Fitting procedure during the wiggle-correction step for some of the wavelength regions in the first exposure of the 4UT observing night of WASP-121b. The fit of the entire spectral range is shown in the last panel. We divide the spectrum into regions (vertical purple lines) in which we define the priors and bounds of the fitting functions. For some regions near the lowest and highest wavelengths, we did not apply the spline fitting because it would not be a correct match to the observed data.