Schematic of the data reduction process for the spectral time series of order 76 in night 2. Each of the six top panels shows the time series of spectra. Each row represents one spectrum in the series. Top panel: original spectra. Second panel from the top: spectral time series after continuum normalisation. Third panel from the top: spectral time series after the mean spectrum was subtracted. Panels 4-6 from the top: Spectral time series after one, two, and three sysrem iterations were run on the time series. Bottom panel: difference in the standard deviation of the residuals between consecutive sysrem iterations as a function of the number of sysrem components. The shaded region highlights the one-sigma deviation in the last four points (to the right of the dashed line), and the black dot indicates the chosen number of iterations (the smaller number), as described in the text.