[AZ-Observing] Twilight SQM measurements
- From: Brian Skiff <bas@xxxxxxxxxx>
- To: amastro@xxxxxxxxxxxxxxx, az-observing@xxxxxxxxxxxxx
- Date: Fri, 12 Oct 2007 18:18:33 -0700 (MST)
I observed the five nights Sunday through Thursday nights (local)
this week. I took a lot of SQM readings, which are posted at the
Lowell nighttime cloudiness page:
http://www.lowell.edu/Research/cloudiness_data/clouds.html
(grab the October 2007 list for details)
The week started looking about like the photo that's at the top of
the page, but warmed up gradually during the week.
Yesterday evening (Oct 12 UT), I had enough time to make SQM
readings at 5-minute intervals starting about 10 minutes after the
end of civil twilight until the end of astronomical twilight,
i.e. about an hour by the clock. The sky brightness declines very
quickly, roughly 1 mag/square arcsec each five minutes until around
10 minutes after nautical twilight, and more slowly thereafter.
UT Date hhmm LST site mu pv dp remarks
(mm) (C)
20071012 0130 1925 Mesa 14.97 2.9 -6 naut twil -21 min, Sun alt -8 deg
20071012 0135 1930 Mesa 16.24 2.9 -5 naut twil -16 min, Sun alt -9 deg
20071012 0140 1935 Mesa 17.22 2.9 -5 naut twil -11 min, Sun alt -10 deg
20071012 0145 1940 Mesa 18.22 2.9 -5 naut twil -6 min, Sun alt -11 deg
20071012 0150 1945 Mesa 19.14 2.9 -5 naut twil -1 min, Sun alt -12 Deg
20071012 0155 1950 Mesa 20.00 2.9 -5 naut twil +4 min, Sun alt -13 deg
20071012 0200 1955 Mesa 20.69 2.9 -5 naut twil +9 min, Sun alt -14 deg
20071012 0205 2000 Mesa 21.13 2.9 -5 naut twil +14 min, Sun alt -15 deg
20071012 0210 2005 Mesa 21.37 2.9 -6 naut twil +19 min, Sun alt -16 deg
20071012 0215 2010 Mesa 21.42 2.9 -6 naut twil +24 min, Sun alt -17 deg
20071012 0220 2015 Mesa 21.51 2.9 -6 astron twil -1 min, Sun alt -18
deg
20071012 0230 2025 Mesa 21.53 2.9 -6
...
...
... and at hourly intervals for the rest of the night.
Some interesting relative comparisons include: the night sky at Full Moon
is roughly mag 18 or so, comparable to the sky well before the end
of nautical twilight, where the sky is starting to lose its blue color.
Similarly, the late-night readings from our Mars Hill campus, directly
above downtown Flagstaff, are about 20.7, similar to 10 minutes after
the end of nautical twilight, when the sky is still noticeably bright
from Anderson Mesa---but still a lot of stars/Milky Way present.
Since I'm used to using the SQM when it's dark, it was amusing
that the readings during twilight took only a single 'beep' of the
device, whereas when it's full dark, the integration time is 6 or 7 seconds.
It's a real privilege to have gorgeous dark sky like this for
several nights running.
\Brian
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