###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.