System Throughput and SNR / Exposure Time Estimation . Below we give further examples of SNR calculations. The Appendix also gives tables of SNR values for a wide range of representative cases. ... At these high SNR levels, it is likely that flat fielding would limit the photometric accuracy, rather than the noise. If we have a look at the terms in the SNR equation, we can see that the Poisson noise dominates; the term containing the sharpness and background noise sources is unimportant. ...