The probability distribution of photon counts from the Optical Monitor on XMM Newton satellite (XMM-OM) and the UVOT on the Swift satellite follows a binomial distribution due to detector characteristics. Incident count rate was derived as a function of the measured count rate, which was shown to follow a binomial distribution.

Discrepancy between the mapping the 1σ Poisson incident count error onto measured count error and the mapping the 1σ binomial measurement count error onto incident counts was illustrated with an example of large counts per frame. Although this discrepancy for small counts per frame was commented to be negligible, the authors urge to use their binomial distribution based formalism to derive the errors in measurements.