Re: Ultra Dark-Adaptation?

Hi,

AstroApp wrote:
So that then leads me to a vague hypothesis regarding these excellent nights that we have been talking about. What if a really good night allows more stars to be visible in the eyepiece, the increased light of which causing a slight shift toward photopic vision, which then enhances the apparent contrast? I already see some problems with this, but it also seems to me that there is *something* here, given the effects I have observed.

Well, perhaps this is indeed true but I am also noticing rare,
particularly superb viewing even with deep dark adaptation that I take
great care to preserve. These kinds of nights are when, say, the
Horsehead really LOOKS like a "head" and not just a barely discerned,
vague darkish region that you can't quite hold with direct vision.

The best I can come up with is this: a night of exceptional transparency (perhaps accompanied by excellent seeing as well) triggers something in the eye-brain that magnifies the effect, resulting in improved contrast. For the borderline objects the effect in the eyepiece is more dramatic than photometry can account for.

But here's another possibility. Contrast is the result of two factors: how much light is being transmitted and the amount of natural skyglow. What if these special nights are simply a rare combination of high transparency (low extinction) and diminished skyglow? Perhaps related to the amount of high-altitude atmospheric dust? It would be interesting to investigate this using one of those all-sky meters.

Greg

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

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