For a given optical configuration (channel, PFOV and Filter or CVF
), using figures 
to , find :
= ordinate of the solid line for the filter or CVF at
In a first approximation, the dark noise, the flat-field noise and the low frequency noise can be neglected.
Taking 
and S/N the expected flux of the source and
the needed S/N ratio of the observation, compute:
If 
this observation should be easy
to do, you can choose the parameter of the figure, or even try to
scale down 
, but not much since the minimum number
of exposures is 4. You could possibly shorten the observation time
by taking a shorter integration time, then 
increases
roughly like 
.
If 
, increasing 
improves S/N like 
as long as long as the low
frequency noise does not get predominant. Checking with the
following detailed calculation is recommended.