Temperature stages within the HFI focal plane unit
This cut-away view of the High Frequency Instrument (HFI) focal plane unit (FPU) shows the location of the different temperature stages within the FPU. The HFI bolometers require an operational temperature of 0.1 K to be able to reach the required detector efficiency.
This extremely low temperature is obtained in several stages. After pre-cooling to 18 K by a sorption cooler, a Joule-Thomson cooler brings the temperature of the HFI FPU to 4 K. A 3He4He dilution cooler provides further cooling down to 0.1 K for the bolometers. The dilution cooler also keeps an enclosure that surrounds the detectors within the HFI FPU at an intermediate temperature of 1.6 K, acting as a buffer towards the FPU's warmer 4 K outer enclosure. The filters and lenses located between the 4 K and 0.1 K feedhorns are also kept at 1.6 K.