Fig. 10. a S/N distribution across the orders for a tungsten spectrum at the location X=500 (solid line) and X=200 (dotted line). b the flux ratio (in percent) between the tungsten spectrum and the inter-order "spectrum" (or background "spectrum"). In the red part, the background level is proportional to the flux in the order which is a characteristic of a local background contamination. The rise of the relative background level in the blue part of the spectrum, mirror image of the flux distribution, is a feature of a high contamination by the diffuse component.