Herschel Focal Plane
The Herschel 3.5 m telescope will focus light onto three scientific instruments inside a cryostat vacuum vessel. To make the instruments as sensitive as possible and to eliminate emission coming from the instruments themselves the detectors inside the three focal plane units (FPUs) are cooled to 0.3 K, using superfluid helium (at about 2 K) and an additional cooling stage inside the FPUs.
In this view, the top of the cryostat vacuum vessel and the optical bench shield have been removed to show two of the three FPUs (SPIRE's FPU is left out to show the light entering the HIFI FPU). The primary mirror is seen at the top, with the collected light passing through a central opening onto the focal plane.