  We demonstrate an atomic bandpass optical filter with an equivalent noise
bandwidth less than 1 GHz using the D$_1$ line in a cesium vapor. We use the
ElecSus computer program to find optimal experimental parameters, and find that
for important quantities the cesium D$_1$ line clearly outperforms other alkali
metals on either D-lines. The filter simultaneously achieves a peak
transmission of 77%, a passband of 310 MHz and an equivalent noise bandwidth of
0.96 GHz, for a magnetic field of 45.3 gauss and a temperature of
68.0$\,^\circ$C. Experimentally, the prediction from the model is verified. The
experiment and theoretical predictions show excellent agreement.
