Weighing performance (instrumental flux calibration) for disk-integrated spectra. Top panel: Overlapping orders in 2D flux spectra (exposure at index 10 in HD 209458 epoch 1). Middle panel: Zoom on the 1D spectrum averaged from the order spectra, when accounting (black) or not (magenta) for instrumental calibration in the weight profiles. Bottom panel: Relative difference between the magenta and black profiles, overlapped with the contribution of the instrumental calibration to the order weight profiles (arbitrary scale).