Etalon cavity map obtained from Gaussian fits to the centre of the Fe I 6173 Å spectral line during HRT ground calibration. The large-scale trend and the slight shift of the two scans shown in Fig. 37 arise from the solar rotation pattern imprinted in the incoming light, since the HRT FOV covers almost the entire solar disc at 1 AU distance, even if the feed optics does not provide a focused solar image.