Do we have any records of the dimmest star a human has ever seen from the surface of the planet? I have heard that either the Sahara Desert or interior Antarctica provide the best opportunities for seeing stars that dim:

https://en.wikipedia...cal_observatory

and

https://en.wikipedia.org/wiki/Ridge_A

that one is wild..... they compare the seeing to the HST!

https://en.wikipedia...e_A#Observatory

I find Antarctica fascinating as it combines my two great loves- astronomy and extreme weather!

#17 Tony Flanders

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Posted 27 June 2013 - 05:16 AM

Hypothetically speaking, if we were able to remove all light pollution (natural and artificial) and have perfect seeing what would be the magnitude of the dimmest stars we could see? Magnitude 8, like the lowest levels on the Bortle Scale talk about?


I'm not really sure what you mean by natural light "pollution;" the phrase seems somewhat self-contradictory.

If you could remove all natural light sources, you wouldn't be able to see any stars at all, since the stars themselves are one of the three major sources of natural skyglow on a moonless night. The others are airglow and the zodiacal light. Airglow can be removed by going into outer space, but you have to get far from Earth to eliminate the zodiacal light.

Most of Earth's surface is in fact free of artificial light pollution -- think oceans. In such cases, the limiting factors are atmospheric clarity, natural skyglow, and each individual's eyes' ability to collect photons. For the faintest stars, we're talking about just a few photons per second falling onto a 7-mm pupil.

Some people have seen stars to magnitude 8.5; Barbara Wilson reports one night in the Andes when she could see every star in the 2nd edition of Sky Atlas 2000.0. John Bortle can (or could) certainly see stars to magnitude 8.0.

Different people have wildly different abilities to see faint stars. That's one of the reasons that John Bortle included multiple different criteria in his scale.

I have seen a few stars in the 7.0 - 7.5 range, but never consistently; I will see one star of mag 7.3 and then fail to see a nearby star of mag 7.1 no matter how carefully I look. This seems to be my own personal absolute limit, since the observations were from superb sites in southeastern Oregon and the Andes.

#18 Ekyprotic

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Posted 27 June 2013 - 06:05 AM

Thanks, Tony. I should have been more specific: if one could remove every source of light except for stars, what would be the dimmest stars one could see? It sounds like 8.5 is about the minimum then? I have searched far and wide and have not seen any reports anywhere about people being able to see stars of Mag 9 or lower. Like you said, there are some reports in the 8-8.5 range, I remember seeing 3 of them (reports, not stars haha), one going back to an old newsgroup, back in the mid-late 90s.

A related question is, since telescope minimum magnitude limits are based on Mag 6-6.5 as the visual viewing limit, is it fair to assume that in an incredibly dark Mag 8.5 sky, we can shift the scope's minimum magnitude limit by 2 levels? Thanks!

Another question I had is, I'm just getting into serious CCD imaging, is there any map source of seeing available, like we have light pollution maps? I'm trying to figure out what kind of pixel pitch I should look for- I live on Long Island very close to NYC and when I look up info online I get various different sources quoting numbers anywhere from 2" to 5" as the number of arc seconds each pixel of the CCD should cover. And applying Nyquist Theorem, divide that by 3. Thanks for any info you can provide on this.

#19 Tony Flanders

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Posted 27 June 2013 - 07:05 AM

Since telescope minimum magnitude limits are based on Mag 6-6.5 as the visual viewing limit, is it fair to assume that in an incredibly dark Mag 8.5 sky, we can shift the scope's minimum magnitude limit by 2 levels?


No. First of all, that mag 6-6.5 naked-eye limit is quite arbitrary. Second, because telescopes reach their limiting stellar magnitude at high magnifications, they're affected less by skyglow than naked eye or binoculars are. That's not to say that skyglow is unimportant for telescopic observation -- that's not true at all. But its less important because the high magnification already spreads out the skyglow.

In any case, take those tables of telescopic limiting magnitude with many, many grains of salt. In other words, they're really not worth much even in the best scenarios.

Another question I had is, I'm just getting into serious CCD imaging, is there any map source of seeing available, like we have light pollution maps?


That would be pointless. Light pollution varies considerably from one night to the next, but there's a certain degree of consistency on a typical night of good transparency.

The same cannot be said for seeing. It varies wildly from one night to the next, and even from one hour or minute to the next. Everywhere in the world has some times with sub-arcsecond seeing, and everywhere in the world has some times with attrocious seeing. The relative frequency varies considerably, but not in a way that can be encapsulated easily in maps.

#20 BrooksObs

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Posted 27 June 2013 - 11:17 AM

There are all sorts of claims as to how extremely faint certain individuals supposedly have seen, but very few of these sightings were done in any verifiable scientific fashion, or include details regarding how the observations were conducted. Even fewer accounts have actually been published in reliable astronomical journals.

Perhaps the faintest naked eye stars ever reported as actually seen and held with certainty was one of +8.3 seen in a series of sky tests conducted by professional astronomer H.D.Curtis during the first half of the 20th century. However, even in that instance his approach was so novel that I feel it to be not altogether trustworthy.

Relating personal experience, in the days long gone by when excellent dark skies were commonplace, I conducted my own series of tests over several years. I found that stars to +7.5 were seen normally from my observing site any good night and occasionally when conditions were outstanding I could glimpse +7.8-8.0 stars.

As a point of information, I do not ascribe to the idea that sensitivity in human vision varies all that much, assuming 20/20 vision and no defects, being governed far, far more by the observer's level of experience. Over the course of half a century I had the opportunity to observe side-by-side with some of the last century's greatest visual observers at excellent sites. Never did I encounter even one that exhibited unique visual sensitivity. In fact, all of them fell within a very narrow range of variation amounting to +/-0.2 magnitudes and all could see no fainter than 7.6-8.0 .

Tony brings up a situation that many observers experience, but few seem to understand. How can one see a 7.3 star, yet be unable to detect a nearby one listed as 7.1 (or even brighter)? The fact is that most visual people put too much credence in modern catalog values. In many instances CCD, or PEP, V magnitudes will correspond fairly well with what the human eye sees. However, "V" is not necessily equal to "v" and it often takes only a small degree of specific unusual emission in a star's spectrum to skew V rather dramatically relative to the response of the human eye. I have seen this exhibited on so many occasions during my association with the AAVSO that I just accept certain comparison stars in a variable's field as having off-kilter catalog values and simply don't use them in making my estimates.

BrooksObs

#21 vsteblina

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Posted 27 June 2013 - 03:12 PM

Where is the most light pollution free zone in the lower 48? I thought it was somewhere in Nebraska based on some LP maps I've seen- but maybe they're wrong?


Going from the maps, we should be starting up an astronomy village in southeastern Oregon.

-Rich


When I was working I had responsibilities for Wilderness management on about 2.4 million acres. At that time, the Forest Service and Park Service were just starting to look at light pollution and Wilderness.

I was curious about "pristine" skies in Wilderness.

I ran into a guy on the internet with a GIS background and he produced a map for me on parts of the country with NO LIGHT DOMES visible. The map is a mathematical model, but my quick checking seemed to indicate that it was fairly accurate.

There are no spots without a visible light dome in the eastern and central part of the country. Western Nebraska is where the first small spots show up.

The bulk of the areas without visible domes were eastern Oregon and northern Nevada.

It was amazing to see how little of the country really is under pristine skies.

Coming back from Arizona this spring we stopped in Alamo, Nevada. Not sure if your beyond the Las Vegas light dome at this point. But a line drawn from Alamo to St. George, Utah would pass through some really dark country.

There was a real weather change at Alamo. Southeastern Oregon is great country but I am not sure I want to spend a winter there.

There was one bankrupt development along 93 just south of Alamo, but I suspect the Las Vegas light dome would be visible from there.

Nevada with no income tax and fairly low taxes otherwise would be a good spot for an astronomical community. Warm weather at this point in life is probably the most important viewing consideration next to sky quality.

#22 Ekyprotic

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Posted 27 June 2013 - 10:59 PM

Thanks Tony, it sounds like the variance for seeing is too high to do some kind of averaging and get a general idea of what to expect as far as climo is concerned about seeing across different parts of the country?

#23 Ekyprotic

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Posted 27 June 2013 - 11:01 PM

The minimum viewable magnitude for different scopes seems to be pretty misleading, I just looked at various telescopes by both Meade and Celestron and their listed mininum viewable magnitudes were off by as much as one whole magnitude for telescopes of the same aperture!

#24 Ekyprotic

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Posted 27 June 2013 - 11:02 PM

Thanks Brooks- it sounds like Mag 8 is the extreme limit..... in your experience, what's the faintest and/or farthest away DSO anyone has seen visually?

#25 BPO

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Posted 28 June 2013 - 04:57 AM

The LP is practically non-existent at my Bortle 1/2 observatory site, but unfortunately my late-middle-aged eyes refuse to believe it.

Even up here at 2,000m ASL in NZ's South Island I still can't see half the things kids in urban areas can.






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