An example of data reduction: the case of the Ap star HD 94660. In the upper panel, the black solid line shows the intensity profile, the shape of which is heavily affected by the transmission function of the atmosphere + telescope optics + instrument. The red solid line is the PV profile (in % units) and the blue solid line is the null profile offset by −0.5% for display purpose. Photon-noise error bars are centred around −0.5% and appear as a light blue background. Spectral regions highlighted by green bars have been used to detemine the ⟨ Bz ⟩ value from H Balmer lines, and the magenta bars highlight the spectral regions used to estimate the magnetic field from metal lines. The four bottom panels show the best-fit obtained by minimising the χ2 expression of Eq. (4) using the PV profiles (left panels) and the NV profiles (right panels) for H Balmer lines and metal lines as described. The field values (⟨ Bz ⟩ ~ −2000 G and ⟨ Nz ⟩ ~ 0 G) are determined with a formal accuracy of ~40 G for Balmer lines and ~25 G for metal lines.
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