Description of step 2 of the data processing algorithm (see middle part of Fig. 2). The data presented here correspond to the 19th sub-band of the data set presented in Fig. 3. The black dotted lines represent the mask around the expected lines. The different colours represent the sub-band at each step of the processing (described in the middle part of Fig. 2). (a) The red (resp. purple) spectrum is before (resp. after) the ‘RFI mitigation 2’ step. (b) The purple spectrum is the sub-band being processed, the black spectra are the neighbouring sub-bands used to assess the general shape. The gray sub-band has been discarded. (c) The pink line is the median of the black and purple sub-band in panel (b), and the black dotted line is the median smoothed by a Savitzky–Golay filter. (d) The purple (resp. navy blue) spectrum is before (resp. after) the ‘flattening 2’ step. (e) The navy blue (resp. azure blue) spectrum is before (resp. after) the ‘RFI mitigation 3’ step. In this case, RFI was removed at 13.92 and 13.96 MHz. (g) The azure blue (resp. light blue) spectrum is before (resp. after) the ‘flattening 3’ step. The light blue (resp. green) spectrum is before (resp. after) the ‘RFI mitigation 4’ step, and the black dotted line is the green spectrum smoothed by a Savitzky–Golay filter. (i) Black line indicates the zero baseline and the olive green spectrum is the final flattened and cleaned sub-band.