Observing conditions (airmass, seeing, and coherence time) at Paranal during the observation run (between 03:09:30.9281 and 07:58:36.2025 UT on August 23, 2016). The large fluctuations in coherence time, τ₀, of atmospheric turbulence show that some images cannot be used, because of small τ₀ values that are associated with fast atmospheric turbulence (the shaded light red horizontal band for which τ₀ < 3.5 ms). The green shaded area represents the time window from which the final images used in our analysis were extracted (see text for explanation). These images correspond to the lowest airmass observing window.