a) Scatter plot of the Dopplergram signal, ⟨v_l⟩ in the quiet Sun, near disc centre (μ > 0.99), in the restored (with K₂) and uncorrected versions of the 720 s Dopplergram. The red circles denote the bin-median (from binning the points by the uncorrected ⟨v_l⟩ in intervals of 20ms^-1) and the blue line the corresponding linear regression fit. b) The factor enhancement of ⟨v_l⟩ in the quiet Sun from image restoration, Δ⟨v_l⟩_QS as a function of μ.