Example of PCA denoising: the original noisy signal O_j(v) (dashed line) for the 612.2 nm line of Ca i, is displayed, together with its projection onto the eigenprofile of matrix C associated with the highest eigenvalue, P_j,1 (full thick line). This last profile already bears a shape very similar to the SLA (or LSD) pseudo-profiles obtained with the whole set of observations, which are displayed in Fig. 1.