Theoretical calculation of an aluminum mirror diattenuation (upper panel) and retardance (lower panel) as a function of wavelength. The four curves represent different values for the thickness of the oxide layer. From top to bottom: 10, 20, 50 and 80 nm, respectively. The curves for 10 and 20 nm overlap in the upper panel. The diamonds show the values obtained from our fit for the turret mirrors (see Fig. 7).