Comparison between Chandra and Swift-XRT instrument background spectra. Upper panel: NXB of the two instruments are plotted per unit area, to show the influence of different environments (orbits plus satellite). Middle panel: NXB are plotted per solid angle, to show the influence of different focal lengths. Lower panel: the ratio between Chandra and XRT NXB calculated per solid angle and normalized by the respective effective areas. This shows the improvement in the signal/background ratio for an extended source using XRT instead of Chandra.