Planck's instruments will operate at a few degrees above absolute zero. To achieve this, a series of cooling stages are required. Without any additional cooling, the spacecraft itself can passively cool to around 50 K (about -223ºC).

For the Low Frequency Instrument (LFI) the focal plane is cooled to around 20 K (about -253ºC). The signals received in the instrument horns are amplified and passed through waveguides to a back-end unit (which is at around 300 K, or about 27ºC) where the signals are processed.

The High Frequency Instrument (HFI) unit is more compact and sits entirely within the LFI housing at about 18 K (about -255ºC). The bolometers (devices for measuring incident electromagnetic radiation) are then stage cooled to just 0.1 K (-273.14ºC) with the detected signals again processed in a warmer back-end readout and data processing unit.

Isolating the various components and cooling only small volumes to the coldest temperatures the optimum efficiencies and mission achieve lifetime.

Credits: ESA (Image by AOES Medialab)

The two instruments detect the collected radiation in different ways. LFI is designed to convert the lower energy microwaves into electrical voltages, rather like a transistor radio. HFI works by converting the higher energy microwaves to heat, which is then measured by a tiny electrical thermometer. The instruments share a common telescope.

Credits: ESA (Image by AOES Medialab)