For a given count rate M and flux N, this can be estimated by P(M,N) = [M!/(N!(M-N)!)](1/M N ), for M>N. The probability indicates that deviations from zero coincidence would be expected despite high count rates and low flux values, requiring careful consideration of the chosen flux for an experiment. This is true if each detector pixel is considered to count with full independence of all others, and more so if events across neighboring pixels are considered correlated during counting (Supplementary Figure ). More specifically, given that each pixel on the Apollo sensor counts at a rate of 2392 e -/pix/s, we can first consider the condition where electrons arrive only at a single pixel and it counts them independently of all other pixels on the sensor. Under these conditions, a 3% CL is expected when detecting a true incident flux of 100 e -/pix/s; a 10% CL is expected for a true incident flux of 500 e - /pix/s (Supplementary Figure ). However, in a more realistic scenario, each electron strikes a cluster of pixels on the sensor, and counts are assessed from 3x3 patches of pixels on the sensor. Then, simulations indicate a lower bound of ~18% CL is anticipated for an incident flux of 100 e -/pix/s, and > 50% CL is expected for an incident flux of 500 e -/pix/s (Supplementary Figure ).