
17.3.
Detection Limits and Signal-to-Noise Ratio 435
to both dark counts and read noise, the SNR for the detector-limited case can be
written as
SNR = SQt/y/Ct-i-R^, (17.2.6)
where C is the dark count per second and R is the rms read noise. If read noise is
negligible compared to dark count, the addition of k separate short exposures
leads to the result given in Eq. (17.2.5). If 7? is dominant, however, then the SNR
for k added exposures is smaller than that of a single long exposure by a factor of
Vk.
This treatment is sufficient to illustrate how system performance is specified in
terms of SNR. We now use this parameter to give a more detaile ...