Left panels: observed (blue) and fitted (green) Stokes I profile for different values of the regularization parameters λ and using an orthogonal wavelet transform as the sparsity-inducing transformation. The dashed red curve shows the inferred systematic effects. The spectral range corresponds to that around the He i multiplet at 10 830 Å. Right panels: first 100 active wavelet coefficients of the set of 512 total coefficients. The percentage of active functions is shown in each panel.