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Fig. 2

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Tracing of the centre of the Hg i arc line at λ = 4358.343 Å on the FORS2 detector in the spatial region where science spectra are recorded. Different colours and line-styles refer to calibration frames obtained on different days and with different positions of the retarder waveplate, as explained in the text. Note that black and red lines are practically superposed, but the line positions on 2011-05-07 are offset by ~0.08 px with respect to those measured on 2011-05-05 and 2011-05-06.

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