Re: Ultra Dark-Adaptation?

Greg,

Wow, do I agree with *everything* you said here!

One of the issues, in fact, has just become obvious to me in the last
few days; before that I was going by intuition and faith, probably,
rather than by actual experience or testing:

On Mon, 22 Jan 2007 12:10:53 -0700, Greg Crinklaw
<theskyhoundyoureye@xxxxxxxxx> wrote:

Hi,

AstroApp wrote:
I look often at the GOES satellite page before attempting to observe,
and download the 'movie' images of the infrared, visible, and water
vapor detectors.
http://www.goes.noaa.gov/

I used to pay a lot of attention to the water vapor image, but over time
it became pretty clear that there wasn't nearly the correlation I had
assumed there would be with transparent skies. That's in the southwest
U.S. though, other locations likely have significantly different
conditions. We also almost never have the jet stream down this far south.


Yes, I *now* think that the water vapor image is probably not nearly
as relevant as I thought before.

On the night of Monday 22 Jan. I was startled to see, looking at the
three west coast satellite images on the GOES page, that there was a
"hole" right over the SF bay area where the water vapor picture was
very dark, i. e. little vapor. There also was high pressure and an
absence of clouds.

I got ready and went up the mountain to observe (at 3,400 feet
elevation) and what I beheld there was not a particuarly superior
night at all! It was not quite as good as it had been on the prior
evening I was there, on 17/18 January. I checked for a confirmation
of M1-18, which I'd spotted that previous night; but on 22 Jan it
wasn't nearly as visible. If I had tried on 22/23 Jan. to get it, I
would have considered it either utterly tentative, or probably a
failed observation; the earlier night -- when the water vapor "hole"
wasn't over the bay area -- it was definitely more distinct, and
repeatable.

So, you're right about this: the water vapor issue is not the thing
that's the magic ingredient!

After much
experimentation, the presence or lack of many stars in the field seemed
to be the only constant; if there were many stars, the contrast was much
better and the view much more pleasing.

How true!

Another observation that I think may likely be related: sometimes when I
run out the back door without any dark adaption to look at the sky,
after a few minutes of rudimentary dark adaption the Milky Way looks
amazing, with bright star clouds with prominent dark lanes. But after I
become fully adapted, the sky looks washed-out, much brighter, and oddly
enough, the Milky Way is much less appealing.

I notice this almost every time I observe at my private property site
in the mountains. Sometimes I really don't look at the sky with naked
eye until AFTER I have put the telescope and the rest of my gadgets
away, with the dome light on in the car. I then turn that off, and
the sky looks FANTASTIC: pitch dark, glorious, and yet star-studded.
I guess that ten minutes or so of putting the scope away, under the
rather indistinct glow of the dome light, isn't enough to completely
ruin my vision; there is still enough sensitivity to the stars down to
about 5th magnitude to give the sky a sparkling appearance with higher
contrast than noticed with profound dark adaptation (when my slightly
light polluted sky looks pretty uninspiring.)

Now I'm no expert on the human eye, but these things together smack to
me of scotopic vs. photopic vision, with the nice contrast coming in the
perhaps partly? photopic case.

I think this is very likely the case.

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.
Such was the night when I finally managed to get it with no filter at
all. That is something I tried to do since about 1985, with no
success; it ALWAYS required a filter. Then, one night there was some
combination of atmospheric phenomena that increased the contrast.
It's probably the confluence of many different things, and would be
plotted if you could quantify half a dozen of the characteristics of
the sky: the thing that nobody does, now that visual astronomy is just
a hobby for amateurs. But I bet that if professional astronomers
weren't doing imaging, they'd by now be so concerned with this that
surely someone would have done a good scientific study.

I have read the precursors of this, back in the late nineteenth
century professional literature. But those articles and papers were
not very well documented nor were the experiments carefully
controlled, and weren't much more substantial than what today's
amateurs could be doing. Usually they were related to site tests,
such as Douglass's work on the air masses in the vicinity of sites
developed by Lowell Observatory.

AstroApp
.

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