Flat field response of the O VI channel along the UVCS slit in the O VI doublet (top-left panel) and redundant H I Lyα (top-right panel) spectral intervals. An example of bias-corrected data is shown in the bottom panel, where the original data are represented by a black curve and the corrected data are shown with a red curve. The data corrected for the detector flat field response have also been normalized to the value measured at the pole, only in this figure for graphical reasons, in order to account for the coronal intensity decrease, when moving from the slit center to parts of the corona at higher distances (see text for correction description).