https://ntrs.nasa.gov/search.jsp?R=20120002550 2019-08-30T19:06:35+00:00Z CORE Metadata, citation and similar papers at core.ac.uk Provided by NASA Technical Reports Server THE ATACAMA COSi'vl0LOGY TELESCOPE: THE RECEIVER AND INSTRUMENTATION L2 D. S. SWETZ , P. A. R. ADE\ j'vI. A !\fIRrl, J. \V'. ApPEL" E. S. BATTISTELLlfi,j, B. BURGERI, J. CHERVENAK', 2 lVI. J. DEVLIN], S. R. DICKER]. \V. B. DORIESE , R. DUNNER', T. ESSINGER-HILEMAN'. R. P. FISHER:', J. \V. FOWLER'. 4 2 1 lVI. HALPERN ,!.VI. HASSELFIELD4, G. C. HILTON , A. D. HINCKS\ K. D. IRWIN2. N. JAROSIK', M. KAlIL', J. KLEIN , . ull 11 I2 l l u 1 J. J\1. LAd "" M. LIMON , T. A. l'vIARRIAGE . D. MARSDEN , K. J\IARTOCCI : , P. J\IAUSKOPF: , H. MOSELEY', I4 2 C. B. NETTERFIELD , J\1. D. NIEMACK " l'vI. R. NOLTA ,L. A. PAGE". L. PARKER'. S. T. STAGGS", O. STRYZAK', 1 U jHi E. R. SWlTZER : , R. THORNTON • C. TUCKER'], E. \VOLLACK', Y. ZHAO" Draft version July 5, 2010 ABSTRACT The Atacama Cosmology Telescope was designed to measure small-scale anisotropies in the Cosmic Microwave Background and detect galaxy clusters through the Sunyaev-Zel'dovich effect. The instru­ ment is located on Cerro Taco in the Atacama Desert, at an altitude of 5190 meters. A six-met.er off-axis Gregorian telescope feeds a new type of cryogenic receiver, the Millimeter Bolometer Array Camera. The receiver features three WOO-element arrays of transition-edge sensor bolometers for observations at 148 GHz, 218 GHz, and 277 GHz. Each detector array is fed by free space mm-wave optics. Each frequency band has a field of view of approximately 22' x 26'. The telescope was com­ missioned in 2007 and has completed its third year of operations. We discuss the major components of the telescope, camera, and related systems, and summarize the instrument performance. Subject heading8: Microwave Telescopes, CMB Observations 1. INTRODUCTION secondary effects on CMB anisotropy, from, for exam­ Measurements of the Cosmic Microwave Background ple, the Sunyaev-Zel'dovich (SZ) effect and gravitational (CMB) provide a wealth of information about the origin lensing, which arc important for understanding structure and evolution of the Universe. Current measurements of formation. the angular power spectrum from tenths of a degree to \Ve have built the Atacama Cosmology Telescope all sky have provided estimates of cosmological paramters (ACT), a eustom six-meter telescope, to address these and have begun to quantify the big bang process (e.g., scientific questions. ACT features a cryogenic receiver, Komatsu et al. (2009), Brown et al. (2009), Chiang et the Millimeter Bolometer Array Camera (:\·IBAC), that al. (2009), Sievers et al. (2009)). Detailed measurements operates at three frequencies: 148 GHz, 218 GHz, and at arc minute scales are placing tighter constraints on the 277 GHz. Each frequency band is imaged by a 32 x 32 standard cosmological model and probe possible devia­ array of transition-edge sensor (TES) bolometers. The tions. These higher resolution measurements also reveal telescope was commissioned at its site in late 2007, at which time the 148 GHz channel was installed in MBAC. 1 Department of Physics and Astronomy, University of Pennsyl­ The remaining two frequencies were installed in June vania, 209 South 33rd Street, Philadelphia, PA. USA 19104 2008. Since then the telescope and completed camera 2 NIST Quantum Devices Group, 325 Broadway :'vlailcode have collected approximately twelve months of data over 817.03, Boulder, CO, USA 80305 two observing seasons. 3 School of Physics and Astronomy, Cardiff University. The Pa­ rade, Cardiff, Wales, UK CF24 3AA This paper presents the instrument and complements a 4 Department of Physics and Astronomy, University of British set of ACT papers presenting the first generation of ex­ Columbia, Vancouver, BC, Canada V6T lZ4 perimental results. The paper is organized as follows. ,5 Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, NJ, USA 08544 In Section 2 we discuss the optical design and struc­ 6 Department of Physics, University of Rome "La Sapienza", Pi­ ture characteristics, site location, and telescope opera­ azzale Aldo l\loro 5. 1-00185 Rome, Italv tion. Section 3 details t.he design of t.he l\IBAC cryostat., 7 Code 553/665, NASA/Goddard Sp~ce Flight Center, Green­ cold optics, and detectors. Telescope control, data ac­ belL MD, USA 20771 8 Departamento de Astronomfa y Astroffsica, Facultad de Ffsica, quisition, merging, and related topics are discussed in Pontificia Universidad Cat6lica, Casilla 306, Santiago 22, Chile Section '4. Finally, we present the image quality of the 9 Kavli Institute for Particle Astrophysics and Cosmology, Stan­ system in Section 5. Companion papers that detail the ford University, Stanford, CA. USA 94305-4085 beams and scientific results are Hincks et a1. (2009) and 10 Department of Physics, Stanford University. Stanford, CA, USA 94305-4085 Fowler et al. (2010). 11 Columbia Astrophysics Laboratory, 550 W. 120th St. Mail Code 5247, New York, NY USA 10027 2. THE ATACAMA COSl\IOLOGY TELESCOPE 12 of Astrophysical Sciences, Peyton Hall. Prince- Princeton, NJ USA 08544 2.1. Construction and Optics Kadi Institute for Cosmological Physics. 5620 South Ellis Ave., Chicago, IL. USA 60637 The dianlE'ter of the six-meter primary refiector vms set 14 Department of Physics, University of Toronto. 60 St. George by the requirement to obtain arcminutc-resolution at the Street, Toronto. ON. Canada M5S lA7 ACT frequencies. The primary and two-meter secondary 1:0 Canadian Institute for Theoretical Astrophysics, University of are arranged in an off-axis Gregorian configuration to Toronto, ON, Canada M5S 3H8 Department of Physics, \Vest Chester University of Pennsyl­ give an unobstructed image of the sky. The primary vania, \Vest Chester, PA, USA 19383 focal length was fixed at 5.2 m. This results in a COIn- 2 D. Swetz et al. pact arrangement between the primary and secondary secondary, it does little to reduce the effective spillover. reflectors, making it easier to achieve the fast scanning Measurements of the detectors indicate that there is as specifications of the telescope (Section 2.3). The design much as 2 3% spillover that does not get reflected to the is dm,cribed in Fowler et al. (2007). The telescope was cold sky. As a result, the secondary bailiing is being re­ built by Amec Dynamic Structures Ltd. (now Empire designed to ensure that the majority of the spillover is Dynamic Structures). redirected to the sky in future observations. Figure 1 shows the major components of the telescope structure and Table 1 the important parameters. To min­ 2.2. Site Location, Band Selection, and Logistics imize ground pick-up during scanning, the telescope has The ACT site is at an altitude of 5190 m near the two ground screens. A large. stationary outer ground peak of Cerro Toco in the Atacama Desert of north­ screen surrounds the telescope. A second, inner ground ern Chile. The telescope location provides visibility to screen connects the open sides of the primary reflector approximately 70% of the sky. The high elevation and to the secondary reflector, and moves with the telescope low precipitable water vapor (PWV) at this location pro­ during scanning. A climate-controlled receiver cabin is vides excellent millimeter and submillimeter atmospheric situated underneath the primary and secondary reflec­ transparency and has attracted several other millimeter­ tors. The telescope was designed to work with MBAC wave experiments. The specific ACT site has also been (Section 3), and also to be able to accommodate future used by the TOCO (Miller et al. 2003) and Millimeter receivers. INTerferometer (Fowler et al. 2005) telescopes. The ACT bands were selected to discriminate be­ TABLE 1 tween the SZ, CMB, and point sources. The bands PHYSICAL PROPERTIES OF THE TELESCOPE AJ\:D OPTICS were also chosen to avoid three large emission features, an oxygen emission line at 119 GHz and water emis­ sion lines at 183 GHz and 325 GHz (e.g, Danese and Telescope height 12 m Altitude 5190 m Partrige (1989)). Using the National Radio Astronomy 67°47'15/1W Ground screen height 13 m Longitude Observatory/European Southern Observatory monitor­ Total mass 52 t Latitude 22°57'31/1S ing data and an atmospheric modeling program devel­ oped by Pardo et al. (200l), the opacity and Raleigh­ Jeans (RJ) brightness temperatures are extrapolated to """WCUll range FOVa 1 deg2 Max. az speed 2°/8 the ACT bands. The level of emission in the continuum Primary reflector Dia 6m Max. az ace. 10°/82 where the ACT bands lie is due to O2 and H2 0 in com­ No. primary panels 71 Elev range 30?5 60° parable parts. Thus, the transmission and absorption in Secondary reflector Dia 2m Max. elev ;;peed 0.2°/s the bands is a function of the FWV in the atmosphere. 11 Seasonal changes in the weather provide sustained peri­ a At telescope Gregorian focus ods with low amounts of water vapor and naturally set the ACT observing season to April through December when the PWV is lowest due to the colder weather. Using the Gregorian design as a starting point, the re­ The ACT site is approximately 50 km from the town flector shapes were numerically optimized to increase the of San Pedro de Atacama (altitude rv 2750m), the loca­ field of view over a classic Gregorian using Code V optical tion of lodging and the main field office. Travel to the design software. 1 At the Gregorian focus before reimag­ site from :.Jorth America takes approximately one day. ing (Section 3.1), the telescope achieves a Strehl ratio The roads are clear year-round with brief periods of in­ greater than 0.9 over a 1 square-degree field at 277 GHz.