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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3125
Loc: Just passing through.....
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Under very dark skies the transparency of the atmosphere can change noticeably over a period of minutes.
This was easily noticeable at the GSSP on Monday night when the Milky Way would ebb and flow in size and structure after midnight. It was commented on by several very experienced observers that I was hanging out with.
Differences in contrast correlated highly with my readings using a Sky Quality Meter (SQM.) Readings of the same part of the sky both away and at the MW showed variability of 0.3 magnitudes over a couple of hours.
This is largely caused by excitation of oxygen in the upper atmosphere and can vary over scales of minutes, much as an aurora display but much more subtle.
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10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT94 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
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Astrojensen
sage
Reged: 10/05/08
Posts: 203
Loc: Bornholm, Denmark
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A couple of interesting links about airglow:
http://en.wikipedia.org/wiki/Airglow
http://www.atoptics.co.uk/highsky/airglow1.htm
Clear skies!
Thomas, Denmark
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3125
Loc: Just passing through.....
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Also see....
The Light of the Night Sky by FE Roach and JL Gordon
1973 ISBN 90 277 0293 4
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10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT94 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3369
Loc: Cambridge, MA, USA
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Quote:
Under very dark skies the transparency of the atmosphere can change noticeably over a period of minutes ...
This is largely caused by excitation of oxygen in the upper atmosphere and can vary over scales of minutes, much as an aurora display but much more subtle.
I don't think that airglow should be confused with transparency. Both of them can vary over short time scales, but they're two unrelated phenomena.
I've never seen a formal definition of transparency, and I doubt that such a thing exists. But it has to do with a reduction or transformation of the light from a celestial object. Extinction is certainly part of it, but maybe not all of it.
Airglow, by contrast, is perhaps best thought of as light pollution -- albeit natural rather than artificial. Same idea as an aurora, the zodiacal light, or scattered moonlight. It doesn't reduce the incoming light, but instead masks it by adding extra light both to the dark and the bright areas of the sky.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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Hrundi
Pooh-Bah
Reged: 02/06/08
Posts: 1159
Loc: Estonia
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I've also seen fascinating degrees of variability.
When I started observing, the winter milky way was a fairly bright, but mostly unremarkable band.
Then suddenly in about 5 minutes it got brighter, had finer filaments visible, and the entire band had a texture similar to marble. I've never seen the winter portion like that before or since that time.
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3125
Loc: Just passing through.....
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OK....let's not use the word "transparency." How about a temporal fluxuation in the signal to noise ratio?
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10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT94 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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7331Peg
professor emeritus
Reged: 09/01/08
Posts: 680
Loc: North coast of Oregon
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I've seen this many times also. Kind of like the idea of calling it a fluctuation in signal to noise ratio - except I like a nice quiet night instead of a noisy one! 
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12168
Loc: Los Angeles
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I saw this the last time out, and it more-or-less corresponded to the momentary changes in transparency.
I think this can also be small scale, too, as I see it often when viewing a large low-surface-brightness object. The details in the galaxy or nebula seem to come and go over a span of minutes.
There is not any noticeable cloudiness when this happens (blue skies at sunset and dawn and no high cirrus passing through)
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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