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Mr. Bill
Carpal Tunnel
Reged: 02/09/05
Posts: 2759
Loc: Just passing through.....
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One of the projects during my recent trip to central Nevada was to trace the edges of the MW which is not as easy as it sounds.
In his book, "Deep-Sky Wonders," Walter Scott Houston talks about this and calls the ambiguity "Houston's Uncertainy Principle."
To quote Scotty..."The next time you're out under a very dark sky, look up at the Milky Way. How wide is it? The answer is not as simple as it seems. The answer depends not only on where you look but how you look."
He further goes on.... "Over the years my conviction has been growing that the atmosphere is really what determines the limits visible to the eye, and the atmosphere can change on a time scale of minutes."
This was obvious when examining the boundry of the Cygnus MW which was overhead.....the width would ebb and flow with the transparency as the 7th and 8th magnitude stars "popped out" of the background stars for several minutes to an hour and then receded.
This correlated with fluctuations in my SQM readings, which would vary as much as 0.05-0.1 magnitudes of time scales of minutes to tens of minutes.
Much of this is due I believe to the changes in oxygen excitation in the upper atmosphere by fluctuations in solar radiation. This really defines the "floor" of the signal to noise ratio that is normally called "contrast."
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stevecoe
Reged: 04/24/04
Posts: 2129
Loc: Arizona, USA
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Bill;
I agree completely. I don't know the cause but the seeing and sometimes the transparency under a clear sky is interesting to watch as it changes during the night. Very weird effect when you are good and dark adapted. I often use the width of the Milky Way to determine the night's rating for transparency. There is a very faint outer isophote of the Milky Way that goes from Lyra to the head of Scorpius, through Ophiuchus and Serpens. When you can see that faint outer band of the Milky Way it is a dark night and you are well adapted to it.
In the Winter I use the brightness and contrast of the Milky Way in Perseus and Monoceros to determine how good the transparency is for that night. If the Double Cluster is within a wide and thick swath of the Milky Way you are in for a good night.
Enjoy;
Steve Coe
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150mm 6" f/8 Celestron Refractor on Sirius Mount
80mmED 3" f/7.5 Orion Refractor
Author "Deep Sky Observing" Springer-Verlag
Author "Nebulae and How to Observe Them" Springer
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Tony Flanders
Carpal Tunnel
Reged: 05/18/06
Posts: 2098
Loc: Cambridge, MA, USA
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Quote:
One of the projects during my recent trip to central Nevada was to trace the edges of the MW which is not as easy as it sounds.
That doesn't sound easy at all! After all, it's quite obvious that the Milky Way doesn't really have an edge at all. Technically, it covers the entire sky. In practice, visually, it doesn't end but rather fades out.
Anywhere you see sharp edges associated with the Milky Way, they're caused by dust clouds.
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Tony Flanders
eyeglasses
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GlennLeDrew
professor emeritus
Reged: 06/18/08
Posts: 624
Loc: Ottawa, Ontario, Canada
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Neglecting man-made causes, the fluctuating brightness of the foreground sky is most often due to airglow. I've seen--and taken--photos of airglow which show that it can have surprisingly small scale structure, usually in bands. So there can be two processes at work; temporal pulsation and place-to-place brightness variation.
The "edge" of the milky way band is a chimera, to be sure. I've found that the smallest changes in sky glow make a substantial change in the faintest "isophote's" location. And as Tony points out, dust clouds do have a part to play in some portions.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
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Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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