The Boomerang dewar
holds 65 liters of liquid helium and 75 liters of liquid nitrogen and lasts
for roughly 12 days with no servicing. The light from the telescope
enters the dewar through a window in the bottom.
The 3He refrigerator
is mounted inside the dewar and cools the detectors. The metal sphere
is the charcoal pump. The 3He pot, which is cooled to 0.28K, is the
small metal cylinder in the lower part of the picture.
This is the focal plane
where the optics, filters, and detectors are located. This whole
structure is cooled to 0.28K.