How do you measure sky brightness in magnitudes per square arcsecond
(MPSA)? Is there some way to relate this measurement to the more
typical zenith limiting magnitude or limiting magnitude of the
observing field?

Bill Ferris' article in this month's Sky and Telescope on Optical
Detection Magnitude (ODM) provides a reference to Bartel's c-code
software to compute the ODM. (
http://www.efn.org/~mbartels/aa/visual.html )

One of the input parameters for this model for the visibility of
extended objects is the background brightness of the sky measured in
magnitude per square arcsecond (MPSA).

I am unable to relate this parameter to my existing knowledge of the
limiting magnitude of the observing field, so I can make estimates of
the MPSA during my observing sessions.

Some of the background internet references related to the article
suggest values like:

Mount Wilson 19.8
Palomar Mountain 21.5
Lick Obs. 20.7
Mount Lemmon 21.5 (near Tucson)
Lowell (Mars Hill) 20.5
Van Vleck 18.7 (Connecticut)
David Dunlap 18.4 (Toronto)
Haute Provence 21.8 (southern France)

Any help on how to estimate the MPSA during my local observing
sessions would be appreciated.

Thanks - Kurt

W. D. Ferris. Dark Skies Rule. Sky and Telescope. 106(2):62 (August
2003).

Brian Skiff. How dark can the sky get. Internet article.
http://www.astropix.com/HTML/L_STORY/SKYBRITE.HTM accessed July 2003
(2001)