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General Astronomy >> Light Pollution

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Darren Drake
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Reged: 10/09/02

Loc: Chicagoland
Bortle scale accuracy?
      #5931865 - 06/20/13 06:52 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...

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mak17
scholastic sledgehammer
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Reged: 02/08/11

Loc: Central Florida
Re: Bortle scale accuracy? new [Re: Darren Drake]
      #5931885 - 06/20/13 07:16 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.

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Tony Flanders
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Reged: 05/18/06

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Re: Bortle scale accuracy? new [Re: Darren Drake]
      #5932527 - 06/21/13 05:41 AM

Quote:

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.


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5932691 - 06/21/13 09:23 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

Edited by BrooksObs (06/21/13 09:38 AM)


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5932729 - 06/21/13 10:07 AM

Quote:

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.


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5932900 - 06/21/13 12:13 PM

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

Edited by BrooksObs (06/21/13 12:19 PM)


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5932940 - 06/21/13 12:46 PM

Quote:

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.

Quote:

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.


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5937666 - 06/24/13 09:36 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

Edited by BrooksObs (06/24/13 01:07 PM)


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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5939133 - 06/25/13 04:05 AM

Quote:

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.


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vsteblina
sage


Reged: 11/05/07

Loc: Wenatchee, Washington
Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5939727 - 06/25/13 12:50 PM

Quote:


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.



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.

Edited by vsteblina (06/25/13 04:10 PM)


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Tony Flanders
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Reged: 05/18/06

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Re: Bortle scale accuracy? new [Re: vsteblina]
      #5939959 - 06/25/13 03:06 PM

Quote:

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.


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: vsteblina]
      #5939972 - 06/25/13 03:15 PM

Quote:

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.

Edited by Tony Flanders (06/25/13 04:13 PM)


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vsteblina
sage


Reged: 11/05/07

Loc: Wenatchee, Washington
Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5940234 - 06/25/13 06:04 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.


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Starhawk
Space Ranger
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: Bortle scale accuracy? new [Re: vsteblina]
      #5942087 - 06/26/13 08:35 PM

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

-Rich


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mountain monk
Carpal Tunnel
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Reged: 11/06/09

Loc: Grand Teton National Park
Re: Bortle scale accuracy? new [Re: Starhawk]
      #5942219 - 06/26/13 09:59 PM

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

Dark skies.

Jack


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Ekyprotic
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Reged: 11/28/12

Re: Bortle scale accuracy? new [Re: mountain monk]
      #5942537 - 06/27/13 05:11 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.org/wiki/Dome_C#Astronomical_observatory

and

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

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

https://en.wikipedia.org/wiki/Dome_A#Observatory

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


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5942574 - 06/27/13 06:16 AM

Quote:

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.


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5942592 - 06/27/13 07:05 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.


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5942622 - 06/27/13 08:05 AM

Quote:

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.

Quote:

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.


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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5943013 - 06/27/13 12:17 PM

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

Edited by BrooksObs (06/27/13 02:48 PM)


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vsteblina
sage


Reged: 11/05/07

Loc: Wenatchee, Washington
Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5943361 - 06/27/13 04:12 PM

Quote:

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?




Quote:

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.


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Ekyprotic
super member
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Reged: 11/28/12

Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5944046 - 06/27/13 11:59 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?

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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5944049 - 06/28/13 12:01 AM

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!

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Ekyprotic
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5944050 - 06/28/13 12:02 AM

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?

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BPO
sage


Reged: 02/23/10

Loc: South Island, NZ
Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5944244 - 06/28/13 05:57 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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BrooksObs
professor emeritus


Reged: 12/08/12

Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5944391 - 06/28/13 08:37 AM

Quote:

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?




That could certainly be a matter of unresolvable controversy. In my experience the catalog V magnitudes of most very faint/exceedingly distant galaxy-like objects are so questionable that it makes it very hard to point to any clear choice.

There is even the problem of observers "thinking" they are detecting some object that seems to be glimpsed at the limits of there instruments. Mistaken sightings of threshold objects are far more common than the amateur community wishes to recognize.

That said, there have been fairly well confirmed sighting of very faint quasars situated far beyond the more normal galaxies.

BrooksObs

Edited by BrooksObs (06/28/13 08:38 AM)


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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5944636 - 06/28/13 11:33 AM

Quote:

Quote:

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?




That could certainly be a matter of unresolvable controversy. In my experience the catalog V magnitudes of most very faint/exceedingly distant galaxy-like objects are so questionable that it makes it very hard to point to any clear choice.




Taking this -- for argument's sake -- to mean visible naked-eye, M81 is almost certainly the winner on both counts. Enough reputable observers have reported spotting it naked-eye so that I don't have any real doubts about its visiblity.

Some people have reported seeing M82, but those sightings are a lot more suspect.

A few studies place Centaurus A (also naked-eye visible) farther than M81, but the consensus appears to be that it's closer. I agree with BrooksObs that V magnitudes of galaxies are suspect, and distances are probably even more suspect.


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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5947889 - 06/30/13 11:29 AM

All excellent points, BrooksObs...

Quote:

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.




This is in complete agreement with my own experience from long ago, at truly dark sites; it should be noted that the slightest increase in atmospheric water vapor seemed to have a disproportionately large effect on these relatively deep limiting magnitudes.

Quote:

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 .




Again, this has been my experience also. It would appear that there is very little difference in perceived limiting magnitude between individuals with good vision, under identical observing circumstances.

Quote:

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.




Precisely. Photometers and CCD's are not the human eye, and their results in regards to visibility of a given star at the limits of perception should not be taken as gospel.

Fred


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Re: Bortle scale accuracy? new [Re: amicus sidera]
      #5948549 - 06/30/13 07:31 PM

The US Armed Forces, among others, have been studying scotopic vision since at least the beginning of WW II, and they and the studies continue. One can find results using an Internet search engine that consistently show scotopic vision is best between ages 22-28, using a number of standard tests.

Scotopic vision tests show variation across test subjects of similar age ranging from 1.3x to 2x, depending on the study and type of test. A few of the tests also revealed substantial (around 20%) variability in each person's test results, based on the season of the year. Apparently summer, with brighter sunlight and longer days, reduces people's ability to dark-adapt.

Around age 20 the average pupil is at its largest. By age 30, scotopic vision has begun to slowly worsen. By age 50, on average, a person has lost one magnitude of light collection (~5mm pupil), and other symptoms of aging eyes are beginning to appear. By age 70, on average, a person has lost another magnitude of light collection (~3.2mm pupil), lost most of the ability to accommodate, and has significant deterioration in at least some of the components of the eye that affect vision.


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amicus sidera
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Re: Bortle scale accuracy? new [Re: derangedhermit]
      #5948599 - 06/30/13 08:00 PM

Quote:

The US Armed Forces, among others, have been studying scotopic vision since at least the beginning of WW II, and they and the studies continue. One can find results using an Internet search engine that consistently show scotopic vision is best between ages 22-28, using a number of standard tests.

Scotopic vision tests show variation across test subjects of similar age ranging from 1.3x to 2x, depending on the study and type of test. A few of the tests also revealed substantial (around 20%) variability in each person's test results, based on the season of the year. Apparently summer, with brighter sunlight and longer days, reduces people's ability to dark-adapt.

Around age 20 the average pupil is at its largest. By age 30, scotopic vision has begun to slowly worsen. By age 50, on average, a person has lost one magnitude of light collection (~5mm pupil), and other symptoms of aging eyes are beginning to appear. By age 70, on average, a person has lost another magnitude of light collection (~3.2mm pupil), lost most of the ability to accommodate, and has significant deterioration in at least some of the components of the eye that affect vision.




Very interesting, Lee, thank you for mentioning it... I believe that Clark used some of that same data in his excellent book Visual Astronomy of the Deep Sky.

One thing touched upon in that material concerns the effect of sunlight on dark adaptation; Clark mentions, and I've found, that essential to reaching the deepest magnitude possible on a given night, shielding ones eyes from sunlight on the day (or better yet, days) prior to observations being made is imperative. This is no doubt a variable that would help to explain the wide discrepancies in limiting magnitudes experienced between observers of similar age and skill level that have occasionally occurred.

Fred


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Re: Bortle scale accuracy? new [Re: amicus sidera]
      #5948960 - 07/01/13 12:59 AM

The next step would be, what steps can we take to keep our vision at an ideal level or, even improve it? I wonder if there will be any surgical techniques or medication available in the next few decades to improve the side effects of aging in these areas (specificially scotopic vision.) I know memory and cognitive research has been in the forefront, hopefully this issue is also getting some attention.

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Re: Bortle scale accuracy? new [Re: amicus sidera]
      #5949195 - 07/01/13 07:30 AM

Quote:

Quote:

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 .




Again, this has been my experience also. It would appear that there is very little difference in perceived limiting magnitude between individuals with good vision, under identical observing circumstances.




I wouldn't dispute either statement -- the operant term being "good vision."

Among the 95% of humanity that doesn't have good vision -- me included -- the range of limiting stellar magnitudes under identical skies is huge -- almost two full magnitudes. I have experienced this first-hand.

The major factor is probably acuity. Good daytime acuity doesn't necessary imply good nighttime acuity. Very few people have really sharp vision when their pupils are wide open, even with the best eyeglass correction available.


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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5949349 - 07/01/13 09:44 AM

"The US Armed Forces, among others, have been studying scotopic vision since at least the beginning of WW II, and they and the studies continue."

Indeed, and I've read some of the papers addressing the results. However...the cross section of those subjects tested did not share any particular degree of training/experience in the detection of extremely faint sources like the real stars. As I pointed out upstream, this is a critical factor and why some long-time observers in specific areas report seemingly seeing so much fainter than the run-of-mill newcomers, or casual observers. Nothing beats long and intense practice and experience in observing.

"Around age 20 the average pupil is at its largest."

Another conclusion that I've often wondered about. Personally, I've long been aware that quite young children seem capable of seeing "things" that are beyond the detection limits of 99.999% of adults.

As a pre-teen I recall viewing the summer heavens from a dark site in the upper Mid-Hudson Valley. I was already fairly well versed in the hobby, so I knew exactly what I was looking at. I will only say that I found a host of Messier objects clearly visible (downright obvious!) to the naked eye and the Milky Way looked to me liked a long exposed photo. Near zenith it spread out westward from the Cygnus Rift, across all of Lyra and as far into Hercules as M13! As an adult I've since been to some of the darkest sites in the world and never saw anything that has come close to those views I had as a kid.

There is also the matter of resolution and acuity. There are numerous reports of children seeing various of the moons of Jupiter without optical aid and before knowing they are even there.

Likewise, when my youngest son was about 7 or 8 I had him out one nice gibbous moonlit night looking at the stars and planets. The moon was high in the sky and far from any earthly reference points. He was looking up at the moon (before we ever got to look through my telescope) and he remarked, "Daddy, how come the moon is moving?" I immediately thought that he must mean across the sky over the course of the night and I started to explain. But he corrected by saying, "No I mean right now as I'm looking at it!" I carefully talked with him about what he was seeing and it became obvious that the moon's diurnal motion in the sky was clearly apparent to him! He related that the moon seemed to move at a speed like the hands of the clock in our kitchen.

Ever since that incident I've had a great deal of respect for what children might tell me about what they see in the environment.

BroksObs

Edited by BrooksObs (07/01/13 11:38 AM)


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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5949793 - 07/01/13 02:27 PM

The studies I read didn't include children. I think scotopic vision rarely gets worse by age 20, since for some reason it is reported that it peaks in the mid-20's. They did not use point sources as part of the testing that I read. They used a variety of target sizes and shapes at a variety of on- and off-axis locations and illumination levels; that is, they used "extended objects". Are you saying the ability to detect the faintest stars is not closely correlated with the ability to see faint extended objects? The Bortle scale directly associates naked-eye limiting magnitude with the ability to see extended objects. Should that connection not exist?

Are you suggesting that extensive training, given to both age groups, in the use of scotopic vision would remove the 2+ magnitude difference between 20 year-old eyes and 70 year-old eyes? Otherwise the amount of training prior to the test is immaterial.

If you saw a lot of stuff naked-eye as a kid that you cannot see as an adult from some of the world's darkest sites, then one may reasonably conclude that young eyes and some experience ("moderately well versed") does beat long experience. Many of the world's darkest sites, even today, have less than 1% light pollution, by direct measurement.


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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5949827 - 07/01/13 02:44 PM

Quote:

The next step would be, what steps can we take to keep our vision at an ideal level or, even improve it? I wonder if there will be any surgical techniques or medication available in the next few decades to improve the side effects of aging in these areas (specificially scotopic vision.) I know memory and cognitive research has been in the forefront, hopefully this issue is also getting some attention.



Adequate vitamin A in the diet is about it. There are already some procedures, like lens replacements, that help in some cases. Replacing the aqueous humour in the eye is a high-risk procedure currently.

Stopping smoking has the biggest effect on improving night vision, once adequate vitamin A is in the diet - on the order of a 20% improvement.

The average eye focal length is about 22mm, so it operates in an f-ratio range of from about f/3 to about f/30.


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: derangedhermit]
      #5949834 - 07/01/13 02:51 PM

Quote:

Are you saying the ability to detect the faintest stars is not closely correlated with the ability to see faint extended objects?




That is my experience. No doubt there is some correlation, but it's weak.

I consider myself quite good at seeing faint extended objects, but only so-so at seeing faint stars. Acuity is an issue for seeing stars, much less so for seeing faint fuzzies.


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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5950003 - 07/01/13 04:26 PM

"Are you suggesting that extensive training, given to both age groups, in the use of scotopic vision would remove the 2+ magnitude difference between 20 year-old eyes and 70 year-old eyes? Otherwise the amount of training prior to the test is immaterial." - derangedhermit

As it happens, I can offer a story that addresses just that. Back in 1968, 69, and 70 I had opportunities to observe side by side with the late Leslie Peltier, America's foremost visual observer of the 20th century. He was 68 and I was 25 (but already a highly experienced observer of variable stars). Both he and I recorded virtually identical limiting magnitudes while observing together. Later, in conversation, Leslie related that in his comet hunting days many years before he could often see stars to about +8.0, just like I could at that time. So, indeed, intense training of the eyes offsets the effects of aging to a large degree.

BrooksObs

Edited by BrooksObs (07/01/13 04:29 PM)


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derangedhermit
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5950040 - 07/01/13 05:08 PM

Quote:

Quote:

Are you saying the ability to detect the faintest stars is not closely correlated with the ability to see faint extended objects?




That is my experience. No doubt there is some correlation, but it's weak.

I consider myself quite good at seeing faint extended objects, but only so-so at seeing faint stars. Acuity is an issue for seeing stars, much less so for seeing faint fuzzies.



That certainly makes sense to me.

Lee


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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5950049 - 07/01/13 05:19 PM

Quote:

As it happens, I can offer a story that addresses just that. Back in 1968, 69, and 70 I had opportunities to observe side by side with the late Leslie Peltier, America's foremost visual observer of the 20th century. He was 68 and I was 25 (but already a highly experienced observer of variable stars). Both he and I recorded virtually identical limiting magnitudes while observing together. Later, in conversation, Leslie related that in his comet hunting days many years before he could often see stars to about +8.0, just like I could at that time. So, indeed, intense training of the eyes offsets the effects of aging to a large degree.

BrooksObs



I'm trying to understand this. You could see ~+8, he could see a magnitude or two less - naked eye. When at the eyepiece, your limiting magnitudes were almost identical. Were you were using telescope and eyepiece combinations such that the exit pupils would put you two on more equal footing (that is, not at very low power)? That seems like one factor that could make a big difference in equalizing what can be seen by young and old when using telescopes.

Lee


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Re: Bortle scale accuracy? new [Re: derangedhermit]
      #5950913 - 07/02/13 08:54 AM

derangedhermit - Re my intercomparison of LM with Leslie Peltier, by the time I first visited with him he was living within the confines of Delphos itself rather than his earlier country location, so skies weren't as dark and pristine as in his earlier years. Thus, my comparison came while employing his 6-inch f/5 comet seeker working at 25x, or a bit less. This offered an exit pupil of slightly over 6mm, quite within the anticipated size of my own pupils at the time.

With this instrument and magnification we both reached the same limiting magnitudes when employing a couple of AAVSO charts. These, I would note, were far better suited to such a task than references to the brightness's of nakedeye stars at that time.

The situation between Leslie and I was hardly unique either. Such results were repeated a number of times over the years with myself and other members of AAVSO widely varying in age (but all highly skilled in their art).

BrooksObs

Edited by BrooksObs (07/02/13 08:58 AM)


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derangedhermit
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5952260 - 07/02/13 11:13 PM

Thanks for the answer, BrooksObs.

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Re: Bortle scale accuracy? new [Re: derangedhermit]
      #5959507 - 07/07/13 07:52 PM

As further documentation concerning the limits of human vision, I just recently came across a couple of published papers presenting results of tests apparently conducted by the military addressing naked eye limiting magnitudes.

One paper, by the Office of Scientific Research and Development, cites a mean of +7.7 as "the faintest star visible with the unaided eye" based on their work.

Another paper, this one by Langmuir and Westendorp, indicated the "average" of their findings to also be +7.7, noting that subjects exhibited a range of 7.4-8.0 magnitude.

BrooksObs

Edited by BrooksObs (07/07/13 08:03 PM)


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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5976757 - 07/17/13 08:20 PM

Thanks, Brooks! That woman you cited, the faintest stars she saw were three near 8.0? I wonder what percentage of the earth's surface actually has the ideal conditions necessary to see that dim.

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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5976794 - 07/17/13 08:39 PM

Being from a highly light polluted area, and never having seen anything better than Mag 5.5 skies myself, when I simulate a Mag 8.0 sky in Starry Night Pro Plus, the sky is littered with stars- there actually seems to be more area covered by stars than there is black sky. Is this how the sky looks to you guys, or can you only see some/few of the 8.0 stars? I wonder what makes only some of the 8.0 stars visible and not the others (since they are all around the same brightness.)

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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5977068 - 07/17/13 11:50 PM

In The Messier Objects, O'Meara says that on Mauna Kea, magnitude 8.5 stars are "within grasp of the naked eye..." (p.29) He also says the he "consistently detected stars as faint as 8.4 magnitude..." (p.31) The subject remains mysterious (for me), but then O'Meara probably sees better than an owl (some humans do).

Dark skies.

Jack


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Re: Bortle scale accuracy? new [Re: mountain monk]
      #5977236 - 07/18/13 03:36 AM

That's fascinating! I've heard that some parts of Antarctica may possibly be even more conducive to visual observation of extremely low magnitude stars (Dome C, Dome A, Ridge A). I wonder if either there or at other locations, with much younger eyes (the story related earlier, about children being able to view the Galilean moons of Jupiter unaided, is fascinating), even dimmer stars than Mag 8.5 could be glimpsed?

Dome C

https://en.wikipedia.org/wiki/Dome_C#Astronomical_observatory

Writing in the Proceedings of the Astronomical Society of the Pacific in 2005, Karim Agabi et al. discuss the suitability of the site for astronomy in terms of the seeing.[1] They determined the median seeing (measured with a Differential Image Motion Monitor placed on top of an 8.5 metres (28 ft) high tower) to be 1.3±0.8 arcseconds. This is significantly worse than most major observatory sites, but similar to other observatories in Antarctica. However, they found (using balloons) that 87% of turbulence was below 36 meters. A telescope built on a tower could rise above this "boundary layer" and achieve excellent seeing. The boundary layer is 200 metres (660 ft) at the South Pole and may be as low as 20 metres (66 ft) at Dome A.
In an earlier (2004) paper, Lawrence et al. considered the site and concluded that "Dome C is the best ground-based site to develop a new astronomical observatory".[2] This team measured superior seeing of 0.27 arcseconds, twice as small as at Mauna Kea Observatory. This figure was taken with an instrument insensitive to near-ground turbulence and so it is comparable to the 0.35 arcseconds Agabi et al. measured for "free atmospheric seeing".

Ridge A

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

Ridge A was identified by a team of Australian and American scientists searching for the best observatory spot in the world.[1] The team leader described the site as "so calm there's almost no wind or weather there at all."[3] Ridge A is a low ridge of ice and has been estimated to have very low disturbances to visibility, such as thick atmospheric boundary layer, amount of water vapour and numerous others.
The site represents the "Eye of the Storm", whereby winds flowing off Antarctica in all directions appear to start from a point at Ridge A, where winds are at their calmest. It is also the site of a vortex in which swirling stratospheric winds high up and calm air at ground level combine to make it a place for viewing into space that is three times clearer than any other location on Earth.[4]
Researchers[who?] on the project suggested that photographs taken through a telescope at Ridge A could be nearly as good as those taken by the Hubble Space Telescope. Despite the difficult conditions on Antarctica and the remote location of Ridge A, construction costs for an observatory there that could match the Hubble telescope could be built at a fraction of the cost of sending Hubble into space.[4]


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Re: Bortle scale accuracy? new [Re: mountain monk]
      #5977473 - 07/18/13 09:08 AM

Quote:

In The Messier Objects, O'Meara says that on Mauna Kea, magnitude 8.5 stars are "within grasp of the naked eye..." (p.29) He also says the he "consistently detected stars as faint as 8.4 magnitude..." (p.31) The subject remains mysterious (for me), but then O'Meara probably sees better than an owl (some humans do).

Dark skies.

Jack




Jack, in point of fact from an observing side-by-side comparison, I've repeatedly found that Steve has no better a detection limit for stars than that of many other highly experienced observers, including myself. This idea that a certain few individuals are born with some manner of special eye sensitivity is a total fiction, although one that is accepted by many hobbyists. The one distinct advantage that Steve does have in seeing particularly faint stars is the occasional opportunity to observe from a MUCH great altitude (Mauna Kea) than nearly any of the rest of us can reach.

As I've pointed out several times previously, any person with normal 20/20 or 20/15 vision is quite capable of matching the best of the best if only they were to gain the necessary observing experience and observe under "excellent" skies...both of which do not seem to prevail for perhaps 95% of today's amateur astronomers.

BrooksObs

Edited by BrooksObs (07/18/13 09:12 AM)


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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5978043 - 07/18/13 02:32 PM

Interesting read...

I spend as many nights in northeastern Nevada as possible during observing season (all except winter) and find the skies to be as dark and transparent as anywhere I've been in the US. Seeing is fair most nights to very good on the best of nights.

My wide angle SQM (I just sent it back and had it recalibrated) shows darkness readings of 21.8-21.95 on good nights and on nights with low particulates the Milky Way does indeed cast shadows on the ground.

There are still dark places out there....you just have to be willing to rough camp to enjoy them.


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Re: Bortle scale accuracy? new [Re: Mr. Bill]
      #5979181 - 07/19/13 05:27 AM

Brooks, maybe it wasn't his eyes that are extraordinary, but maybe the viewing location? Do you think a site like Mauna Kea or one of the locations listed in Antarctica could offer even dimmer stars for naked eye viewing? Or maybe, as you stated earlier, children and pre teens might be able to see dimmer stars because of their greater visual acuity?

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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5979339 - 07/19/13 08:34 AM

As I indicated, based on a direct comparison with others, Steve essentially doesn't "see" fainter than anyone else of equal experience and with good eyesight. So, given that he does have access to and often observes from sites of outstanding quality, it is logical to conclude that his gains are due to the skies, not his eyesight. At considerable elevation above sea level (as from somewhere up on Mauna Kea) the air is relatively thin, while also containing far less particulates from outside sources due to the vast surrounding ocean, and would offer much better transparency than the rest of us encounter.

However, I would still contend that many very young individuals with good eyesight are likely capable of seeing more than the best adults, based on empirical evidence.

BrooksObs

Edited by BrooksObs (07/19/13 08:37 AM)


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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5980901 - 07/20/13 05:04 AM

Thanks Brooks. I assume the same would be true of those sites in Antarctica or the Andes site that Tony Flanders was referring to, where Barbara Wilson (I hope I got the name right) saw every star in the second edition of Sky Atlas 2000.0

http://www.cloudynights.com/item.php?item_id=172

That would take us all the way down to Mag 8.5. But I wonder if the atlas went down to 9.0, maybe she would have been able to say that she saw stars even dimmer?

The reason I ask is that I am trying to put together a presentation on light pollution using Starry Night Pro Plus and I wanted to illustrate the different levels of pollution from the areas with the most light pollution to the areas with the least, and with the greatest transparency. I have even have panoramas of each specific site or city I am going to use (one for each level of the Bortle scale.) I'm going to use the Andes site that Tony Flanders mentioned as an example of the "perfect" dark site (although I could use Mauna Kea or Antartica also- as all the panoramas are either built into the program or can be made.) Absent of any proof that anyone has seen any stars of dimmer than Mag 8.5, I think I'll set that as the minimum magnitude for this site, just so people can have an idea of what the sky looks like from an amazing location like this. I was just wondering if very young eyes or an even better location (like the Dome A and C and Ridge A in Antarctica) could offer even deeper seeing, closer to Mag 9.0 visually. The number of visible stars rises exponentially even with small increases in limiting magnitude.

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

6.0 0.40% 4 800
No 7.0 0.16% 14 000
8.0 0.063% 42 000
9.0 0.025% 121 000
10.0 0.010% 340 000

Based on that CN review I linked to in the book, there are over 81,000 stars down to Mag 8.5..... if we could get it down closer to Mag 9.0 we would exceed over 100,000! It would be hard to comprehend what that would look like without actually being there, but a computer simulation helps. It's amazing that what we consider a dark site here in the northeast, that lets us see down to Mag 6.5, "only" lets us visually observe 9,500 stars, which is still only about 10 percent of what the perfect dark site would be capable of!


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #5980948 - 07/20/13 06:57 AM

Quote:

Thanks Brooks. I assume the same would be true of those sites in Antarctica or the Andes site that Tony Flanders was referring to, where Barbara Wilson (I hope I got the name right) saw every star in the second edition of Sky Atlas 2000.0.




Lake Titicaca is much higher than where Steve O'Meara observes in Hawaii. Even the top of Mauna Kea is very low altitude for the Andes.

Mind you, we're quibbling over small percents. BrooksObs is claiming 8.1 or 8.2, and Steve O'Meara and Barbara Wilson 8.4 or 8.5. I don't doubt any of these world-class observers, nor do I much care about the difference between 8.1 and 8.5.

I also don't believe that 95% of the world's population is capable of seeing stars of magnitude 7.5 even in ideal conditions.

Nor do I believe that light pollution is the main obstacle, for two reasons. First of all, sites that are effectively free of artificial skyglow at the zenith are actually very easy to find in the American West. And for me, skyglow becomes a fairly small obstacle to limiting magnitude even at fairly high levels of skyglow. I can't see significantly fainter stars from the black zone than from the blue zone.


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BrooksObs
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5981149 - 07/20/13 10:19 AM

Quote:

I'm going to use the Andes site that Tony Flanders mentioned as an example of the "perfect" dark site (although I could use Mauna Kea or Antartica also- as all the panoramas are either built into the program or can be made.) Absent of any proof that anyone has seen any stars of dimmer than Mag 8.5, I think I'll set that as the minimum magnitude for this site, just so people can have an idea of what the sky looks like from an amazing location like this. I was just wondering if very young eyes or an even better location (like the Dome A and C and Ridge A in Antarctica) could offer even deeper seeing, closer to Mag 9.0 visually. The number of visible stars rises exponentially even with small increases in limiting magnitude.




Use caution in setting your planetarium magnitude limit. Computers are "dumb" when it comes to such situations and will produce exactly what is asked for without even considering mitigating circumstances.

Appreciate that even for highly experienced observers seeing stars of magnitude 8.0 or 8.5 is not a simple matter of just glancing skyward and there they are. It requires giving careful, sometimes lengthy, attention to spot such faint examples. Setting a planetarium to the same limits will likely generate a sky with FAR MORE stars than a real observer would perceive under the best of conditions.

For a more realistic depiction of the sky at a glance for such individuals I'd suggest making settings at least half a magnitude, or even a bit more, brighter than any absolute limit. Even then, as Tony points out, the majority of observers today are probably lacking in the needed observing experience necessary to "see" to their full potential. Proceed with care!

BrooksObs

As something of a P.S., let me also note that once the observer exceeds an altitude of say 11,000-12,000 feet lack of oxygen actually reduces just how faint he sees. Not all that much higher results in serious physical problems. There are stories of astronomers working at Mauna Kea without keeping their 02 masks handy finding themselves suddenly standing staring a the blank observatory walls right infront of them!

Conversely, use of 02 may enhanse the vision of some, resulting in either seeing fainter...or the illusion there of!

Edited by BrooksObs (07/20/13 10:33 AM)


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Mr. Bill
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5981150 - 07/20/13 10:20 AM

Let's remember that there is a limit to altitude and gains in visual acuity....anoxia trumps gains in scotopic vision.

My personal experience is spending time observing at the Barcroft Research Station in the White mnts. east of Bishop. It is 12500 ft altitude. I found that I could discern more subtle detail in low contrast extended objects when observing at the Grandview campground (8600 feet)located below the station. At that time I was using a pair of 25x150 Fujinon binoculars.


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mountain monk
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Re: Bortle scale accuracy? new [Re: Mr. Bill]
      #5981333 - 07/20/13 12:14 PM

I would say the limit is 18000 feet. You can acclimatize to that altitude; people are born and live at that altitude. Their eyesight is acute. If they do have problems it is because of exposure to too much UV, though that was more of a problem in years past when they did not have access to sunglasses.

I spent about four years of my life at elevations from 11700 to 16000 feet and often went higher. I've lived at 6400 to 6800 feet under black and gray zone skies for decades. I am of the school that says the higher you go the better it gets and the more you see--if you are acclimatized.
I've been fortunate enough to observe from some very dark skies in the U.S.--the "boot heal" of New Mexico (the Gray Ranch), northern Death Valley (Eureka Dunes), and Beartooth Pass (on the border of Wyoming and Montana). I've also spent months at high elevation on the border of Peru and Bolivia, and in the western ranges of the Himalaya--the Kun Lun, Karakoram, Hindu Kush, and Pamirs. Much of the time I was at elevations of 15000 to 16000 feet. My judgement--and it can be no more than that since I was not carrying meters, etc.-- was that I saw more stars, a greater density of stars, at the higher elevations. I was well acclimatized--months at 12000 to 15000 feet.

I usually had clients or fellow mountaineers or military folks with me- probably a fairly normal cross-section of humanity. None has special training in astronomy or observing, but there were decided differences in how much they could see, either naked eye or through my 10x Zeiss binoculars.

Of course, this is merely anecdotal. In her book Celestial Sampler, Sue French has a story of an early explorer in Greenland who for a few moments could see more and fainter stars than the he could usually see--even under those (I assume) very dark skies. I have always believed in that possibility. Perhaps 9.0 is possible under very rare conditions, but I am aware of no proof, and I'm happy to accept other, more expert, opinions. As for subtle distinctions--between 8.1 and 8.4--I cannot say. I sure can't do that now. And in the past? Well, memory plays games with the mind.

Thanks to all for the interesting thread.

Dark skies.

Jack


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Mr. Bill
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Re: Bortle scale accuracy? new [Re: mountain monk]
      #5981589 - 07/20/13 03:09 PM

Quote:

I would say the limit is 18000 feet.
Jack




That's an astounding statement....

Detection of stars is one thing, stars are point sources, I was referring to edge detection of very low contrast extended objects...structure in dark nebulae.


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BrooksObs
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Re: Bortle scale accuracy? new [Re: Mr. Bill]
      #5981651 - 07/20/13 04:05 PM

One must be cautious about just how much additionally the visual threshold increases with altitude beyond a certain point.

Indeed, there are small societies that survive up to altitudes of 15,000 to maybe 18,000 feet, but I cannot cite any observers from among those populations who can report about the advantages.

As far as the rest of us are concerned, I would note that above 10,000 feet the partial pressure of oxygen begins reducing to the point that it adversely affects the normal activities and functioning of the human body. In fact, the reactions of the average person begin to be impaired at an altitude of about 10,000 feet and for some people as low as 5,000 feet. The potential of altitude sickness and other effects quickly overcomes any visual advantage.

At 10,000 feet the observer is above 50% of the Earth's atmosphere, most of its moisture, and the vast percentage of atmospheric pollutants. Venturing higher probably would gain one very little further advantage and still have them remain fully functional. Thus, I would contend that 10,000 feet above sea level is about the break even point for "most" observers in gaining the greatest advantage in seeing absolutely as faint as possible.

BrooksObs

Edited by BrooksObs (07/20/13 04:15 PM)


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Mr. Bill
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5981670 - 07/20/13 04:27 PM

That confirms my experience and those of whom I have first hand knowledge of.

Maybe Lance Armstrong and folks bred in extreme altitudes for generations have different results, but that is a very select population.



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JayinUT
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5981812 - 07/20/13 06:38 PM

That goes with my experience though Jack by far has more experience than me at high elevation. I live at 5046ft and work at the same level. My observing sites are 1. 5336 ft and impacted to the NE by the Salt Lake City Light Dome, and it continues to grow and impact. The light dome from the Provo Metro area is also started to spread out over this site. It is a quick and easy site to hit with SQM-L measurements at around 21.53 to 21.55 at the darkest area. If I point toward the dome the iSQM-L drops to the mid 21.40's, so yes, the dome impacts the site. It's like having the summer Milky Way overhead and I won't share that impact to the SQM-L when I have the LP and the summer Milky Way.

2. Next site is my preferred observing area at 6097ft or 6396 ft (two locations down the dirt road but I call any gain form 200 feet minimal). and far enough from the Salt Lake Light Dome that the only impact is to 10 to 15 degrees if you go searching to the NE. SQM at Zenith and to the south at this site come in between 21.67 to 21.73 depending on the time of year. This site is a hour and twenty minutes form home and is U.S. Forest Land so it works nicely with unimproved camping but with toilets at a reservoir about 3 miles away. If I point at the SQM-L at the SLC Light Dome the reading decreases by about 5-7 points depending on the reading, based on my log records. This by far is the best site to observe at near to the Salt Lake City area. The question is for how long? I expect my observing career at age 48 will endure it for my lifetime but in the lifetime of my children I expect this site to drop off as development creeps westward from the Provo area and that light dome carries over to impacting the site more like the first site I listed. I also have another site down the dirt road that is at 6396 ft and adds a point or two on the SQM-L.

3. I have two sites in the Unita Mountains, one at 9978 ft and the other at 10203 ft. I usually use the 9978 ft site as its easier to get to, a shorter drive and during the week, I'm usually alone. Salt Lake City is about 100 miles west of the site and yes, it is now impacting the darkness of this site. SQM at this site is 21.60 to a high of 21.65 but usually around 21.63, that is where most of my recordings come in. It use to be higher, by about 6 to 8 points 10 to 12 years ago, but the light dome from SLC does impact. A good reason is these sites are closer to Salt Lake City than my favorite site and though higher and looking through less atmosphere, there is that LP impact. It seems a trade off of elevation for more LP. It's one reason I don't go there anymore that often. I will also say though that these readings are coming when the Summer Milky Way is above (can only use the sites in the summer) and so I can't measure what a fall, winter or spring reading would be. The site is open for snowmobiling but I don't do that hobby nor would I be up there on a cold winter's night, though I go to my 6096 site in the winter . . . and its cold there, but usually far less snow, like none (melts in a couple of days usually).

So I may gain over 3000 to 4000 ft between my primary sight and the mountain site, I'm not sure of that benefit. It prefer he darker skies with less impact from the SLC dome. I'll state up front living at 5000 ft means I don't find an adjustment for myself even at 10,000 feet. I do believe that helps and I believe one of the altitude projects in CO has done a study on this that if you live at altitude, you accumulate easier to a higher altitude than if your coming from sea level or near sea level.

So based on SQM-L readings do the light domes impact my observing? Surely. I can tell when I go to the first sight listed, 45 minutes from home over the darker site 1 hour and 20 minutes from home. Fainter objects have slightly more detail and at the other site, I have to work for that detail a lot harder. Fainter objects I wouldn't try at the first location like Hickson 79 which m 14" pulled in easily at the darker location that has far less impact from the SLC Light Dome.

My best location in Utah? Are there dark sites? Yep. The National Parks do a great job, Bryce, Arches, Canyonlands, Capitol Reef, and there is plenty of BLM land and Forest Land to go to that many outside of the Utah amateurs don't know about. I know many think these spots are great, and they are, and yes, they are dark. It's also a good 4-5 hour drive so that is a weekend trip with Friday off and driving down on Thursday for me. There is BLM land out by Canyonlands that I like but that is also getting an impact somewhat from Moab, not too bad, but it is increasing. I'll still go there as well or to Canyonlands.

In terms of dark areas though, I personally know many sites are kept quite here in Utah, just to keep people out of our favorite haunts. IF people ask, were happy to share and to be fair to me, I have a Google Map that shows the observing areas here in Utah, well most of them. A few I haven't put on yet. Two favorite that are within 4 hours, well more than two, Capital Reef, BLM Land in central Utah; BLM land near Canyonlands, and then The Wedge Overlook and Notch Peak (zero impact for any LP source and the highest SQM readings I've had). However, be ready to drive a good 3 to 5 hours to get to them and to me that means a long weekend of observing (which isn't a bad thing). However, that takes time and money and the average backyard and club outreach observer isn't going to do that on a regular basis and thus they never improve their observing skills. For that matter, in the first couple of site I mentioned, all are within 1 to 1.5 hours of Salt Lake City and yet many don't drive out to use these sites who are visual observers.

So I guess my point is that if your a serious observer and want to find dark skies or the darkest skies in your area, you find them. I also don't think too many people publish them and I think for the most part, the average person in the hobby to those well versed in the hobby won't go to those sites on a consistent basis and thus they fail to develop the experience needed to know when light pollution is impacting. That and I believe they don't comprehend why some people can spend forty-five minutes to an hour and a half on an object or set of objects to observe all the detail they can. They find it, view the object and off they go. Yes, a SQM meter and other tools can help to quantify the impact, but in the end, I think the experience and serious observer knows when LP is impacting their views. As time goes by it will become harder and harder for the visual observers of tomorrow, if there are enough to matter, to find sites like what have been and are available. I am convinced that the darkness of many sites now used will continue to degrade as we move closer to the year 2100. In truth, I wonder if the serious visual observer is going the way of the dinosaur, extinct?

One last item. In my own observing I have made pretty through notes of not only the impact of LP, but of haze and pollution on observing. It seems to me and I know the 3 years I've been gathering the data isn't long enough to quantify it, that pollutant particles that linger and hang in the air are also impacting the quality of observing. Has anyone else noticed that or is it just a local thing? Our population is expanding and will expand by around 60% by 2042. Utah is not good on decreasing pollution and so it lingers reducing observing conditions and again making it to where if you really want those views, you have to travel far away. Again, is it just local or anyone else seeing this and do you feel it also degrades the view of observing? For me it does. Thanks.


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5981882 - 07/20/13 07:25 PM

Quote:

At 10,000 feet the observer is above 50% of the Earth's atmosphere.




You are mis-remembering. Only 25% of the atmosphere is below you at 10,000 feet. The 50% mark is reached at 18,000 feet -- coincidentally around the limit of long-term human survival.

Even people with normal sea-level genetics can adapt pretty well to 10,000 feet in a few weeks. However, I have to suspect that even for people who are genetically adapted to high altitude, it's impossible to perform quite as well at 15,000 feet as at 5,000 feet. Any way you look at it, the body has to spend a bigger fraction of its energy budget pumping air through the lungs and blood.


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mountain monk
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Re: Bortle scale accuracy? new [Re: JayinUT]
      #5981900 - 07/20/13 07:43 PM

Gentlemen: You all know far more about astronomy than I will ever know. I will only comment on "small societies..."

Because we live where we do, we have a somewhat limited idea of life at high altitude. A few factoids about places I have been, or been near to.

La Rinconada, Peru is a city of 30,000+ inhabitants at 16700 feet. Cerro de Pasco, Peru is a city of over 70,000 inhabitants at 14200 feet. Rongbuk Monastery, on the way to base camp for the north side of Everest, lies at 16340 feet. When I visited, it was still mostly ruins (destroyed by the Cultural Revolution), but was being rebuilt and is now occupied and can be reached by road. When the first British Everest expedition reached Rongbuk, in 1922, it housed 450 monks. In his book Himalayan Pilgrimage, David Snellgrove (London School of Oriental and African Studies and perhaps the preeminent authority on Tibetan culture of his generation) describes his travels to Dolpo, part of Nepal but Tibetan in culture. He describes villages at 16500 feet and says that several are higher. Petter Matthiessen describes them in his book The Snow Leopard, which won the National Book Award. I have spent time with Peruvian Indians and Tibetans and Sherpas at these altitudes and I have found no reason to believe their vision is inferior to our own. Going high with acclimatization, or living high, does not negatively affect vision. The body adapts. But that's only one half of the argument.

The other half is the argument that you can see more. Yes, of course, the law of diminishing returns sets in above...what? I cannot prove that you can see more, obviously. I will note that the Kun Lun Range is just south of the Taklamakan (various spellings) Desert, one of the driest places on the planet and very cold--well below zero in the winter. (And remote from light sources.) There, and in the Karakoram, you use camels, not yaks. I wish we had observations from there, but to my knowledge, we don't. I'll bet my cookies on a Tibetan kid born at 18000 feet and given a bit of training at observing... I wish I could do just that.

I concur with everything that Jay says.

Dark skies.

Jack

Edited by mountain monk (07/20/13 08:46 PM)


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BrooksObs
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5982044 - 07/20/13 09:18 PM

Quote:

You are mis-remembering. Only 25% of the atmosphere is below you at 10,000 feet. The 50% mark is reached at 18,000 feet -- coincidentally around the limit of long-term human survival. - Tony




From 3 separate citations I consulted: "6000-10,000 feet is the altitude range some people may start to experience problems related to altitude. AMS, or Acute Mountain Sickness symptoms may set in at any altitude above 6,000 feet. At 10,000 feet, the atmosphere is only 50% of that found at sea level."

BrooksObs

Edited by BrooksObs (07/20/13 09:19 PM)


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mountain monk
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5982074 - 07/20/13 09:43 PM

Yes, but the key words here are "some" and "may."

Our company (I worked for them for over thirty years and was president) takes people up the Grand Teton. The typical client flies in from sea level or near sea level to 6000-7000 feet, spends two days in climbing school at 7000 feet, goes up to our hut at 11700 feet on the third day and climbs the Grand Teton--13700 feet-on the fourth day and returns to 6000-7000. Our rate of difficulties with AMS, after eighty years in business, is a small fraction of one percent. This is not to deny the possibility of AMS or its seriousness. I've had both forms--pulmonary and cerebral. It's serious but rare at the elevations you mention and not, I think, something that should concern most astronomers.

Dark skies.

Jack


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5982184 - 07/20/13 11:12 PM

Thanks Brooks, Tony and everyone else! Lots of great info here :-) I'm going to set the planetarium to Mag 7.5 based on this. This answers my other question about elevation- would some place even higher than what the Andes can provide (like, near Mt Everest), be even better? (There's a Mt Everest panorama in SNPP.) I take it the answer is no, but not for the reason I thought..... I figured the light pollution from India to the south would be a problem. The Andes are in an ideal location, being close to the Pacific Ocean.

A related question to this I had was what is the FOV of the human eye? Based on what I've read it's close to 180 x 180 degrees, but I also read that to visually view stars that are dimmer than Mag 6, you need to concentrate on a much smaller FOV (I surmise this was along the lines of what Brooks was saying regarding what's necessary to see very dim stars.) It was recommended to make the starting FOV in the program 120 degrees.... which is where I have it. In any case, the program suffers from distortions if the FOV is greater than 167 degrees.

Sources:

http://clarkvision.com/imagedetail/eye-resolution.html

http://www.astronomyforum.net/astronomy-beginners-forum/116992-human-eye-fov....

http://www.astronomyforum.net/amateur-astronomy-forum/38747-h-alpha-detection...

160 x 175

http://en.wikipedia.org/wiki/Naked_eye

http://en.wikipedia.org/wiki/Human_eye#Field_of_view

I found the discussion about Peru particularly interesting because I was wondering if ancient cave art depicted astronomical objects that we can only see with telescopes now. I remember reading that Sirius B may be such an object.

I also read that the Nazca Lines may have been inspired by constellations (maybe Orion) and perhaps even the dark lanes in the Milky Way.

https://en.wikipedia.org/wiki/Nazca_Lines#Purpose

my favorites are condor, giant, spider, dog and the monkey

https://en.wikipedia.org/wiki/Sirius_B#Etymology_and_cultural_significance

Dogon[edit]
See also: Nommo
The Dogon people are an ethnic group in Mali, West Africa, reported to have traditional astronomical knowledge about Sirius that would normally be considered impossible without the use of telescopes. According to Marcel Griaule's books Conversations with Ogotemmęli and The Pale Fox they knew about the fifty-year orbital period of Sirius and its companion prior to western astronomers. They also refer to a third star accompanying Sirius A and B. Robert Temple's 1976 book The Sirius Mystery, credits them with knowledge of the four Galilean moons of Jupiter and the rings of Saturn. This has been the subject of controversy and speculation. According to a 1978 Skeptical Inquirer article it is possibly the result of cultural contamination.[115] Some have suggested the contaminators to have been the ethnographers themselves.[116][117] Others see this explanation as being too simplistic.[118]



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JayinUT
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5982265 - 07/21/13 12:45 AM

I'll share a couple of links on altitude and people can read and do what they want with the info. I think we need to get back to discussing dark sites.

Wm Keck Observatory Link.

Institute for High Altitude Medicine Link

BaseCampMD.com

The site above lists this: High Altitude is 5000 to 11500 feet; Very High Altitude as 11500 to 18000 feet; Extreme Altitude above 18000 feet.

UIAA website


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #5982457 - 07/21/13 06:09 AM

Quote:

From 3 separate citations I consulted: "6000-10,000 feet is the altitude range some people may start to experience problems related to altitude. AMS, or Acute Mountain Sickness symptoms may set in at any altitude above 6,000 feet. At 10,000 feet, the atmosphere is only 50% of that found at sea level."




Gotta check your sources! That's especially true on the internet, where incorrect information propagates like wildfire. The first two statements are correct, the third is not.


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #5984097 - 07/22/13 07:46 AM

This is more of an academic question but does anyone have any idea what the limiting magnitude may be from outer space? Assuming that neither the sun nor the moon is visible from this position in space.

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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6000965 - 08/01/13 03:25 AM

Are there or have there been any reported naked eye sightings of the planet Neptune? I figured this would be a good target for those in areas of very little artificial light pollution as it is close to Mag 8 (7.78-8.00).

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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6000993 - 08/01/13 04:34 AM

Found this post from 1999 about a guy who can see down to Mag 8.5

http://tech.groups.yahoo.com/group/amastro/message/898


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JayinUT
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6001341 - 08/01/13 11:40 AM

One thing I have found in the hobby is that for 1 person that is vocal on their abilities, there are usually 2 or 3 that are not. So is viewing from 8.0 to 8.5 as rare as we might thing for the serious visual observer? Isn't it just possible that someone can do it and just isn't vocal about it, doesn't feel the need to share or just doesn't want to prove that they can do it. They know they can do it, they've done it and so for them that is enough? How many of these people are there in the hobby I wonder?

Edit: I don't mean to imply that being vocal or sharing is bad, or bragging. I think many people just share what they find and what they are capable of. However, I am suspicious that there are more people who can and do wonderful things and just don't publish them for whatever reason. Much like I know of several people that have done 12 or more of the AL programs but never put in for the reward. They know what they can do. Their close group of observing friends know what they can do and that is all that matters to them. So I think there are more out there than we may think who can push the limits.

Edited by JayinUT (08/01/13 11:44 AM)


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: JayinUT]
      #6001410 - 08/01/13 02:21 PM

Quote:

So is viewing from 8.0 to 8.5 as rare as we might thing for the serious visual observer?




Yes. Amateur astronomy is a small world; if there were lots of people who could do this, I would know about it.

Quote:

I think many people just share what they find and what they are capable of. However, I am suspicious that there are more people who can and do wonderful things and just don't publish them for whatever reason. Much like I know of several people that have done 12 or more of the AL programs but never put in for the reward. They know what they can do. Their close group of observing friends know what they can do and that is all that matters to them.




Sure, but among that close group of friends would be somebody I know, or somebody who knows somebody I know.


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #6002210 - 08/01/13 10:01 PM

Tony, I thought it was interesting how the person who wrote that piece said that his co-observer could see half a magnitude to a full magnitude lower than he could, at the same site, at the same time. So maybe there are some significant variances between people.

I've been scouring the net looking for any reports of naked eye observing of Neptune and have yet to find any. I have a thought......

Isn't it true that the human eye is particularly sensitive to red light? So is it possible that those very dim stars that some people can see are red stars? Maybe the reason why there are no reports of naked eye seeing of Neptune is because it's blue-green and the human eye isn't as sensitive to that color light as it is to red? Just an idea.....


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JayinUT
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6002476 - 08/02/13 01:33 AM

There's a lot out there on the limits of naked eye DSO's.

First is Brian Skiff's list of objects that have been seen at this link.

There is this threat over at Astronomy where a poster states that he can see M101 and M51 naked eye and the rebuttals to that claim. Link 2 I find that thread an interesting read.

In this report at this link, it is reported that Brian Skiff has seen M81 as "a threshold object."

Brian Skiff reports that from a true dark site the limit (for observing naked eye) from dark sites seems to depend strongly on visual acuity. I'll provide that link at the bottom on his comments on altitude.

In terms of Neptune, Brian Skiff at this link failed to find it in 2005, though he believes it should be "straightforward with patience from Chile or elsewhere in the south."

My take from this and from someone who regular observers at sites form 21.64 to 21.77 is that it takes a dark site like this and observing there from a regular basis to really begin to train the eye to see to its limits. That limit will vary depending on the person. I think you can gain experience from observing as often as you can, and for a long time, the more you do it the better trained the eye is. It helps when you start going to a dark site. However, regular and consistent training of the eye at a dark site, which Brian Skiff states is 21.5 arch seconds or better, enhances the training. I'd be most interested in hearing from Tony how observing in light pollution in his opinion also helps in visual observing.

In terms of observing at altitude I share what I found there from Brian. He states:

"However, for visual observing, if you go too high, you'll lose visual sensitivity simply because not enough oxygen is getting to your brain. The optimum altitude range seems to be from about 1500 up to perhaps 3000 meters (5000 to 9000 feet). Below 1500m, the amount of crud increases dramatically, and above 3000m most people have at least mild effects from lack of oxygen. Visual observing from Mauna Kea without bottled oxygen is pretty crummy. Remember that astro-observing is mostly at the threshold of acuity, so even small physiological effects from altitude (or ill health etc) will have pronounced effects on your vision in these circumstances. (When you're observing sometime at high power, try exhaling and not taking a breath for a good chunk of a minute: before you get dizzy, the eyepiece view will fade out and go grainy.) For what it's worth, acclimation to altitude appears to be independent of age, gender, or physical condition---but the genetic engineering necessary for high altitude living has already been worked out: they're called Bolivians!"

That quote and the one above is at this link. His findings go with what I find, though I don't find an impact on myself until I go well over 10,000 feet. That is why my FOR me, my maximum height is between 9000ft and 10000ft. I often observe between 6000 and 7000 feet.

Anyway, just sharing what I found in spending some time looking around and I look forward to others insights.

Edited by JayinUT (08/02/13 01:35 AM)


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: JayinUT]
      #6002562 - 08/02/13 03:48 AM

Thanks! This is exactly the kind of research I find most interesting! What's the highest level of seeing that any site that you know of has attained? 22? Also, I find it quite interesting that it seems like a number of people have reported being able to see to Mag 8.5 but no one any higher than that, not even 8.6 or 8.8? Did you find any reports for anyone claiming to have gone beyond 8.5?

Another question I have is how does myopia affect the dimmest objects we can detect? My vision is 20/60 (20/35 in my right eye and 20/80 in my left eye)...... would this affect the minimum magnitude I can see by a significant amount? Unaided or with glasses? The funny thing is when I shut my left eye and just look through my right eye it actually feels like I am wearing glasses because my vision seems so much sharper.


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6002570 - 08/02/13 03:57 AM

haha that was an interesting thread- that guy was talking about how someone having a porch light on 5 miles away drives him nuts and he doesn't have any sources of artificial light pollution within 10 miles of his house.

further down in the thread he said this:

One night here I went to ~900X with my 12.5" on Saturn and saw near photographic detail. I consider seeing like that night a once in a lifetime thing, but I still have my notes and remember it vividly.

and someone responded that he'd read that 9th mag stars had actually been seen visually from Mauna Kea?

Dave - I don't want to hyjack the thread but in Stephen O'Meara's book "The messier Objects" he mentions that he was able to see stars of 8.2 with the unaided eye while doing his research in HI on Mauna Kea (at 9,000'). He mentions that others with him have made similar (8th-9th) sightings in the world's best observing sites. So I'm assuming that for extended objects in the same seeing conditions, could it be possible to include M101, even with its very low surface brightness, as a naked eye object under the best of conditions? Mr Q



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derangedhermit
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #6002590 - 08/02/13 04:24 AM

Quote:

Yes. Amateur astronomy is a small world; if there were lots of people who could do this, I would know about it.

[...]

Sure, but among that close group of friends would be somebody I know, or somebody who knows somebody I know.




So astronomy's version of "Six Degrees of Kevin Bacon" is "Two Degrees of Tony Flanders".


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6002595 - 08/02/13 04:35 AM

This thread here indicates SQM readings of 22.1 or so consistent in the outback of western australia and namibia- higher readings also possible, but only under overcast skies. What does 22.1 translate to in terms of limiting magnitude?

http://www.iceinspace.com.au/forum/archive/index.php/t-71823.html


from that site

I note the highest reading from Australia is 22.07 from outside Mullawa in WA, which is about 100km West of Geraldton and 500km North of Perth. I have no doubt it's right. There are 3 men and a dog per 1,000 square km out there. But not close to the equal of several readings in CONUS? I note the highest reading from New Zealand is 22.02 from Benmore Peak Observatory. Again I have no doubt it's correct as indicated in one of my earlier posts.

and

12-03-2011, 12:20 AM
Hi all,

It was me who submitted the SQM-L 22.07 reading from outside Mullewa, W.A. I visited a remote farm together with Finnish amateur astronomers in December 2009 to do some really dark DeepSky-observations. They had also a SQM which showed 22.09 as best. In a series of three readings I got 22.01, 22.02 and 22.07. That was when I pointed to a relatively "blank" region of the sky, not the Milky Way or the zodiacal light. From here we observed the Light bridge between LMC and the Milky way. See my thread

http://www.iceinspace.com.au/forum/showthread.php?t=68088

From my regular observing site here in Sweden, the SQM-L has shown 21.48 at most and my naked eye limiting magnitude was 7.4. My Australian site is way darker than that!

/Timo Karhula

This is from the second link- much more on the site!

I'm an amateur astronomer from Sweden who regularly makes a trip to Western Australia, because I have an apartment in Geraldton after my late father. Last year, in November - December, I made a trip 130 kms to the inland where light pollution is unknown. My naked eye limiting magnitude was 7.9 and the SQM-L meters showed 22.09 magnitude per square arc-seconds! For the first time in 13 trips south of the equator, I observed a bridge of light between the LMC and our Milky Way. I have written a lengthy article of my astro-trip for the Deep-Sky Observer magazine of the Webb Society which will be published this winter or next spring. Here is an excerpt from my article:

"The light bridge of the Large Magellanic Cloud

The journey’s most exciting and unexpected sky phenomenon that we observed, was without a doubt, the light or materia bridge (as I call it) of the Large Magellanic Cloud, LMC. It is not even scientifically studied yet and only a handful of people have reported it in the literature!

......

I suspect a sky capable of showing stars to magnitude 7.5 or has a darkness of about SQM 21.9 is necessary to show the light bridge between the LMC and the Norma Starcloud. Has anyone on this forum seen the bridge of light? It would be very strange if nothing more is known of this naked-eye feature!

Clear Skies!

Timo Karhula


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6002597 - 08/02/13 04:44 AM

hmmm this cant be right, this site indicates that a limiting mag of 8.5 equates to an SQM reading of 28?!

http://www.cruxis.com/scope/limitingmagnitude.htm

If you enter a specific SQM reading it calculates the limiting magnitude automatically


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: derangedhermit]
      #6002639 - 08/02/13 06:26 AM

Quote:

So astronomy's version of "Six Degrees of Kevin Bacon" is "Two Degrees of Tony Flanders".




Make that "two degrees of Sky & Telescope." But the same could be said of many of the people who post here, who are equally embedded in the amateur-astronomy community. It really is a very small world.


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6002642 - 08/02/13 06:29 AM

Quote:

This thread here indicates SQM readings of 22.1 or so consistent in the outback of western australia and namibia- higher readings also possible, but only under overcast skies. What does 22.1 translate to in terms of limiting magnitude?




It doesn't. The correlation between SQM readings and limiting magnitude is extremely tenuous. In my opinion, essentially non-existent for SQM readings darker than 21.6 or thereabouts.


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6002658 - 08/02/13 06:51 AM

Quote:

How does myopia affect the dimmest objects we can detect?




Not at all if it's fully corrected. But uncorrected myopia has an enormous effect. Even 1/2 diopter off the optimal correction can easily cut 0.5 off your naked-eye limiting magnitude. And the optimal correction for nighttime isn't necessarily the same of the optimal correction for daytime. It's easy to experiment with a cheap pair of drug-store reading glasses; try it for yourself!

Astigmatism and higher-order aberrations also have major effects on NELM. My guess is that these are the biggest causes of variation in NELM among experienced observers. I am right up there with the best when it comes to seeing faint fuzzies, but easily a full magnitude off the best as far as seeing faint stars is concerned.

That's why I find BrooksObs's claim that everybody with normal vision can reach the same NELM with training isn't very helpful. Sure, it's true -- but only if you define "normal" to include a very small percent of the population. Which defies the common use of language.


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George N
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #6002874 - 08/02/13 10:00 AM

Quote:

Quote:

This thread here indicates SQM readings of 22.1 or so consistent in the outback of western australia and namibia- higher readings also possible, but only under overcast skies. What does 22.1 translate to in terms of limiting magnitude?




It doesn't. The correlation between SQM readings and limiting magnitude is extremely tenuous. In my opinion, essentially non-existent for SQM readings darker than 21.6 or thereabouts.




The only place in NY or PA where I've gotten an SQM reading darker than 21.70 was in my windowless bathroom at midnight with the lights off!

I do remember an article by Walter Scott Houston (in S&T of course) where he notes that while observing from a very dark location in southern Mexico, he was able to see dimmer stars while looking at a patch of sky up thru a hole in the forest canopy versus walking out into an open area.

I've recently (finally?) started using an observing hood to block out side light and I've found that it really helps, even when at a very dark location (SQM = 21.5 or so).


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Re: Bortle scale accuracy? new [Re: George N]
      #6003104 - 08/02/13 12:06 PM

Quote:

Quote:

Quote:

This thread here indicates SQM readings of 22.1 or so consistent in the outback of western australia and namibia- higher readings also possible, but only under overcast skies. What does 22.1 translate to in terms of limiting magnitude?




It doesn't. The correlation between SQM readings and limiting magnitude is extremely tenuous. In my opinion, essentially non-existent for SQM readings darker than 21.6 or thereabouts.




The only place in NY or PA where I've gotten an SQM reading darker than 21.70 was in my windowless bathroom at midnight with the lights off!

I do remember an article by Walter Scott Houston (in S&T of course) where he notes that while observing from a very dark location in southern Mexico, he was able to see dimmer stars while looking at a patch of sky up thru a hole in the forest canopy versus walking out into an open area.

I've recently (finally?) started using an observing hood to block out side light and I've found that it really helps, even when at a very dark location (SQM = 21.5 or so).




Indeed, the use of vision "limiters", or "concentrators" having a restricted field of view can (in the hands of an experienced observer) definitely increase one's naked eye limiting magnitude to a degree.

At the same time, it demonstrates that the natural background light from even a "dark sky" site causes a slight contraction of the pupil's diameter. Use of a vision concentrator was one way that H.D.Curtis managed his sightings of +8.3 magnitude stars.

At the same time, I personally have little confidence in SQM reading as truly depicting the sky situation, particularly in relation to limiting magnitude. When I was in Australia and observing from the outback I found the sky background far brighter than I had anticipate, given that I was hundreds of miles from any mentionable light sources. If fact, the sky background was no where near as dark as I had experienced years before on Nantucket Island in the U.S.A. Odd I know, but nevertheless unquestionably true.

BrooksObs

Edited by BrooksObs (08/02/13 12:08 PM)


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: BrooksObs]
      #6003246 - 08/02/13 01:56 PM

Quote:

Indeed, the use of vision "limiters", or "concentrators" having a restricted field of view can (in the hands of an experienced observer) definitely increase one's naked eye limiting magnitude to a degree.

At the same time, it demonstrates that the natural background light from even a "dark sky" site causes a slight contraction of the pupil's diameter.




I think that's unlikely. For one thing, most people's eyes seem to open fully even in quite bright surroundings. For another, there seems to be little correlation between people's pupil diameters and their limiting magnitudes.

More likely, the light from the sky background affects the chemical adaptation of the retinal cells. Or possibly it's a purely perceptual effect.

Any way you slice it, though, a so-called "dark sky" really isn't dark at all. The total light of a hemisphere of stars is many times brighter than Venus, which is famously bright enough to cast shadows. No wonder a "dark sky" hampers dark adaptation!

Quote:

At the same time, I personally have little confidence in SQM reading as truly depicting the sky situation




Definitely not. SQMs measure what SQMs measure, which doesn't correlate perfectly to anything of interest to stargazers. Among many other things, it's famously "fooled" by the Milky Way. The only way to accurately measure the background sky brightness between the stars is with a CCD camera.

Quote:

When I was in Australia and observing from the outback I found the sky background far brighter than I had anticipate, given that I was hundreds of miles from any mentionable light sources. If fact, the sky background was no where near as dark as I had experienced years before on Nantucket Island in the U.S.A.




I certainly cannot correlate two subjective, non-quantitative impressions made many years apart. In this case, I suspect a difference in the color of the sand underfoot rather than the sky overhead. But it could be due to solar activity. Airglow is famously variable on every scale of time and space.

One thing's for sure -- in the middle of the Australian outback it wasn't due to artificial light pollution!


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Ekyprotic
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #6004222 - 08/03/13 01:03 AM

Thanks all, that was some very useful information- Tony, I am also inclined to believe that at least some of the reason we see variations in limiting magnitude between different people at the same site, at the same time, is because of uncorrected vision- I didn't know that even if vision is properly corrected for daytime "normal" viewing- that doesn't mean it's corrected for night time stargazing!

Based on everything I've read, it seems like the darkest sites in the world are (in no particular order):

1) In the Andes, near the Atacama Desert, the driest place in the world

2) Mauna Kea, Hawaii

3) Dome A or C or Ridge A in Antarctica, the coldest places in the world

4) the outback of Western Australia

5) Namibia in southwest Africa

But based on what Brooks has said, it appears some sites in North America should be on this list also? Which of these should rank first in the world? The thread Jay referenced, indicates that a 65 yr old from Wyoming (John M), could see M51 and M101...... did he ever post a follow up to that thread (he said he would post more details in August of the year he made the claim, but I haven't been able to find anything on it.) Is it possible to see all the Messier objects naked eye from the darkest locations?


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Tony Flanders
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Re: Bortle scale accuracy? new [Re: Ekyprotic]
      #6004384 - 08/03/13 06:54 AM

Quote:

I didn't know that even if vision is properly corrected for daytime "normal" viewing- that doesn't mean it's corrected for night time stargazing!




See skypub.com/spectacles.

Quote:

Based on everything I've read, it seems like the darkest sites in the world are (in no particular order):

1) In the Andes ...




No, what you have listed is the best sites, not the darkest sites. Those are two very different things!

Most of Earth's surface is completely unaffected by artificial light pollution. That's probably even true if you're talking about most of Earth's land surface. There are vast uninhabited areas -- Northern Canada, Siberia, the Gobi, Sahara, Antarctica, and so on.

However, darkness is only one thing that makes a site good, and probably not the most important. The things that make Chile, Namibia, and Baja California so good are infrequent cloud cover, superb transparency, and steady seeing.

Good transparency actually results in a brighter sky, not a darker one, because it increases the amount of starlight.

Quote:

Is it possible to see all the Messier objects naked eye from the darkest locations?




Absolutely not, not even close. Most of them simply don't put out enough photons per second to excite your retinal cells.


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ggalilei
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Re: Bortle scale accuracy? new [Re: Tony Flanders]
      #6005089 - 08/03/13 04:18 PM

I have a related (I think) question on this subject: if from a dark site one can easily see stars of magnitude 7 and 8 and beyond, how come the ancient (thus "dark site") star magnitude scale - still used today, I believe - only goes to stars of magnitude 6?

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GlennLeDrew
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Re: Bortle scale accuracy? new [Re: ggalilei]
      #6005244 - 08/03/13 07:12 PM

Regarding vision 'constrictors.' By presenting to the great majority of one's field of view an essentially zero illumination scene, there is less scattering occurring within the eye, which reduces veiling glare and so improves contrast. Our eyes are not as transparent as we might like to think, and more so with advancing age.

At the risk of embracing a kind of stridency I normally eschew, I reject out of hand the notion that at a dark site the light of a starry sky will induce any real constriction of the iris. From my own personal observation (and taking into account accounts of similar behavior I've read of over the years), my pupils expand to maximum well before this level of dimness, and feel it's safe to say this should apply to all (except perhaps the literal one in a million.)

And yes, natural air glow can vary rather considerably at small scale in both time and space. At any one place I suspect the variance can be as much as a half magnitude.


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derangedhermit
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Re: Bortle scale accuracy? new [Re: ggalilei]
      #6005398 - 08/03/13 09:09 PM

Seems related to me.

Of the stars in the catalogs of Brahe (1598) and updated by Kepler (1627) (without telescope), the approximately 1000 stars are given magnitudes of from 1-6. Of the 200-odd stars designated as mag 6, only about 50 are mag 6 according to Hipparchos; the rest are actually mag 5.

Hevelius (and his second wife, Elizabeth Koopman) improved on and expanded Tycho's catalog, again using naked-eye instrumentation. Koopman published their catalog of about 1500 stars after Hevelius' 1687 death. Elizabeth Koopman is considered to be the first female astronomer. The catalog, I believe, included 6 stars labeled as mag 7, but Hip measurements are mag 6.

In both cases, the catalogs are incomplete, beginning around mag 3 or mag 4.

One would assume they only included stars in the catalogs they could hold steadily in direct vision for their positional measurements. And they could probably only use one eye, but I don't know.

There are a couple of great papers, suitable for amateur astronomers of any level to read, by Verbunt and Gent, analyzing the catalogs of Brahe+Kepler and Hevelius. They are in pdf format, free on the web and easily found by search.

Flamsteed used telescopes built into his instruments for measuring star positions and creating his list of stars. The leading British astronomers of the time tried to convince Hevelius to use telescopes in his positional measurements, but they failed.

I think it would be fun to design and build naked-eye positional tools using modern deisgn and manufacture, and astronomical knowledge, and repeat the exercise. I just have no idea how.

There must be some verified recordings of the faintest stars viewed with naked eyes in days predating, or in the early days of, the telescope; anyone know of any?


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