The following figures give an overview of the Planck-HFI cryogenic chain, the last one gives a view of the Russian doll architecture.

Artist view of the Planck satellite showing the thermal grooves that allow to passively cool the mirrors and the first stages of the instruments to about 40K.

Schematic layout of the Planck cryogenic chain.

Instrument cutaway showing the Russian doll architecture.

The cooling chain is devided in 4 parts:
 The 50K stage is obtained by passive radiative cooling into space.
 The precooling of HFI at 18K is obtained thanks to the sorption cooler.
 A helium Joule-Thomson cooler based on the use of frictionless mechanical pumps provides the cooling power at 4.5K.
 The lowest temperature (0.1K) is provided by the 3He/4He open loop dilution cooler, while the 1.6 K cooling capability is obtained by Joule-Thomson expansion of the 3He/4He mixture. A prototype of this cooler has been successfully used for astronomical observations from the ground and with the balloon-borne experiment Archeops.