  In recent years, detectors with sub-electron readout noise have been used
very effectively in astronomical adaptive optics systems. Here, we compare
readout noise models for the two key faint flux level detector technologies
that are commonly used: EMCCD and scientific CMOS (sCMOS) detectors. We find
that in almost all situations, EMCCD technology is advantageous, and that the
commonly used simplified model for EMCCD readout is appropriate. We also find
that the commonly used simple models for sCMOS readout noise are optimistic,
and recommend that a proper treatment of the sCMOS rms readout noise
probability distribution should be considered during instrument performance
modelling and development.
