Howdy Brian, et al;
I need a reminder of the value for the numbers given, please. Is "21.75" what
you would call "true dark" skies? What is that number? Does someone have
some values for the SAC sites?
Thanks;Steve Coe
On Thursday, February 4, 2016 10:46 AM, Brian Skiff <bas@xxxxxxxxxx> wrote:
I recently visited Kitt Peak for four nights
to participate in an observing run with the
WIYN telescope:
http://www.wiyn.org/About/overview.html
The black circular hub in the right-most photo at
the link above is the Hydra multi-fiber spectrograph
that I was using, mounted at a Nasmyth focus.
It can take up to 90 spectra in a single exposure
(about 50 stars in M67 for this project).
  Since the Moon would be gradually receding
during the run, I brought along my SQM box in hopes
of making some sky-brightness readings. On Tuesday
night, following the intense winterstorm passage,
the sky was especially transparent, though the
aerosols looking horizontally were not completely
absent. Importantly, the KPNO nighttime all-sky
camera showed that the night airglow was completely
quiescent.
  Thus I think the SQM measurements copied
below are representative of Kitt Peak's minimal
sky brightness as per my specific SQM meter,
now 10+ years old. The readings are in the
column 'muV' which is nominally the V-band sky
brightness in units of magnitudes per square arcsecond.
The big drivers of the sky brightness at dark sites
are the state of the airglow and the sidereal time
(i.e. whether there's Milky Way or less-starry sky
overhead), so there's a comment added about the first,
and the LST given in its own column along with the UT time.
  Having not been to the site in 27 years,
it was distressing (but not unexpected) to see
the growth of both the Phoenix skyglow nearly 200km
north and the now-pervasive lights (all low-pressure
sodium) to the west of the mountain on the Tohono O'Odham
reservation. The zodiacal light after dusk was
nevertheless spectacular viewed from the southwest-aiming
promontory that the WIYN sits on.
  The numbers show that Kitt Peak is still a
plenty-dark site. The first few "bright" entries were
taken during twilight. You can compare these figures
with recent measurements from the Lowell Anderson
Mesa site at my nighttime cloudiness page:
http://www2.lowell.edu/Research/cloudiness_data/clouds.html
  The isolated and beautiful sky-island site
also provided excellent views of the triangular
mountain-shadow at sunset, terrific relief in the
basin-and-range desert landscape, and dramatic cloud
scenery at close proximity.
\Brian
UT Date hhmm LST site muV trans remarks
20160203 0202Â 0325Â KPNOÂ 20.35Â 4Â Â WIYN telescope
20160203 0220Â 0345Â KPNOÂ 21.37Â 4Â Â very transparent
20160203 0225Â 0350Â KPNOÂ 21.39Â 4Â Â Sun at -18deg alt
20160203 0230Â 0355Â KPNOÂ 21.45Â 4
20160203 0240Â 0405Â KPNOÂ 21.47Â 4Â Â airglow quiescent
20160203 0300Â 0425Â KPNOÂ 21.46Â 4
20160203 0400Â 0525Â KPNOÂ 21.54Â 4
20160203 0540Â 0645Â KPNOÂ 21.64Â 4
20160203 0600Â 0725Â KPNOÂ 21.66Â 4
20160203 0700Â 0825Â KPNOÂ 21.73Â 4
20160203 0800Â 0925Â KPNOÂ 21.75Â 4
20160203 0830Â 0955Â KPNOÂ 21.69Â 4
20160203 0900Â 1025Â KPNOÂ 21.75Â 4
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