A sketch of the cool-down sequence, indicating when the main instrument and cryo-chain-related activities took place in the early phases of the mission. The coloured curves trace the temperature of each stage in the cryogenic chain. The arrows indicate some of the key events in the sequence, as described in the text. (Note: SCS = sorption cooler system; FEM = LFI front-end modules; FPU = focal plane unit; and HS = heat switch.) The events are colour-coded by sub-system: blue for the sorption cooler; pink for LFI; and yellow for HFI. “LFI tuning” refers mainly to the optimisation of the bias settings of the FEMs, whereas “HFI tuning” refers mainly to optimisation of the thermal control loops at each low-temperature stage.