Example of optimal aperture radius selection and spectra rejection criteria using the AB Pic b dataset. In each panel, each color represents a different exposure of AB Pic b. Left: values for the fitted continuum slope between 2.05 and 2.2 µm as a function of aperture radius. Two distinct domains are observed: for radii smaller than the optimal value, the slope increases steeply; for radii larger than the optimal value, the slope still varies but becomes more stable. Center: S/N estimated within the same wavelength range as in the left panel, plotted as a function of aperture radius. For each observation, we highlighted the maximum S/N value with a square. The aperture radius corresponding to this maximum is our final selected radius each time. Right: derivative of the slope (reported in the left panel) with respect to the aperture radius. Some targets exhibit a non-smooth, decreasing derivative; we reject those epochs. For AB Pic b, this resulted in the rejection of the entire dataset for the second night.