Example of wavelength calibration of CaII K spectral line. The red crosses represent the observed disk-integrated intensities S _obs (Eq. (1)). The black lines are the scaled smoothed reference spectrum (Eq. (5)). Top panel: S _obs and are plotted as a function of the intensities of the smoothed reference spectrum (Eq. (3)). Middle and bottom panels: S _obs and are plotted as a function of wavelength. The coefficients a, b, c and d of the fit are shown. The vertical blue line gives the line core position at λ _c = 3933.663 Å. The corresponding FITS index is given by CRPIX3. The interval over which the reference spectra is smoothed is given by δλ. WSSR is the resulting weighted sum of squared residuals. The top and middle panels give the fit with the initial parameters and the bottom panel gives the fit and parameters after optimization (lower WSSR).