Our local club has 3 Unihedron SQM meters and they usually seem to be within a
tenth of each other when taking readings together. Just moving the meter a few
degrees can make a difference. So, it's sometimes tough to know if they are
different readings, or if the slight movement of each is causing the different
readings. We haven't tried mounting them yet together to see if there is really
We've been taking readings in the Verde Valley for about 10 years and so far
there has been no lower readings that are constant. No appreciably more light
pollution. Building has been picking up, so that may change in the years to
The Phoenix light dome seems to be brighter and larger. And, it's worse with
moisture and dust. That seemed to happen after the computer billboards were
installed on I17. Could be coincidence.
Date: Fri, 10 Apr 2015 17:26:15 -0700
Subject: [AZ-Observing] Re: Sky brightness versus aerosols
On 04/10/15 16:43, Brian Skiff wrote:
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
I wonder how many of those aerosols are suspended high enough above
the mesa to have a direct line of sight to the millions of lights here
in the Valley. They probably wouldn't have to be very high above ground
level for that to be the case. Also, how well is the performance of a
typical SQM determined as a function of temperature? If the air
temperature was different by even a few degrees between the two nights,
it could affect the accuracy of the readings.
Anyone ever compared the readings of two or more SQMs at the same
time, to see how much variation there is from one unit to the next. I
know they claim to be calibrated, but it might be interesting to know
Thanks for the report, Brian. I'd have expected a bit more of a
difference than your numbers indicate.
-- Mike --
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