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General Astronomy >> Light Pollution

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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Is a little light pollution good?
      #5573175 - 12/15/12 08:39 PM

There have been a couple posts by seasoned observers saying that there comes a point where the eye functions better (or at least just as well) with a little more light. One example: observer says he can see faint stars just as well at 21.4mag skies as at 21.9 mag skies. Another: Glen LeDrew's chart and statements about total light v. object size and brightness.

Adding to this, the existing light pollution charts based on the Italian work emphasize small differences in brightness levels between 22.0 and 21.0. This, especially combined with the above, seems misleading to me.

Or?


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Tony Flanders
Postmaster
*****

Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Is a little light pollution good? new [Re: derangedhermit]
      #5573276 - 12/15/12 09:38 PM

Quote:

There have been a couple posts by seasoned observers saying that there comes a point where the eye functions better (or at least just as well) with a little more light.




As far as I know, the only case where the eye functions better is when observing bright objects such as the Moon and (perhaps) Jupiter. Venus, of course, is best observed in bright twilight or even daylight.

Quote:

Adding to this, the existing light pollution charts based on the Italian work emphasize small differences in brightness levels between 22.0 and 21.0. This, especially combined with the above, seems misleading to me.




The difference between 21.0 and 22.0 is enormous. At 21.0, the Milky Way is severely washed out. Still plenty easy to see, but much of the subtle detail is gone. Even some fairly bright DSOs, such as Barnard's Galaxy, are genuinely tough to spot.

Moreover, light pollution is inevitably stronger toward the horizon. So if the zenith glows at magnitude 21.0 per square arcsecond, it will be 20.0 25 degrees above the horizon. That's particularly unfortunate for northern observers, who can never see the central Milky Way high off the horizon.

A lot depends on the direction of the main light source. If it's north, 21.0 may not be so bad, because everything that's low in the northern sky gets higher at some other time. But if it's in the south, you really lose a ton.

I'm not sure the Light Pollution Atlas needs quite so many gradations; perhaps four zones could be compressed into three. But that's not much of a difference.


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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Is a little light pollution good? new [Re: Tony Flanders]
      #5574100 - 12/16/12 12:34 PM

Glenn's chart and explanation can be found at http://www.cloudynights.com/photopost/showphoto.php?photo=17726&password=... It seems to indicate that the eye needs an overall minimum amount of light, below which detection of objects of various sizes becomes more difficult, even if the contrast between the object and the sky is increased by the darker background.

I think natural sources of night light also increase toward the horizon.

Is anyone aware of tests of NELM (or similar) of astronauts with dark-adapted eyes? I wonder what the eye sees without the natural sky brightness.


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Tony Flanders
Postmaster
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Is a little light pollution good? new [Re: derangedhermit]
      #5574438 - 12/16/12 04:30 PM

Quote:

Glenn's chart and explanation can be found at http://www.cloudynights.com/photopost/showphoto.php?photo=17726&password=... It seems to indicate that the eye needs an overall minimum amount of light, below which detection of objects of various sizes becomes more difficult, even if the contrast between the object and the sky is increased by the darker background.




Of course that's true! Any detector has a minimum threshold below which it won't work. Your eyes won't detect any light if they're only picking up one photon per hour.

But you seem to be extrapolating that to indicate that decreasing the background brightness doesn't help. No doubt that becomes true at some level. But that level is far below anything ever seen on Earth -- or in outer space, for that matter.

Quote:

I think natural sources of night light also increase toward the horizon.




The three major natural sources are airglow, the zodiacal light, and starlight. Of those, airglow increases toward the horizon and the other two decrease.

Being in outer space cuts out airglow, but of course the zodiacal light and starlight are brighter.


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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Is a little light pollution good? new [Re: Tony Flanders]
      #5576427 - 12/17/12 09:09 PM

I am not extrapolating anything from the chart. The need for the eye to have more light is already apparent in the range of typical observing skies, according to the chart. The notes say there is more info in the RASC handbook.

It is possible I *misunderstand* the chart, though.


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Glen A W
professor emeritus


Reged: 07/04/08

Loc: USA
Re: Is a little light pollution good? new [Re: derangedhermit]
      #5577042 - 12/18/12 10:13 AM

I was very used to a mag 4.5 sky. Now that I live under a mag 6.5 sky, I actually would like to go back to the sky I had before! Somehow, it just looked better to me. Some of the constellations nearly disappear into the masses of stars when you live in the dark areas. Maybe I was just so used to the LP'd sky that I think that is how it is meant to be. I thought it was more beautiful, believe it or not. I mostly did lunar and planetary work, and variable stars, and that sky just seemed "right" to me. Very strange. GW

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Tony Flanders
Postmaster
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Is a little light pollution good? new [Re: Glen A W]
      #5577173 - 12/18/12 11:39 AM

Quote:

I was very used to a mag 4.5 sky. Now that I live under a mag 6.5 sky, I actually would like to go back to the sky I had before! Somehow, it just looked better to me. Some of the constellations nearly disappear into the masses of stars when you live in the dark areas.




Other people have reported that, but I find all the constellations easier to make out in dark skies. Much easier, in fact.

Quote:

I thought it was more beautiful, believe it or not. ... Very strange.




Strange indeed! But of course you still get to see your 4.5-magnitude sky every month at full Moon. And for a few minutes in twilight every dusk and dawn.

As for being more beautiful, the obvious retort is:

But what about the Milky Way?


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Starman81
Carpal Tunnel
*****

Reged: 03/06/08

Loc: Metro Detroit, MI, USA
Re: Is a little light pollution good? new [Re: Glen A W]
      #5602295 - 01/03/13 02:01 AM

Quote:

I was very used to a mag 4.5 sky. Now that I live under a mag 6.5 sky, I actually would like to go back to the sky I had before! Somehow, it just looked better to me. Some of the constellations nearly disappear into the masses of stars when you live in the dark areas. Maybe I was just so used to the LP'd sky that I think that is how it is meant to be. I thought it was more beautiful, believe it or not. I mostly did lunar and planetary work, and variable stars, and that sky just seemed "right" to me. Very strange. GW




I have such a sky and would like to trade... Now to figure out how to actually do it.

Yeah, what about the great beauty of the Milky Way??


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planet earth
Carpal Tunnel
****

Reged: 09/07/10

Loc: Ontario Canada
Re: Is a little light pollution good? new [Re: Glen A W]
      #5602370 - 01/03/13 05:07 AM

Quote:

I was very used to a mag 4.5 sky. Now that I live under a mag 6.5 sky,




Mag 6 or better is a dream made in heaven for DSO's.
Maybe your observing interests will change.
I find it amazing when I travel North to mag 6 plus skies from mag 3-4 skies. At first it's hard to find the constellation Orion.
All those stars, gives you a very strange feeling.
But then after 2-3 days it's back to the city.
Sam


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