Illustration of various cuts through the trace in detector X direction (cross-dispersion). Top left panel: nine trace profiles shown in terms of raw counts, i.e., C_yx. The central cut lies exactly at the peak of a FP line. The others are offset in detector Y (wavelength) direction by up to ±4 pixels. The saturation of the line color indicates the offset from the line. Top right panel: same as top-left panel but all profiles are normalized to unity flux, highlighting the different shapes. Bottom left panel: trace profiles extracted from a spectral flatfield frame at the same positions as the curves in the top panels, i.e P_yx. No significant dependence on the position is observed. Bottom right panel: raw counts from the FP frame normalized by the corresponding trace profiles, i.e., the quantity C_yx/P_yx. The gray dotted curve shows the inverse-variance weighting (arbitrary scale). The solid gray curve represents the full weighting function including the window function , which limits the extraction to ±5 pixels (vertical dotted lines).