Flux envelope and background light characteristics of FP and LFC calibration light sources and the line centroid differences between the fits with and without background light subtraction. The top panels show for each spectral order the median and 95% range of flux envelope and background light, i.e., the statistical properties of flux and background light shown explicitly for two orders in Fig. 7. The bottom panels display per order the scatter of the centroid shifts, computed from the 16th–84th percentile range. For simplicity, only Slice a of Fiber A is shown.