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What are N_pix, epsilon_f and epsilon_f(1)?

     ###Exposure Time Calculator###

     * In calculating exposure times and source counts in
     imaging observations, what are N_pix, epsilon_f and epsilon_f(1) ?
     The counts/second from your source is calculated over some
     selected area of N_pix pixels, where a signal of an electron
     on a CCD is equivalent to one count. Default values for N_pix
     are 5x5, 9x9, 15x15, for WFC, HRC and SBC, respectively.
     epsilon_f is the fraction of the point source energy
     enclosed in N_pix (this is approximately 80% for the default values).
     epsilon_f(1) is the fraction of energy within the peak pixel.