Full width optimal extraction apertures (top panel) and their associated flux losses (bottom panel) in echelle order 10 (UVB) for the star HD 38237. The extraction apertures resemble a discrete step function, as the S/N only changes at the transition between adjacent pixels. The corresponding flux losses follow this step function, but show an underlying smooth component that arises from the continuous PSF shape variation within the echelle order (see bottom panel of Fig. 10).