Principle of flat-relative optimal extraction (FOX). Each of the two upper panels shows a 400 pix × 13 pix section of the reddest HARPS order (fibre A). Pixel-to-pixel variations, fringing, order tilt/curvature, and the spatial profile are present in both the master flat F_x,y (top) and the stellar raw spectrum S_x,y which has additional (telluric) absorption lines (middle). The extracted spectrum s_x / f_x (bottom) is the scaling factor between both spectra for each column within the extraction mask (red lines).