[AZ-Observing] Re: Trip to NM

Stan raises a lot of interesting points about sky darkness.  Without actual 
measurements, I have noticed that the skyglow component at really dark sky 
sites seems to vary during the night and, probably, with date, season, and 
whoknowswhat.  I find it useful to note how the gray glow varies as you go from 
zenith to horizon (in a zone free of Milky Way, Zodiacal band, and city light 
domes).  The night always starts off with noticeable glow extending up about 30 
degrees from the horizon, the "crud".  On good nights, the height lowers and is 
down to about 10 degrees several hours after sunset.  It can remain all night, 
but some nights it vanishes altogether. I always feel that an hour or so after 
midnight gives the least visible sheen.  

In 2002, I was in the deep outback of Western Australia with the nearest little 
light bulb over 200 km away and witnessed the most amazingly dark sky I had 
ever seen.  A re-visit in 2005 at the same time of year was disappointing; 
there was a gray sheen over all the sky and lots of horizon crud even though it 
was really clear.  However, it was slightly more humid on the return trip, so 
maybe that is a more important factor than we think.  

The Sun blasts UV into the top of the atmosphere all day and, no doubt, causes 
things to crackle and pop long after sunset in response to electrons dropping 
into lower energy states. The midnight hours may put us in the darkest shadow 
for things going on way up there?  

Could ramble on with lots of anecdotal comments, but I'm sure someone in this 
group has some actual scientific wisdom on the matter.  Much of the action on 
those sky-glow meters seems to be in the decimal places.  Maybe there are 
serious photometric studies with science-based interpretations?  Thanks for 
bringing up the skyglow issue.


Paul


-----Original Message-----
From: az-observing-bounce@xxxxxxxxxxxxx on behalf of Stan Gorodenski
Sent: Mon 5/25/2009 1:23 PM
To: AZ-Observing
Subject: [AZ-Observing] Trip to NM
 
I made my trip to my lot in NM. During the ride from Datil to my lot one 
gets to see the Plains of San Augustin against a backdrop of mountains. 
This is one of my most favorite natural sceneries, maybe the most 
favorite. It has such a serene feel to it.  On Tuesday last week, the 
day before I left Arizona, the weather forecast did not look good and so 
I didn't take my telescope. I went anyway because the purpose of the 
trip was for other reasons. It is a good thing I did not take my 10" 
LXD75 because the forecast called for partly cloudy days and nights for 
the week, 50% chance of rain on Tuesday, tapering off to about 20% for 
the remainder of the week, but it was completely clouded over Thursday 
and Friday, with a 100% chance of rain both days. It rained a lot. I got 
a peek of some night skies early in the night on Saturday night.

Although I brag about the location of my lot being in a premier dark sky 
location (and it is in one of the best dark sky locations in the SW), I 
am not that convinced the skies are that significantly darker. There is 
no light pollution from man made sources. That is an absolute. However, 
it just doesn't seem the skies are as dark as they should be. Whenever I 
mention that they look gray from zenith to horizon, the response is that 
this is a sign of a good site because the eyes are dark adapted so the 
skies look lighter than one would expect. However, this just does not 
feel to be the correct explanation. For example, the glimpse I got of 
the sky through the clouds on Saturday night did not look that dark and 
this was after emerging from my tent with a light inside, and lights 
outside to collect moths, and so eyes were definitely not dark adapted. 
Also, some claim that at 8,000 ft ele there are physiological effects 
that make the sky appear brigher, but I do not think this is the correct 
explanation either. When I was at my lot in September, 2007 I took some 
dark sky readings and it verifies, to me, that the skies are not that 
dark. The following readings were taken in 2007 and are Arizona times. 
NM time is one hour ahead that time of the year. They were taken with 
the same dark sky meter and so there is no between instrument 
differences. I recall raising this issue in the past, but I do not 
remember if I presented the following data.
Last Frontier
   Sept 13  9:10  21.75 (ave of 3)
   Sept 13 10:05  21.70 (ave of 3)
   Sept 13 11:05  21.65 (ave of 3)
   Sept 14  2:50  21.84 (ave of 3)
   Sept 15  9:40  21.61 (ave of 4)
   Sept 15 10:55  21.42 (ave of 3)
   Sept 17 11:05  21.42 (ave of 3)
   Sept 18 11:15  21.54 (ave of 6)
   Sept 18  3:55  21.54 (ave of 3)
   Sept 19  2:30  21.59 (ave of 3)

Blue Hills Observatory (Dewey, Az)
   Aug   8  9:15  21.06 (one reading)
   Aug   8 11:30  21.14 (one reading)
   Aug   9 10:45  21.14 (ave of 3)

Cherry Road Site
   Aug   9 10:00  21.49 (ave of 3)

There was one night it my NM lot where the dark sky reading was a nice 
21.84. This was at 2:50 AM on Sept 14, but it was not duplicated on Sept 
19. On that night the reading was taken at 2:30 AM and the reading 
dropped to 21.59. It can be seen that although the Cherry Road site was 
generally lower than the Last Frontier readings, there were a couple of 
nights at Last Frontier that had lower readings than the Cherry site.

What could cause the readings not being higher at Last Frontier than I 
had expected? One possible problem is that I only have one 3 reading 
average for the Cherry Site. I understand that pointing at different 
parts of the sky will make some difference, but if I recall, it was not 
that much different unless one purposely compared a region with no stars 
to a section of the Milky Way.

I am wondering if the Plains of San Augustin to the south of me, hidden 
from view by Horse Mountain (a heck of a location name for a possible 
observatory site), could be a cause. It is a wide expanse of very flat 
grassland that is very light beige in color. It probably does a good job 
of reflecting light back into the sky, but I cannot imagine back 
reflected starlight being the cause of the lower readings than expected. 
One wild idea is this. Is it possible that light from surrounding 
cities, like Gallup, Socorro, even Albuquerque, etc., is somehow 
dispersed horizontally by some atmospheric anomaly so that it gives the 
appearance of air glow without an obvious point source? Alternatively, 
are there too many variables I was not able to control that accounts for 
the small difference between the Last Frontier readings and the Cherry 
Rd site readings?
Stan




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