"David Knisely" wrote in message
...
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).

Some charts for this purpose are in my book, Celestial Objects for Modern
Telescopes, though you can use any charts that give you the information.
Mine are spaced so that one of them will always be within 40 degrees of the
zenith -- which is not always close enough for a good determination,
depending on your site. Next time they'll be closer...


--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope