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General Astronomy >> Light Pollution

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Tony Flanders
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Loc: Cambridge, MA, USA
Re: Introducing DarkSiteFinder.com new [Re: krp]
      #6023153 - 08/13/13 02:49 PM

Quote:

The only way to see how dark a site truly is, is to visit there on a good night.




Amen. The maps are tremendously useful, but at best they can only be approximations of reality.


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derangedhermit
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Reged: 10/07/09

Loc: USA
Re: Introducing DarkSiteFinder.com new [Re: krp]
      #6023549 - 08/13/13 05:31 PM

Quote:

Quote:

Kevin, have you discussed using Dave Lorenz' maps with him?



I have attempted to but I got no reply. But since he has the PNG images available for download on his website, I don't think he would have a problem with it. I made sure to give him credit.



Alternate e-mail address for Dave sent via PM.
Lee


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derangedhermit
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Re: Introducing DarkSiteFinder.com new [Re: Tony Flanders]
      #6023568 - 08/13/13 05:37 PM

Quote:

Quote:

In fact, it might be good to intentionally use a number of levels that clearly distinguishes this from the Bortle scale, to prevent further confusion.




Attempting to mimic the Bortle scale would simply increase confusion.






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derangedhermit
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Re: Introducing DarkSiteFinder.com new [Re: Tony Flanders]
      #6023963 - 08/13/13 09:06 PM

Quote:

I deeply regret the original publication of the correlation by the North Virginia Astronomy Club, although their intentions were entirely legitimate. And its propagation through Wikipedia is even more unfortunate. However, the cat's out of the bag now; the harm cannot be undone.




The Wikipedia entry for "Bortle Scale" is now fixed.


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Alein 1630
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Loc: Dutchess county NY
Re: Introducing DarkSiteFinder.com new [Re: Daniel Guzas]
      #6024132 - 08/13/13 10:49 PM

Quote:

Would their be any reason why the light pollution map wouldn't show up on an IPad? I can get the map ( underlay) to show up fine but no light pollution map.

Any thoughts? Or is this something you already are aware of? I would love to take a look at your maps!




Same here. color doesn't come up??


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Alein 1630
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Loc: Dutchess county NY
Re: Introducing DarkSiteFinder.com new [Re: krp]
      #6024140 - 08/13/13 10:53 PM

Quote:

Quote:

Would their be any reason why the light pollution map wouldn't show up on an IPad? I can get the map ( underlay) to show up fine but no light pollution map.

Any thoughts? Or is this something you already are aware of? I would love to take a look at your maps!



Sorry I didn't really optimize the website for mobile devices. I don't have an Ipad to test with either. Maybe someday I'll learn how to build an app though.






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Tony Flanders
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Re: Introducing DarkSiteFinder.com new [Re: derangedhermit]
      #6024498 - 08/14/13 05:22 AM

Quote:


The Wikipedia entry for "Bortle Scale" is now fixed.




Wow, that's pretty drastic! I did add a caveat to that article, but I'm reluctant to remove information even if it's only approximately correct.


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derangedhermit
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Re: Introducing DarkSiteFinder.com new [Re: Tony Flanders]
      #6025675 - 08/14/13 05:50 PM

Quote:

Quote:


The Wikipedia entry for "Bortle Scale" is now fixed.




Wow, that's pretty drastic! I did add a caveat to that article, but I'm reluctant to remove information even if it's only approximately correct.




Given John Bortle's own statements that he was *not* involved, and that there would be considerable differences between the two, and that the Bortle Dark Sky Scale is intended for use for on-site evaluation, not for use in creating maps (and your own respected expression of deep regret), it seems perfectly appropriate to me.

It's a simple matter of respecting John Bortle's wishes. I think you said it was a shame it had happened, but could not be undone. I am willing to undo it where possible; it's a small effort made to show some respect and remove some of the confusion that we see often here on CN.

If you like, we can add a short summary of the other main LP measurement systems / night sky evaluation methods there and point to Wiki pages on them. That might be even more helpful - to remove the false connection, but to show the way to additional information. There is already a question in "talk", unanswered, about the relationship between the Bortle Scale and SQM readings.

Does that sound worthwhile to develop?

Lee


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Tony Flanders
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Re: Introducing DarkSiteFinder.com new [Re: derangedhermit]
      #6026576 - 08/15/13 07:02 AM

Quote:

Quote:

Quote:


The Wikipedia entry for "Bortle Scale" is now fixed.




Wow, that's pretty drastic! I did add a caveat to that article, but I'm reluctant to remove information even if it's only approximately correct.




Given John Bortle's own statements that he was *not* involved, and that there would be considerable differences between the two, and that the Bortle Dark Sky Scale is intended for use for on-site evaluation, not for use in creating maps (and your own respected expression of deep regret), it seems perfectly appropriate to me.




You're probably right; I should have been more decisive when I edited that article some years ago. No matter how many caveats you add, people will take the chart as gospel.

I do have an article on the S&T website discussing the relationships, here.


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derangedhermit
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Re: Introducing DarkSiteFinder.com new [Re: krp]
      #6027266 - 08/15/13 02:02 PM

Quote:

Please let me know how you like the website and if you have any suggestions to make it better.




Ken,

First, let me get something out of the way. Your site is using Dave Lorenz' work. He programmed the algorithms from the research papers from scratch, a significant amount of work. I know he has them for download, but there is no statement of release for various uses, especially for web sites that may be considered partially commercial. I will feel a lot better talking about the site here (CN is quite sensitive about copyright) and encouraging others to use it once you have Dave's explicit permission.

OK, now for the suggestion: I think the 8 colors can be improved on, to solve two problems.

1) Since the colors are solid and bright, people simply remember "green" is "green", when in fact there is a 3x difference in light pollution between the green/blue boundary and the green/yellow boundary, and this makes a very large difference in practice.

2) The color ranges do not directly correspond to magnitudes. This is unfortunate since we have a meter, the SQM, and at least one app, that allows one to measure sky brightness in magsas directly.

The natural sky brightness, without light pollution, varies by up to about one magnitude (not counting the effect of the moon) anyway due to several natural effects. So the map is relative to a baseline dark sky which varies, between perhaps 21 magsas and 22 magsas.

My first suggestion is to use the 255 grayscale map to create a new color mapping. It uses 7 colors:

clear - no light pollution, natural pristine skies (e.g. 22 magsas)
gray - 0-1 magsas brighter than natural, pristine skies (21-22)
blue - 1-2 magsas brighter (20-21)
green - 2-3 magsas brighter (19-20)
orange - 3-4 magsas brighter (18-19)
yellow - 4-5 magsas brighter (17-18)
white - 5+ magsas brighter (17+)

My second suggestion is to help people intuitively and also quantitatively see a finer gradation of sky brightness. The 8-color scale is insufficient, the 16-color scale is better, I think this is much better still: For each of the 5 colors besides clear and white, draw a contour line at the boundary and for each .25 magsas, and label them with the magsas (e.g 21.25). This is exactly like the contour lines on a topo map. This gives 3 lines within each color. This is needed since there is a large difference in one whole magnitude, and it would be helpful to remind people and show people where the changes are, down to a quarter of a magnitude. Along with this, I would greatly reduce the saturation of the colors to make the map look more "smooth". I have an example of this last, if you PM me an e-mail address I will send it to you.

Regards,
Lee

Edited by derangedhermit (08/15/13 02:04 PM)


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Tony Flanders
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Re: Introducing DarkSiteFinder.com new [Re: derangedhermit]
      #6027414 - 08/15/13 03:11 PM

Quote:

My first suggestion is to use the 255 grayscale map to create a new color mapping. It uses 7 colors:

clear - no light pollution, natural pristine skies (e.g. 22 magsas)
gray - 0-1 magsas brighter than natural, pristine skies (21-22)
blue - 1-2 magsas brighter (20-21)
green - 2-3 magsas brighter (19-20)
orange - 3-4 magsas brighter (18-19)
yellow - 4-5 magsas brighter (17-18)
white - 5+ magsas brighter (17+)





Aside from the practical problem of calibrating it, there's a more fundamental problem with this idea.

What's interesting is the amount of artifical skyglow, not the total skyglow. That's precisely the problem with the SQM; it measures total skyglow, and at the darkest levels (21.5 - 22.0) the variation is caused almost entirely by variations in natural rather than artificial sources.

Put another way, the difference between 18.0 and 19.0 is quite substantial, but by no means overwhelming. The difference in observing experience between 20.0 and 21.0 is much bigger, because at 21.0 quite a lot of the total light is coming from natural sources. And the difference between 21.0 and 21.5 (where less than half the glow is artificial) is even bigger than the difference between 20.0 and 21.0.

That's why the system of the original Light Pollution Atlas, which works in terms of artificial rather than total skyglow, makes more sense to me.


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derangedhermit
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Re: Introducing DarkSiteFinder.com new [Re: Tony Flanders]
      #6027795 - 08/15/13 06:24 PM

Perhaps. First, the website creator's intention is to help people find dark sites, not demonstrate the level of light pollution specifically, as was the intention of the World Atlas, So let's keep that goal in mind, in case it makes a difference.

First, natural skyglow does vary, as we have both said above, so the map is necessarily relative to the natural sky brightness. I hesitated putting fixed numbers with the scale because of this. If we should take away the direct association with integer mpsas values, that's OK with me.

The World Atlas works not in terms of artificial sky brightness alone, but in terms of a ratio between natural sky brightness and artificial sky brightness. Evidently they took this scheme exactly from Garstang's work, except for the addition of 0.1-0.10 dark gray.

My understanding is that the key to deep-sky observing is the contrast between extended objects' surface brightness and the background sky brightness. I suspect that is what the current scale attempts to capture, and the main thrust of your objection.

Yet it cannot hurt to review that scale to see if it remains the most effective way to indicate a significant difference between areas. I do not understand why the divisions used since the 1960's accomplishes that. The "3x" factor seems completely arbitrary to me.

I'm happy to work on the basis of changes in contrast, if that is the way to go; I suspect we can refine your example values.

I will PM Glenn LeDrew and ask him to join in, since he will certainly say something if he chooses to on the subject of contrast, and then maybe some of us can decipher what he might have meant. His mind works differently than mind (yeah, and better, too, I admit).

Regards,
Lee


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GlennLeDrew
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Re: Introducing DarkSiteFinder.com new [Re: derangedhermit]
      #6028163 - 08/15/13 10:20 PM

At Lee's behest...

First, I should put it out there that I don't put quite so much stock in the vaunted LP maps, which is why this topic did not catch my eye. Why my aloofness regarding this map? I live near the edge of a 'red' zone in west Ottawa, yet on a really good night, from my *stupendously* light-awash apartment balcony I can just glimpse the 'Mexico' portion of the North Amarica nebula with unfiltered 10X50 binos. This object has a surface brightness of perhaps 24 MPSAS, implying that the sky SB must be no more than 3-4 magnitudes brighter, or 20-21 MPSAS. Further, my NELM on such a night is about 5.8m. In (admittedly near the edge of) a *red* zone?!

Near sizeable populated centers, someone not too near the center of the light dome will often have a not insignificant gradient in sky brightness. For that reason it's hardly worthwhile to more finely quantize the brightness steps, at least for the brighter 1/2 or 2/3 of the scale (green--certainly yellow--and brighter.) But it might be worthwhile to more finely establish the divisions for the darker levels (due to the usually gentler brightness gradient), *if* the data permit this by not having significant error or noise.

Now, I'm not dismissing the LP map out of hand. It is a most useful indicator of at least relative sky brightness variation. And it permits to identify populated places of even quite small size (when seeking to avoid all light sources possible.) What I 'object' to is that it's too easily taken as a precise indicator, resulting from not appreciating sky brightness gradients and the inherent variability of atmospheric aerosols/transparency.

About the scaling of the map. It seems to quantize in factors of 3, where a factor of 2.5 (more precisely 2.512) would correspond to 1magnitude; more directly applicable to astronomers.

Could a correlation between LP map brightness, Bortle scale, SQM and (possibly??) NELM be feasible? As long as certain assumptions are made and clearly stated, perhaps. For one thing, a standard baseline for a pristine sky is a given, which presupposes a reasonably clean and dry airmass. For another, the mean air glow brightness by latitude should be established (it does brighten toward the poles.) And for NELM, the most variable and contentious 'standard', some acceptable mean must be established.

I don't use the Bortle scale, although I have a barely passing familiarity with it. But I do use my own variation of it, under dark skies, where certain milky way features are used as a gauge of sky quality. I find this more reliable than the SQM, as the latter merely measures brightness, which is not the same as clarity.

This is admittedly something of a jumble of first thoughts on the matter. Perhaps I can focus more coherently on specific aspects...


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derangedhermit
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Re: Introducing DarkSiteFinder.com new [Re: GlennLeDrew]
      #6028352 - 08/16/13 12:37 AM

Thank you Glenn. I thought of you since I think the optimum display scale will have a known relationship with the contrast of the sky brightness v extended objects, and I think I have learned from your chart that it is not linear with magnitude.

This site has DaveL's 15-color map (minus snow), overlaid on Google Maps, just in case you wanted to double-check your exact location.

The Bortle Scale
----------------
The Bortle Scale does include NELM as one of its several attributes of each class, so that is done. Other than that, John Bortle himself has said his scale is often substantially off compared to the World Atlas (or the other way around), and his scale was not intended for use as a map input. He appeared to not want to be associated with the World Atlas (his posts are in another recent thread here, judge for yourself). He made similar remarks regarding SQM readings. So, I think we respect his preferences if we leave his scale as a standalone measure, to be used on-site each night.

I myself would like to be able to compare the LP maps with SQM readings and the iPhone app (mentioned elsewhere on this LP forum). We know they won't match, but by doing better work with more, and more recent, data, each tool can be improved. IMHO it is worth the attempt.

The statement that the existing scale quantizes in a factor of 3 is, I think, an oversimplification. I suggested just using magnitudes (and decimal fractions thereof) and we can read Tony's feedback.

The equation that is used to assign color in the World Atlas is:
(nsb = natural sky brightness, asb = artificial sky brightness)
color range
black/clear...1.0nsb
gray...1.0 nsb to 1.10nsb (asp < 0.10 nsb)
blue...1.1 nsb to 1.33 nsb (asp < 0.33 nsb)
green..1.33nsb to 2.00 nsb (asp < 1.0 nsb)
yellow.2.0 nsb to 4.0 nsb (asp < 3.0 nsb)
orange.4.0 nsb to 10.0 nsb (asp < 9.0 nsb)
red....10.0 nsb to 28 nsb (asp < 27.0 nsp)
white..>28 nsb (asp >27 nsp)

Please correct me if I am wrong.
I don't think these come close to the same scale as magnitudes. i don't know how to convert this chart to magnitudes to find out.

The original research paper stated it used 21.6 magasa I believe as an average nsb.

BTW, there is no reason there should be just one chart, if it is helpful to have several or have one that is configurable.

Regards to all,
Lee


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derangedhermit
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Re: Introducing DarkSiteFinder.com new [Re: GlennLeDrew]
      #6028412 - 08/16/13 01:33 AM

Quote:

But it might be worthwhile to more finely establish the divisions for the darker levels (due to the usually gentler brightness gradient), *if* the data permit this by not having significant error or noise.




DaveL has a 256-level grayscale map that is smooth (low noise). He does not mention applying additional smoothing techniques to it I believe the grayscale image is linear. I have played with it, and you can d/l it from his site. I think it is suitable, if 250 levels is enough. I'm not sure I see much of a gentler brightness gradient, though.

Quote:

A standard baseline for a pristine sky is a given, which presupposes a reasonably clean and dry airmass. For another, the mean air glow brightness by latitude should be established (it does brighten toward the poles.)





I'm not sure we can do anything here without substantial effort. There was some follow-up work being done by the same WAofNSB fame, considering altitude, shadowing by hills, variations in airglow, etc. But it seems to have stopped.

Go where you will with this, but one question I would direct toward you is if it make sense to display a map that shows divisions based on the range of extended object contrasts (average, or what we have to work with that makes sense). By this I mean I've gotten the vague impression that matches in quality, if not in figures, what Tony says above: the difference you can see at the eyepiece between 21.5 and 21.4 is much larger than the difference you can see between 20.5 and 20.4 - or maybe the opposite is true, and a baseline of 21.5 is too dark, once you put some magnification on an object. Looking at your chart, I can't tell.

Yes, I'm lost, but I sense an opportunity to help prevent others in the future from being quite so much so. Give us a hand, mates.

Lee


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GlennLeDrew
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Re: Introducing DarkSiteFinder.com new [Re: derangedhermit]
      #6028469 - 08/16/13 02:38 AM

Lee,
I can't seem to get the LP overlay to display on my phone...

The advantage of dealing with magnitudes is that no matter where you are on the scale, a given difference in values is the same brightness ratio.

For example, between 20 and 21 magnitudes is the same brightness ratio as between 8 and 9, or -1 and 0.

The brightness figures the zones are based on seem to be the linear brightness ratio. To convert to surface brightness ratio in magnitudes:

SB = LOG(brightness ratio) * 2.5

For example, the green zone ranges from 1.33 to 2.0 times brighter than natural sky brightness. In magnitudes, this is 0.31 to 0.75 magnitudes. To make the math clear, for a brightness ratio of 2.0:

SB = LOG(2.0) * 2.5
SB = 0.301 * 2.5
SB = 0.753

If we assume this brightness is the *combination* of both natural and artificial light, thus being the expected brightness we would measure with a meter, then we subtract these calculated values of SB from 21.6. And so the sky surface brightness range in the green zone is expected to be 21.3 to 20.9 MPSAS.

Or consider the orange zone, where the linear brightness ratio is 4.0 to 10.0. The magnitude equivalents are 1.5 and 2.5 magnitudes, respectively. Subtracted from 21.6, the orange zone's sky surface brightness is thus expected to range from 20.1 to 19.1 MPSAS.

Lastly, the white zone's sky is more than 28 times brighter than a pristine sky. This is 3.6 magnitudes, making the sky brighter than 21.6 - 3.6 = 18 MPSAS.

If the brightness zones are instead *just* the contribution of artificial light, which then must be added to natural air glow, that requires additional math. If it comes down to that.


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derangedhermit
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Reged: 10/07/09

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Re: Introducing DarkSiteFinder.com new [Re: GlennLeDrew]
      #6030108 - 08/16/13 11:39 PM

Cinzano et al picked 21.6mpsas as a representative average number from a site under the atmosphere with no light pollution. (This is at V band, at sea level, no shadowing.)

Brian Skiff wrote up a post explaining at least one source for 22.0 mpsas as "as dark as it ever gets, during the lowest part of the solar cycle" when looking through the atmosphere at zenith. I will pick that for this post. It will be something different every night, of course.

So this is the existing World Atlas scale with corresponding MPSAS:

Code:

LP* MPSAS Color code
0.00 -0.01 22.0 black/clear
0.01 -0.11 22.0 to 21.9 gray
0.1 to 0.3 21.9 to 21.7 blue
0.3 to 1.0 21.7 to 21.3 green
1.0 to 3.0 21.3 to 20.5 yellow
3.0 to 9.0 20.5 to 19.5 orange
9.0 to 27.0 19.5 to 18.4 red
27.0+ 18.4- white



*LP in column one: a value of "1" means the artificial sky brightness at zenith equals the natural sky brightness at zenith, as seen from the earth's surface.

So we get these ranges:
0.01 mag (black)
0.1 mag (gray)
0.2 mag (blue)
0.4 mag (green)
0.8 mag (yellow)
1.0 mag (orange)
1.1 mag (red)
(and then white for everything brighter than 18.4 mpsas)

Neither the lack of alignment with the magnitude system nor the formula used to calculate the ranges seem as helpful as they might be to amateur astronomers.

One thing that is needed is some indication of what level of accuracy we think is appropriate to display. Empty accuracy is misleading. The World Atlas says little about error; what they did show suggests 0.1 mag accuracy is not there. I think 0.25mag might be better as a minimum band size.

The World Atlas paper states that in dark areas errors are significant, since they represent a much larger component of total sky brightness. They also state that any error n their work is less than the variability in natural night sky brightness (something between 0.5 and 1 mag).

I still think my suggestion of 1 color per magnitude, with indicators at 0.25 mag intervals, makes sense. I think I read some agreement and some disagreement. After doing the table above, I would modify it to be:

Code:

clear/black - natural pristine skies with no light pollution
gray - 0-1 mag of LP, with 4 shades to indicate 0.25mag contours
blue - 1-2 mag of LP
green - 2-3 mag of LP
yellow - 3-4 mag of LP
white - 4+ mag of LP
with contour lines at 0.25 mag intervals.



This has the benefits of aligning the map with integer magnitudes and showing the distinctions in the dark zones that do likely exist.

Comments? Would anyone want to see what such a map would look like?


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GlennLeDrew
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Re: Introducing DarkSiteFinder.com new [Re: derangedhermit]
      #6030141 - 08/17/13 12:01 AM

Lee,
I feel that the regime of the darker skies needs to be more finely differentiated than the more light polluted regime, because the contrast index is logarithmic, and the natural sky glow is a bias toward which the contrast index effectively converges asymptotically.

Think of it this way. When you go from 3 to 4 magnitudes (delta = 1mag.) of man-made light on top of a dark, 22 MPSAS sky, the difference in the view is not really large. But go from 1 to 2 magnitudes (delta again 1 mag.) of light on top of that same dark sky, and the difference is much more profound. And from 0 to 1 magnitude (the same 1 mag. delta) of artificial light would be the proverbial night and day difference!

This is why the scale currently in use is more compressed toward the bright end and expanded toward the faint end. One could work out the basis of this by plotting the contrast index defined by the ratio of artificial to natural sky glow on a logarithmic scale. Or something like that; I've not worked on this problem myself.


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derangedhermit
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Reged: 10/07/09

Loc: USA
Re: Introducing DarkSiteFinder.com new [Re: GlennLeDrew]
      #6030231 - 08/17/13 01:21 AM

Quote:

Lee,
I feel that the regime of the darker skies needs to be more finely differentiated than the more light polluted regime, because the contrast index is logarithmic, and the natural sky glow is a bias toward which the contrast index effectively converges asymptotically.

Think of it this way. When you go from 3 to 4 magnitudes (delta = 1mag.) of man-made light on top of a dark, 22 MPSAS sky, the difference in the view is not really large. But go from 1 to 2 magnitudes (delta again 1 mag.) of light on top of that same dark sky, and the difference is much more profound. And from 0 to 1 magnitude (the same 1 mag. delta) of artificial light would be the proverbial night and day difference!

This is why the scale currently in use is more compressed toward the bright end and expanded toward the faint end. One could work out the basis of this by plotting the contrast index defined by the ratio of artificial to natural sky glow on a logarithmic scale. Or something like that; I've not worked on this problem myself.




Great! Thanks, Glenn, I think I understand that, and it matches what Tony was explaining but I couldn't understand.

I think you're saying that 22mpsas light pollution double the (best possible) natural sky brightness, while another 22mpsas only makes it another 50% darker. And the thinking then is that does what to your ability to see fainter objects?

My understanding is that as the sky brightness decreases, then you can see extended objects with lower surface brightness. There is some relationship there, between darker skies and our ability to see fainter objects, correct?

If the relationship is one-to-one, then for every magnitude decrease in sky brightness you could see extended objects that are one magnitude fainter, right? Like this:
Code:

SB OB (OB=Object brightness)
19 23
20 24
21 25
22 26


Right? That would be one-to-one. What you two seem to be saying is that it is not 1:1; the current ratio is 1:3, and that seems about right to you. That going from 21.9 to 22.0 is as important as going from 21.3 to 21.7, or from 19.5 to 20.5. Right?

For each magnitude decrease in sky brightness you could see extended objects that are three magnitude fainter, right? Like this:
Code:

SB OB (OB = object brightness)
19 23
20 26
21 29
22 32


Yet at this chart, it looks like things are going in the other direction, by a ratio of about 3:4:
Code:

19 23
20 23
21 24
22 25


Where as the skies darken one magnitude, the eye can see fainter objects, but only objects that are about 0.75 mag fainter.

I'm confused now, and will retire to read my RASC to see if it helps.

Lee


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nexrad1
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Loc: Frederick, Maryland
Re: Introducing DarkSiteFinder.com new [Re: derangedhermit]
      #6034365 - 08/19/13 11:20 AM

Thanks for the map, Kevin, I think it does a great job of combining the data with Google maps and is very useful as a initial guide to finding local dark regions. One suggestion I have...does the US border need to be white? Look in the Washington, DC/Maryland eastern shore area. I know some coastal sites are white areas due to heavy coastal development, but the white boarder makes all coast lines look like Atlantic City.

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