Boomerang Photo Gallery: Instrumentation
Boomerang has a long history - over 5 years in the making. A Long Duration Flight around Antarctica was successfully completed in January, 1999.  The experiment is designed to measure anisotropies in the Cosmic Microwave Background Radiation.  To do this Boomerang contains an array of detectors cooled to 0.28 K in a large, long duration dewar.  This dewar is mounted at the focus of a 1.3m diameter telescope and flown on a pointed gondola from a NASA high altitude balloon.
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.

This is the boomerang gondola missing most of its outer shielding.  The dewar is positioned in the center of the gondola, with the primary mirror (underneath a blue protective cover in this photo) near the bottom.  The gondola hangs from the balloon during the flight, and is able to scan back and forth repeatedly over a selected observing field. 
Here is a "CMB photon's view" of the telescope.  The primary mirror in the center focuses light up into the detectors in the dewar.  One of the windows to the dewar can be seen imaged in the primary mirror.  The large panels that are around the mirror and attached to the sides of the gondola keep light from the earth and sun from interfering with our measurements.