Post-processing of the data, on the example of Cassiopeia A. Top panel represents the whole spectrum averaged over all observing blocks. The gray area is an example of three stacking selections. Middle panel is a zoom on the gray area. Vertical dashed lines encompass the three selections of lines to be stacked. The vertical spikes are Cα RRLs. Bottom panel shows the three stacks computed from the selections. Stacking allows for the detection of three velocity components. Their expected positions are determined from previous studies (Oonk et al. 2014, 2017; Salas et al. 2017) and are marked by the vertical coloured lines: blue (−47 km s⁻¹), green (−38 km s⁻¹) and red (0 km s⁻¹). The mean S/N for these three stacks is about ~100 at ~1 km s⁻¹ resolution.