Millimeter-wave bolometer array receiver for the Atacama pathfinder
  experiment Sunyaev-Zel'dovich (APEX-SZ) instrument
  The Atacama pathfinder experiment Sunyaev-Zel'dovich (APEX-SZ) instrument is
a millimeter-wave cryogenic receiver designed to observe galaxy clusters via
the Sunyaev-Zel'dovich effect from the 12 m APEX telescope on the Atacama
plateau in Chile. The receiver contains a focal plane of 280 superconducting
transition-edge sensor (TES) bolometers instrumented with a frequency-domain
multiplexed readout system. The bolometers are cooled to 280 mK via a
three-stage helium sorption refrigerator and a mechanical pulse-tube cooler.
Three warm mirrors, two 4 K lenses, and a horn array couple the TES bolometers
to the telescope. APEX-SZ observes in a single frequency band at 150 GHz with
1' angular resolution and a 22' field-of-view, all well suited for cluster
mapping. The APEX-SZ receiver has played a key role in the introduction of
several new technologies including TES bolometers, the frequency-domain
multiplexed readout, and the use of a pulse-tube cooler with bolometers. As a
result of these new technologies, the instrument has a higher instantaneous
sensitivity and covers a larger field-of-view than earlier generations of
Sunyaev-Zel'dovich instruments. The TES bolometers have a median sensitivity of
890 uKcmb-sqrt(s) (NEy of 3.5e-4 sqrt(s)). We have also demonstrated upgraded
detectors with improved sensitivity of 530 uKcmb-sqrt(s) (NEy of 2.2e-4
sqrt(s)). Since its commissioning in April 2007, APEX-SZ has been used to map
48 clusters. We describe the design of the receiver and its performance when
installed on the APEX telescope.
