[AZ-Observing] Sky brightness versus aerosols

  • From: Brian Skiff <bas@xxxxxxxxxx>
  • To: <az-observing@xxxxxxxxxxxxx>, <amastro@xxxxxxxxxxxxxxx>
  • Date: Fri, 10 Apr 2015 15:34:41 -0700

On the last two nights at Anderson Mesa I was
able to make SQM meter sky brightness measurements
prior to Moonrise. Both nights were cloudfree
before Moonrise (clouds came in around 2am and
1am on the successive nights). The Kitt Peak
all-sky 'red' images showed that the night airglow
was quiescent on both nights.
Generally from Anderson Mesa --- and presumably
from any dark site --- the main drivers of the
sky brightness are the state of the night-airglow and
the sidereal time (i.e. looking more at the Milky Way
or more toward the galactic poles). I have
hitherto considered that the low-level aerosols
(general crud in the atmosphere from dust, pollens,
even just humidity) had a relatively small influence.
And it is uncertain whether more aerosols makes the
sky darker from obscuring distant light sources or
whether there's more scattered starlight as a result,
so the sky is brighter. The actual situation is
probably more complicated than this simple idea.
The main visual difference between the two
nights was that the second night had more 'stuff'
in the air, so that the view horizontally was
more hazy than the first night. Both nights had
dewpoints around -10C, so it was quite dry.
Here's the numbers:

--UT Date-- muV comment
Apr 9 0305 21.21 Sun at -15 deg
Apr 10 0306 21.16

Apr 9 0321 21.45 Sun at -18 deg
Apr 10 0322 21.39

Apr 9 0330 21.51
Apr 10 0330 21.44

Apr 9 0400 21.54
Apr 10 0400 21.47

Apr 9 0500 21.66
Apr 10 0500 21.56

I make the measurements in a highly consistent
fashion, and the SQM diode detector is shielded from
strong light and the box kept at room temperature except
when in use. The values repeat at the ~0.03 mag level.
These nights had the zodiacal light and winter stars
conspicuous at the start, and similarly Jupiter was
near the center of the SQM's very wide field-of-view
(it makes essentially no difference in the numbers).
The first measurements are nominally at the
mid-point of astronomical twilight, where the
sky is still changing relatively quickly, so a
timing difference of under a minute can make a
modest difference in the SQM readings. Knowing that,
those readings were taken within a few seconds of
the start of the minute shown.
The sky gets gradually darker as the evening
goes on from the setting of the zodiacal light and
then looking at progressively high galactic latitude.
Anyway, it looks as though there was a small effect,
possibly from the increased aerosols, such that the
sky was 5 or 10 percent brighter on the second night.
I don't think this is something you can actually
detect visually, mainly from the vargaries of one's
own vision (really hard to calibrate!). You'd need
a lot more sample nights to test this better, but the
trend suggests the sky is very slightly brighter
on nights with higher aerosols.


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