On 11/17/2010 11:16 AM, glen youman wrote:
Rick -

How dark is your sky, I ask because your exposure time seems to
indicate dark sky's and the ClearSkyChart for Mantrap Lake doesn't
have a light pollution map.

I would love to live in a dark sky area but the boss (my wife) has
veto power on those type decisions.

Below is an out of date LP map for my location. There has been
significant development to the west in the last few years, greatly
adding to the LP. I pretty much limit my imaging to northeast
quadrant and even then I'm fighting significant gradients.


I'm in the middle of no where. Back before cataracts I could easily see
6.5 magnitude at the zenith. Younger eyes were reaching 7. I still
don't have glasses since the surgery as one eye remains too swollen to
fit for glasses so my vision isn't great. I was 20:1200 without glasses
without cataracts I was so nearsighted. Also have several cylinders of
astigmatism 3.5 left eye and 4.25 right. That isn't corrected though
the lenses do give me nearly correct vision otherwise. Still too poor
due to the astigmatism to know how it has helped me see the sky. Still,
in summer I can see my fuzzy shadow on the deck and side of the house
cast by the light of the Milky Way.

My wife grew up in pot hole country of North Dakota where the nearest
town was 20 miles away and had 120 people. So living here is just what
she likes. She hated living in the city when we were working full time.
So the move up here was not a problem. Though she might have
preferred the family farm in ND. It's a 4 hour drive west from here so
she gets back from time to time. Their skies aren't as dark however.
Every farm has several "security lights" that, of course, provide just
the light a crook needs to do his thing yet hide in the stark shadows
while doing it. So crime is up about 500% since they went in! But
farmers like them for working in the yard at night. Result is about 5.5
magnitude skies even though the population is still low. Not as low as
here however.

This time of the year all have gone south. There's only 4 families that
live in this township all winter for a population of 18. Being heavily
wooded the few "security lights" here are all well below tree line which
is another 40 foot higher and blocks their light almost totally. While
I can see nearly three miles across the lake I can see only ONE home or
cabin, summer or winter. The only lights it has in summer are well
shielded trail lights lighting the path from cabin to dock. They do run
all night -- to let the bears find the water? In any case they aren't a
problem but can be seen dimly. Other than that, even from atop my hill
which is the highest around the lake, I can't see any other lights.
Park Rapids has a rather nasty light dome to the SW so avoid imaging
there. It has a population of only 3000 but is a tourist town. Each
business out-lights the next trying to attract customers that don't
exist this time of the year as only locals left now that deer season is
over. Some snowmobilers will show once we get a snow base but they are
weekenders from Fargo or the Twin Cities. All businesses complain about
the high cost of electricity but yet waste it thinking lighting bellies
of flying ducks attracts customers. It is so well lit that most drive
at night without their lights on! You can see to drive fine but if your
car is dark no one sees you. Result is lots of nightly accidents.
Editor of the paper has constantly pleaded with folks to turn on their
car lights at night and can't fathom why they don't. Comparing the
famous night sky photo for Park Rapids at 3000 to towns back in Nebraska
with populations of 30,000 you find Park Rapids brighter (though a
smaller foot print but not by much). No business will be the first to
cut back. Bemidji is 10 miles more distant to the north but has
virtually no light dome from here. It too is a tourist town but hasn't
gotten into the light trap of Park Rapids, besides, thanks to my Polaris
Tree (actually 2 trees) I can't image to the north anyway.

My main problem is air glow. It is far worse here than in Nebraska. It
is really bad in winter. When it is low my typical background in a 10
minute 2x2 binned image is 250 ADU after dark subtraction. In winter it
can hit 1200 or more. Far stronger in green and sometimes red. I've
noticed the new Generation 2 filters have reduced the red portion
considerably yet seem to have more red sensitivity to red stars and
nebula making color balance far easier when I do have air glow problems.
Even in green it isn't as much of a problem. No change in the L
channel however.

Now that the sun is getting active I expect air glow to worsen and faint
all sky aurora to start to be a problem. So far I've lost only a couple
good nights because of it. When it hits my background in L hits 4000
and I shut down. I expect that to start to be a problem.

Most calculators for sky limited exposures say I need subs of over 1
hour. That's more than my normal total exposure time and would so
saturate stars there'd be no color left in them. I figure with that low
of sky noise I'm still ahead of many as far as total noise goes. I do
use 30 minutes for H alpha. Even then my background is usually about 40
ADU after dark subtraction. I'd need 24 hour subs at that rate!

Rick
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