Visualisation of the CCF residuals profiles obtained before (top panel) and after (bottom panel) the decorrelation by the S-index for HD4628. The colour code is the Z-normalised S-index vector (in arbitrary units). The stellar activity signature, initially visible in the top panel by the blue-to-green colour gradient at the line centre (v = 0 km s⁻¹), is removed, whereas the clear instrumental defocus is preserved. The instrumental time-domain vectors can be extracted after performing a PCA decomposition of those profiles (see Fig. C.2).