Cryogenic Far-IR Laser Absorptivity Measurements of the Herschel Space Observatory Telescope Mirror Coatings

Far-infrared laser calorimetry was used to measure the absorptivity, and thus the emissivity, of aluminum-coated silicon carbide mirror samples produced during the coating qualification run of the Herschel Space Observatory telescope to be launched by the European Space Agency in 2007. The samples were measured at 77 Kelvin to simulate the operating temperature of the telescope in its planned orbit around the second Lagrangian point, L2, of the Earth-Sun system. Together, the telescope equilibrium temperature in space and the emissivity of the mirror surfaces will determine the far-infrared/submillimeter background and thus the sensitivity of two of the three astronomical instruments aboard the Observatory, if stray light levels can be kept low relative to the mirror emission. Absorptivities of both clean and dust-contaminated samples were measured at 70, 118, 184 and 496 mu m. Theoretical fits to the data predict absorptivities in the range 0.2 -- 0.4% for the clean sample and 0.2 -- 0.8% for the dusty sample, over the spectral range of the Herschel Space Observatory instruments.