Local normalisation around a spectral line. Left: a median filtered array (middle blue line), the filter width was chosen to be 1.25 times the full width at half maximum (FWHM), typically FWHM ~ 0.4 Å, which corresponds to 50 pixels given the instrument resolution. Pixels within 3 × FWHM and with intensity beyond 3 Poisson-statistics sigmas from the median filter (red curves) are discarded. Middle: zoom to the median filter. The accepted pixels were used to fit a third order polynomial. Right: the normalized spectrum after division by the polynomial and the Voigtian fit (grey) to the stellar line.