Object surface brightness (MPAS) =
or
from object magnitude
and size in arc-minutes
by
Sky background brightness (SQM MPAS reading) =
Results
Interpreting the results
MPAS stands for Magnitude Per Arc-second Squared.
Sky background brightness is 21.5 MPAS for a dark site, 18.5 MPAS for a city site.
Detectable contrast for objects of large apparent size is 3-6%.
Very faint objects smaller than five degree apparent size can be impossible to see; the larger the apparent size the better as long as the apparent size does not exceed the eyepiece's apparent field of view.