Extraction of the instrumental systematics basis for HD4628 by performing a PCA decomposition on the M(t, v_i) matrix (as defined in the text). The first five principal components P_j(v_i) are displayed (left panel) with an arbitrary offset as well as their time-domain coefficients U_j(t) and their corresponding periodogram (right panel). The grey periodograms were derived after applying a high-pass filter on the time-series, while the black ones do not correspond to any filtering. The S-index (top row) is also displayed for comparison. The ratio score R is displayed for each component. Only the first two components (blue and orange) are found symmetric (R₁ = 91.0 and R₂ = 15.1). The first one clearly shows the instrumental defocus during the first part of the instrument’s life (1st and 2nd seasons), whereas an instrumental refocus is also visible at the middle of the penultimate season, which corresponds to a manual intervention on the instrument on the 4th of October 2021 to fix the HARPS-N cryostat (priv. communication).