Wavelength solution fit for spaxel #501. Left: fitted centroids of Hg, Cd and Xe (see legend) arc lamp emission lines (in pixel-row from the right) as a function of their expected wavelengths. The orange line is the best fitted fifth order polynomial enabling the conversion of wavelength to pixel and conversely. Right: blue curves represent the arc lamp spectra (in flux per pixel-row from the right). The black lines are the best fitted “Gaussians + third-order polynomial-continuum”. Vertical thin gray lines are the initial guess positions. See details in Sect. 2.1.2.