MrB wrote:

I have seen several posts that indicate the mag of the sky in the viewing
area. How is this determined? Is it subjective?

It takes a little care to do it, but it isn't exactly difficult. All you do
is look up into the sky to an area near the zenith (ie: straight-up). You
carefully scan an area and see which are the *very* faintest stars you can see
with the eye (get dark adapted and use averted vision). You then locate these
stars on an atlas and get the data on exactly how bright they are from a
catalog (star-charting software is often good for this). The faintest star
you see is then your personal "Zenith Limiting Magnitude" (ZLM). The figure
you get will vary, depending on sky conditions and the observer. Moonlit
nights will often have a ZLM of 4.2 to 5.5, as will non-moonlit nights in
suburban environments, while a dark night well away from city lights might
have a ZLM as faint as 6.5 or even 7.0. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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