Measured background power in the bolometer bands from the primary and secondary mirrors of the Planck telescope. The solid line through the data is a best fit of a constant emissivity, ϵ = 0.07% times the computed blackbody power from the primary mirror at  ~36.2 K and the secondary mirror at  ~39.4 K in the measured bandpass for each detector. The dashed line is the best case performance estimated before flight, which corresponds to a mirror with ϵ = 0.6% at a temperature of 40 K. The scatter in the measured power is largely due to uncertainty in gradients at the level  < 1 mK in the 100 mK plate.