On Sep 24, 11:16 am, Greg Crinklaw
wrote:

NELM seems a valid way to compare skies as long as it is
the same observer and done in a consistent way.

Agreed -- with a couple of caveats.

First of all, I find that determining NELM to a high accuracy is very
tedious and time-consuming -- it takes me a minimum of a half hour,
and even then I wouldn't give it better than +-.2 mag reliability. So
in practice, I usually make do with a casual, non-numeric assessment.

Second, like Bill Greer, my NELM tops out in skies that are clearly
less than ideal, and simply doesn't get any better no matter how dark
or transparent the sky is. I've been to many places where I could see
some stars fainter than mag 6.5, but I've never been anywhere where
I've seen every mag-7.0 star that I've looked for.

So while I find NELM quite useful for a quick assessment of light-
polluted sites -- or for assessing transparency at a pretty-good site
-- I find it useless for distinguishing good skies from great skies.
For that purpose, I find the best criterion is the visibility of naked-
eye dark nebulae in the Milky Way. Unfortunately, there's no easy way
to make that quantitative!