ESA Science & Technology - Herschel Build-up

Animation of the Herschel spacecraft build-up, showing the different components from the inside out. Starting with the optical bench and the focal plane units (FPUs) of the three instruments (HIFI, PACS and SPIRE), located in the top of the cryostat vacuum vessel.

Cooling system
Helium gas boils of from the superfluid helium II in the helium II tank and flows into pipes around the FPUs to cool them down to between -271 °C & and -256 °C (2 to 15 K). The combined optical bench and helium II tank are surrounded by 3 thermal shields. After cooling the FPUs, the helium gas is directed through pipes between the three thermal shields which are cooled down by the gas to between -240 °C and -210 °C.

The external shield of the cryostat vacuum vessel, not being cooled down, stays at ambient -200 °C. At the end of the circuit the helium gas vents into outer space.