My 15 degree SQM has hit 21.73 when aimed at the darkest part of the sky a
number of times, maybe even higher (would have to dig through more records).
The first reading is usually higher than the next three taken immediately in
sequence. It has been as high as 21.77 but dropped on the immediately
subsequent readings to the low twenty one point seventies. The readings are
hand held and thus in a slightly different area of the sky. These readings
have been both at Hovatter Norte (1500') and at the North Rim near Jacob Lake
(8300'), usually about 2 am. Not sure how they calibrate these things, but
this is what the little red numbers said when the clicking stopped.
Paul Knauth
________________________________________
From: az-observing-bounce@xxxxxxxxxxxxx [az-observing-bounce@xxxxxxxxxxxxx] on
behalf of Robert Ayers [astroayers@xxxxxxxxx]
Sent: Friday, February 12, 2016 8:56 PM
To: az-observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Re: SQM readings -- was KPNO
A "perfectly dark" site with zero light pollution, at sea level, reads
about 21.8 on a Unihedron SQM. (private communications, Unihedron) The
reason that it is not "22.0" is fairly straightforward, but wordy --
details on request. (Different SQM meters will also exhibit a decent
amount of scatter.)
The reading is in "mags", just like stellar mags. That is a log scale,
with a difference of 1.0 being a factor of 2.512. That means that a small
delta of light pollution affects values down near 21 much much more than
the same delta affects numbers around 18.
Bob Ayers
​
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