Re: Ultra Dark-Adaptation?

On Mon, 22 Jan 2007 17:43:03 GMT, AstroApp wrote:

Years ago here there was a discussion of sky "background" and its
visibility, to which I contributed a comment or two. I notice even
with small aperture scopes that on some occasions there is a very
distinctly visible, palpable irregular background that does not look
to me like an average of the indiscernible light of faint stars, but
rather like a cloudy nebular glow. Of course it is more apparent near
the Milky Way than away from the bulge of our galaxy, so I think that
when THIS phenomenon is visible, we get the best nights for seeing
faint objects. But, when it's visible I can usually detect at least
some of it EVERYWHERE across the whole sky.

Perhaps a measurement of the exit-pupil transition point where the
background sky darkness matches and merges into the darkness on the
other side of the field stop would prove useful. Though this would
most likely only be relevant under *dark* sky conditions with a fully
dark-adapted observer.

Unlike the normal recommendation for deep sky observers usually given
by most experts -- to use low power -- I tend to use the highest power
possible (except, of course, when looking for extremely large angular
diameter objects): so I find myself employing, say, 466x which is
convenient for me with my C-11 with a 6 mm wide field ocular. On
those nights when the "background" is quite noticeable, compared to
the occasions where it is just a bland dark "mush", just about
everything looks really great, and more detail is seen.

I think the 'use low power' advice has been fairly well discounted
now. I use low power to see large DSOs in their totality; but in
general, considerably higher powers will show much more detail.

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/

That would be interesting information to correlate with amateur
observations. I may have to create another bookmark . . . OTOH, don't
I already have *enough* to do to prepare for a 'recreational'
observing session? ;-)

It also occurred to me that Todd Gross probably knows exactly what is
going on. It has been some time since I've read him here; he has an
elaborate website --
http://www.weatherman.com/
-- and also has written some interesting articles about the
meteorological impact on observing, such as this one about the jet
stream:
http://www.cloudynights.com/item.php?item_id=1482

Ah yes, Todd Gross, another of saa's former luminaries. He is missed
here!

With regard to the jet stream, it seems to me that when I look at it
using satellite imagery, it's almost ALWAYS bad and flowing right over
my geographical location. Maybe this is one of the most important
mechanisms that affects those good vs. bad vs. average nights.

The jet stream certainly effects seeing conditions . . .

Meanwhile, I still tend to be surprised by things that I can see when
the weather does not seem remotely cooperative, such as one of my best
views of IC-10 when I struggled to overcome light pollution in some
regions of my local sky, high winds, and the unpleasant and upset
weather conditions near a storm front. Though I've seen the galaxy
better in a darker sky, I can't ever recall detecting more of the
*granularity* of its apparent "surface" -- yet I almost did not bother
to take out my telescope and observe.

Such nights are indeed odd. My California Nebula night was such a
night. I didn't bother setting up a telescope because the night was
'supposed to' be poor (below average to poor transparency with some
clouds) for astronomy!
--
Bill
Celestial Journeys
http://cejour.blogspot.com
.

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