Motion in time of the circular aperture for the four stars studied in this work. Left panel: Position of the center of the circular aperture along the X-axis as a function of time for all four targets. If the sequence was interrupted (because of an instrumental problem or zenith avoidance region), each sequence is plotted in a different panel. The error bars correspond to the 1σ uncertainties. The colored solid circles correspond to science frames and the colored solid triangles to the sky frames. Right panel: Same as the left panel, but for the Y-axis.