The first step in calculating the observing time needed for an LWS observation is to calculate the S/N that would be achieved on a given ramp for a given signal. This can be done using the following formula:
for continua and
where 
is the S/N on a single ramp,
is the continuum flux,
is the line flux
(see the LWS Observers Manual for a discussion on the calculation
of CFLUX and LFLUX),
and 
is the EFFECTIVE duration of
the ramp (i.e. 0.15 sec for bright sources and 0.4 sec for other
sources (see above). The achieved S/N per ramp is then given
by (given a maximum S/N per ramp of 100):
