Example of the artificial instrumental errors. The original HARPS spectrum of Gl 526 (Karmn J13457+148) is displayed at the bottom as a black line, while the polynomials with coefficients stated in Sect. 2.3 and the resulting spectra are shown in blue, red, and green, respectively. The shaded gray areas mark the regions used for the extraction of in this publication, whereas the blue regions highlight the bands N_H, N_K, N_V, and N_R commonly used for the determination of the S index.