###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.