Sample properties and observing conditions. (a) Histogram of the star magnitudes (G, H, and K) for the sample considered in this study. (b) Scatter plot showing the coherence time (τ₀) against the seeing with the Strehl ratio color-coded. Data points without Strehl information are colored in white. The boxes delimit the ESO observing categories. (c) Scatter plot showing the 1 sigma raw contrast measured at a separation of 0.5″ (directly measured on the coronographic frame, before post-processing) with respect to the K-band star apparent magnitude. (d) Scatter plot displaying the Strehl ratio as a function of the star apparent magnitude in the G band.