###Exposure Time Calculator###
* In calculating exposure times and source counts in
spectroscopic observations, what is (epsilon_Nspix)' ?
And how is the S/N calculated ?
(epsilon_Nspix)' is the fraction of the point source energy
within N_spix pixels in the spatial direction. For
spectroscopic observations, S/N calculations are performed
over N_spix x N_lpix pixels, where N_spix is already defined
and N_lpix is the number of pixels for a resolution element
in the dispersion direction, i.e. 2 px for STIS and ACS.
