Located in the focal plane of the telescope, Planck’s Low Frequency Instrument (LFI), and the High Frequency Instrument
(HFI), are equipped with a total of 74 detectors covering nine frequency channels. These detectors must be cooled to
temperatures around or below 20 K so that their heat does not swamp the faint microwave signals they are designed to detect.

Planck’s active cooling system consists of a three-stage refrigeration chain which takes over after the passive cooling system cools the telescope to about 50 K. The first stage makes use of liquid hydrogen to reduce the temperatures to 20 K. The
second stage is a mechanical cooler (a pump) that uses liquid helium (4He) to bring the temperatures down to 4 K. The third
stage makes use of a mixture of Helium 3 and Helium 4 (3He and 4He) to reach an amazingly low temperature of just 0.1 K.

This image shows the path that microwave light collected by telescope follows to reach the instrument detectors via the conical feed horns (for HFI on the top sequence, for LFI on the bottom sequence). The bolometric detectors of the HFI, located behind the horns, absorb the light and heat up slightly. A thermometer reads the temperature rise and converts it to an electrical signal which travels down wires connecting the low- and high-temperature ends of the instruments.

For the LFI, the process is similar, but the conversion to an electrical signal takes place further down the line, beyond the
waveguides that connect the focal plane unit to the LFI electronics placed in the service module.