Illustration of the subtraction of the positive peak from the cross-correlation. Left panel: north-south cut of the cross-correlation, IC(τ), of the image at line cord level 5 in the FeI line at 603.15 nm, with the image at the continuum level, for μ = 0.82. The negative peak of cross-correlation arising from the contrast inversion of the images is partly blended with the positive peak. Right panel: result of the subtraction IC(τ) − αC(τ), with α = 0.12. We notice the shift of the negative peak when the correction is applied.