Figure 8 plots the log(Exposure time (secs)) needed to reach a given
Figure 8: Calculated exposure times for various conditions.
magnitude at a S/N of 5 and 20, for seeing conditions of 0.564''\
(ie 1 
'', ie for surface brightness),
0.8'' and 1.2'', and for apertures
of radius 2.5 
seeing, observing in ND_STARE mode.