Comparison of NEFD estimates using two methods on observations of HLS J0918+5142. Left panel: evolution of the 1σ flux density uncertainties as a function of the effective integration time t_eff, as defined in Eq. (27), for A1 (cyan), A3 (dark blue), the combination of A1 and A3 (medium blue), and A2 (red). The solid black lines are the best-fit models using . Right panel: NEFD as a function of the measured line-of-sight atmospheric opacity using the same colour code as in the left panel. The solid black lines are the theoretical fits of NEFD_τν,  x = NEFD e^τ_ν x and give the NEFD when extrapolated to τ_ν/sin(el) = 0.