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Tony Flanders
Carpal Tunnel
Reged: 05/18/06
Posts: 2202
Loc: Cambridge, MA, USA
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Re: Ground Truth for the Bortle Scale
07/09/08 09:15 PM
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Here's a semi-dark site that's particularly useful for callibration purposes, because many people go there: Stellafane, just south of Springfield, VT.
The Clear Sky Chart page for Stellafane places it in the yellow zone, but I think that's its much more typical of the green zone. I attribute the discrepancy to two causes. First, this is an area with a sharp skyglow gradient, and a trivial change in callibration would put Stellafane on the green side of the border. Second -- a more telling problem with the color-zone map -- Stellafane's light pollution is strongly asymmetric. It's worst to the north -- where nobody needs to look and not nearly so bad to the south, the direction that really matters.
Even so, Stellafane has plenty of light pollution; there's light welling up from the horizon in every direction. And disturbingly, the light dome of Greater Boston, more than 100 miles to the southeast, is quite obvious. The southern Milky Way is seriously affected; for instance, the Pipe Nebula, though visible naked-eye, is by no means easy to see, as it is at any truly dark site.
Near the zenith, though, the sky's not half bad. I get SQM readings around 21.5 there, and telescopic observing isn't seriously compromised. I've never looked for the zodiacal light at Stellafane, but I assume that it's pretty easy to see -- at the right time of year.
All in all, Stellafane fits the criteria for Bortle class 3 pretty well.
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Tony Flanders
eyeglasses
6x15 and 8x32 monoculars
8x25, 7x35, 10x30 IS, 10x50, and 15x70 binoculars
70mm and 100mm achromatic refractors
4.5", 7", and 12.5" Dobs
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