6.3.3 Count Rates for Emission Line Sources

The count rate in units of e^- s^-1 for a monochromatic emission line is given by

where F is the emission line flux in units of ergs cm^-2 s^-1, and is the wavelength of the line in Angstroms. The quantity QT is the (system + filter) quantum efficiency at the wavelength of the line, which can be determined from inspection of the figures in F622W, F631N, F656N. For lines near the maxima of the filter transmission curves, it should be sufficient to use QTmax from Table 6.1. Note that the integrated filter efficiency is not relevant for the signal calculation.

In cases where the width of the line approaches that of the filter, it will be necessary to convolve the line shape and filter bandpass using either the SYNPHOT or XCAL programs.

For example, H emission at 6563Å, with total source flux F=10^-16 erg s^-1 cm^-2, observed through the F656N filter (total system throughput T=0.11 from the plots F622W, F631N, F656N), will produce a target count rate Robject=0.17 e^- s^-1 integrated over the entire source.