Signal-to-noise for Models I (solid lines) and II (dashed lines) as a function of ΔE keeping E fixed to 3.0 keV and allowing F_lim to take values 10^-13 (upper curves), 10^-14 (middle curves), and 10^-15 erg/s/cm² (lower curves), while keeping the flux to counts conversion factor the same, i.e., 500 counts at 10^-13 erg/s/cm². The dotted lines are for Model I with a three times narrower line, i.e., 15 eV FWHM.