Relationship among N_e, Δt, and m. Blue triangles and red circles represent the data captured with Zyla and iXon, respectively. The lines fitted using Eq. (9) are drawn separately for the Zyla and iXon data. The data points at a certain x (= Δt · 10^−m/2.5) exhibit a vertical scatter depending on the atmospheric clearness. To avoid underestimating k, the fitting was performed considering N_e/x as weights. The lower k for Zyla than iXon is presumably owing to the shorter effective exposure time of Zyla (~3.4 ms for Zyala vs ~4.8 ms for iXon) and the lower quantum efficiency of Zyla (~80% for Zyla vs ~95% for iXon at the wavelength of ~650 nm).