Author Topic: Bortle scale accuracy?  (Read 527 times)

asagnata

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Re: Bortle scale accuracy?
« Reply #15 on: January 16, 2018, 11:26:22 PM »
Where is the most light pollution free zone in the lower 48? I thought it was somewhere in Nebraska based on some LP maps I've seen- but maybe they're wrong?

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

https://en.wikipedia...cal_observatory

and

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

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

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

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

unllamerblood

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Re: Bortle scale accuracy?
« Reply #16 on: January 20, 2018, 04:43:23 PM »
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.

buddderpdrivla

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Re: Bortle scale accuracy?
« Reply #17 on: January 21, 2018, 03:23:31 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.

ticploodunghen

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Re: Bortle scale accuracy?
« Reply #18 on: January 23, 2018, 12:28:34 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.

<p class="citation">QuoteAnother 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?[/quote]

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.

Alex Strouth

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Re: Bortle scale accuracy?
« Reply #19 on: January 23, 2018, 03:09:05 AM »
There are all sorts of claims as to how extremely faint certain individuals supposedly have seen, but very few of these sightings were done in any verifiable scientific fashion, or include details regarding how the observations were conducted. Even fewer accounts have actually been published in reliable astronomical journals.

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

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

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

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

BrooksObs

Jay Cole

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Re: Bortle scale accuracy?
« Reply #20 on: January 25, 2018, 07:06:31 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?

<p class="citation">QuoteGoing from the maps, we should be starting up an astronomy village in southeastern Oregon.

-Rich[/quote]

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.

olaralal

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Re: Bortle scale accuracy?
« Reply #21 on: January 25, 2018, 11:10:41 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?

Daniel Horton

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Re: Bortle scale accuracy?
« Reply #22 on: January 26, 2018, 01:41:41 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!

precaregmo

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Re: Bortle scale accuracy?
« Reply #23 on: January 31, 2018, 03:29:28 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?

Ligon Payton

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Re: Bortle scale accuracy?
« Reply #24 on: January 31, 2018, 07:42:42 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.

Praveen Mac

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Re: Bortle scale accuracy?
« Reply #25 on: January 31, 2018, 01:08:58 PM »
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

hiswacoka

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Re: Bortle scale accuracy?
« Reply #26 on: February 02, 2018, 04:35:25 PM »
Quote
<p class="citation">QuoteThanks 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.[/quote]

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.

rubnirootcount

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Re: Bortle scale accuracy?
« Reply #27 on: February 02, 2018, 06:08:46 PM »
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.<p class="citation">QuoteAs 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 .[/quote]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.<p class="citation">QuoteTony 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.[/quote]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

halubicom

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Re: Bortle scale accuracy?
« Reply #28 on: February 03, 2018, 04:00:36 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. 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.

lehroldwebbdep

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Re: Bortle scale accuracy?
« Reply #29 on: February 08, 2018, 05:45:20 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 <em class="bbc">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