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What is (epsilon_Nspix)' and how is the spectroscopic S/N calculated?

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  * 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.