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General Astronomy >> Light Pollution

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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
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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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Reged: 11/28/12

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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Reged: 11/28/12

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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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
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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amicus sidera
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Reged: 10/14/11

Loc: East of the Sun, West of the M...
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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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
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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Reged: 10/14/11

Loc: East of the Sun, West of the M...
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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Ekyprotic
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Reged: 11/28/12

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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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
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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BrooksObs
professor emeritus


Reged: 12/08/12

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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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
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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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
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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Reged: 05/18/06

Loc: Cambridge, MA, USA
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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BrooksObs
professor emeritus


Reged: 12/08/12

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
Pooh-Bah


Reged: 10/07/09

Loc: USA
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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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
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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BrooksObs
professor emeritus


Reged: 12/08/12

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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