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
â
--
See message header for info on list archives or unsubscribing, and please
send personal replies to the author, not the list.
