FORS2 observations of the magnetic Ap star HD 188041. In the upper panel, the black solid line shows the Stokes I spectrum (uncorrected for the transmission function of the atmosphere + telescope and instrument optics); the red solid line shows the reduced Stokes V spectrum, P_V = V/I (in % units), and the blue solid line is the null profile offset by − 1.25% for display purpose. The scattering of the null profile about zero is consistent (although sometimes slightly higher than) the 1σ photon-noise error bars, which are also shown centred around − 1.25% and appear as a light blue background to the null profile. Spectral regions highlighted by green bars (at the top and at the bottom of the panel) have been used to determine the ⟨ B_z ⟩ value from H Balmer lines, while the magenta bars highlight the spectral regions used to estimate the magnetic field from He and metal lines. The four bottom panels show the best-fit obtained by minimising the χ² expression of Eq. (1) using the P_V spectra (left panels) and the N_V spectra (right panels) for H Balmer lines (upper panels) and metal lines (lower panels).