| Mr. Bill |
| (Post Laureate) |
| 06/28/09 01:24 PM |
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Transparency variability.....
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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.
| Astrojensen |
| (sage) |
| 06/28/09 03:23 PM |
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Re: Transparency variability.....
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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
| Mr. Bill |
| (Post Laureate) |
| 06/28/09 03:44 PM |
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Re: Transparency variability.....
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Also see....
The Light of the Night Sky by FE Roach and JL Gordon
1973 ISBN 90 277 0293 4
| Tony Flanders |
| (Post Laureate) |
| 06/29/09 07:03 AM |
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Re: Transparency variability.....
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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.
| Hrundi |
| (Pooh-Bah) |
| 06/29/09 07:27 AM |
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Re: Transparency variability.....
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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.
| Mr. Bill |
| (Post Laureate) |
| 06/29/09 09:51 AM |
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Re: Transparency variability.....
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OK....let's not use the word "transparency." How about a temporal fluxuation in the signal to noise ratio?
| 7331Peg |
| (professor emeritus) |
| 06/29/09 06:53 PM |
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Re: Transparency variability.....
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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!
| Starman1 |
| (Vendor - Scope City) |
| 07/04/09 02:54 PM |
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Re: Transparency variability.....
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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)