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 (NEy of 3.5 × 10−4 ). We have also demonstrated upgraded detectors with improved sensitivity of 530 (NEy of 2.2 × 10−4 ). 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.
We thank the staff at the APEX telescope site, led by David Rabanus and previously by Lars-Åke Nyman, for their exceptional support, and the machine shop staff of the University of California, Berkeley for their assistance in designing and work in fabrication of the APEX-SZ receiver system. We also thank LBNL engineers John Joseph and Chinh Vu for their work on the readout electronics. We thank Bryan Steinbach for calculation of ac-biased detector responsivity.
APEX-SZ is funded by the National Science Foundation under Grant Nos. AST-0138348 and AST-0709497. Work at LBNL is supported by the Director, Office of Science, Office of High Energy, and Nuclear Physics (ATL and HS), and by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division (JC, collaboration on development of SQUID multiplexer), of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Work at McGill is supported by the Natural Sciences and Engineering Research Council of Canada, the Canadian Institute for Advanced Research, and Canada Research Chairs program. N.W.H. and M.D. acknowledge support from Alfred P. Sloan Research Fellowships. C.H. and D.J. acknowledge financial support from the Swedish Research Council. R.K. acknowledges partial financial support from MPG Berkeley-Munich fund. F.P. acknowledges support from Grant No. 50 OR 1003 of the Deutsches Zemtrum für Luft- und Raumfahrt.
I. INTRODUCTION II. APEX A. Telescope B. Atacama Site III. OPTICS A. Tertiary optics B. Filters and band C. Focal plane optics IV. DETECTORS V. FREQUENCY MULTIPLEXED READOUT ELECTRONICS VI. CRYOGENICS A. Cryostat B. Pulse-tube cooler C. Helium sorption refrigerator D. Millikelvin stage E. Thermometry VII. SYSTEM PERFORMANCE A. Detector yield B. Beams 1. Gaussian fit 2. Beam profile C. Band D. Calibration 1. Calibration procedure 2. Atmospheric opacity 3. Calibration stability E. Pointing F. Instrument efficiency G. Optical loading H. Noise and sensitivity 1. Instrument noise 2. Detector sensitivity VIII. OBSERVATIONS AND ANALYSIS A. Scan strategy B. Cluster map making C. Mapping speed IX. CONCLUSIONS