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General Astronomy >> Light Pollution

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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
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
Carpal Tunnel
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Reged: 05/19/06

Loc: Binghamton & Indian Lake NY
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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BrooksObs
professor emeritus


Reged: 12/08/12

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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Reged: 05/18/06

Loc: Cambridge, MA, USA
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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Reged: 11/28/12

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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Reged: 05/18/06

Loc: Cambridge, MA, USA
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
scholastic sledgehammer
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Reged: 09/01/11

Loc: Kentucky
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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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
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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Reged: 10/07/09

Loc: USA
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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