Telescopes and Instrumentation APEX–SZ: The Atacama Pathfinder EXperiment Sunyaev–Zel’dovich Instrument Daniel Schwan1 9 Materials Sciences Division, phenomena in great detail, for example, Rüdiger Kneissl2, 3 Lawrence Berkeley National Labo- cluster formation and dynamics, cluster Peter Ade4 ratory, Berkeley, USA mergers and their associated shock Kaustuv Basu 5 10 Department of Physics, McGill waves (which are sites of cosmic ray Amy Bender 6 University, Montreal, Canada acceleration), the cosmic ratio of dark Frank Bertoldi 5 11 Department of Physics, University of matter to baryonic matter density, active Hans Böhringer7 Colorado, Boulder, USA galactic nuclei feedback, the evolution of Hsaio-Mei Cho 8 12 Department of Earth and Space galaxies in the ICM, heat transport pro- Gayoung Chon7 Sciences, Onsala Space Observatory, cesses, and plasma instabilities (Sarazin, John Clarke1, 9 Chalmers University of Technology, 1988). Matt Dobbs10 Sweden Daniel Ferrusca1 13 Department of Physics, Columbia In addition, galaxy clusters offer a unique Daniel Flanigan1 University, New York, USA probe into the composition and evolu- Nils Halverson 6,11 14 Physics Division, Lawrence Berkeley tion of the Universe. They are the most William Holzapfel1 National Laboratory, Berkeley, USA massive gravitationally-collapsed objects Cathy Horellou12 15 Max-Planck-Institut für Radioastro- and directly map large-scale structure Daniel Johansson12 nomie, Bonn, Germany over a large range of redshifts. Conse- Bradley Johnson1,13 16 I. Physikalisches Institut, Universität zu quently, measuring the evolution of the James Kennedy10 Köln, Germany cluster number density places powerful Zigmund Kermish1 constraints on the parameters that Matthias Klein 5 govern the growth of structure. These Trevor Lanting10 The APEX–SZ instrument was a milli- parameters are, notably, the mass energy Adrian Lee1,14 metre-wave (150 GHz) cryogenic re­­ density and dark energy density of Martin Lueker1 ceiver for the APEX telescope designed the Universe, the dark energy equation of Jared Mehl1 to observe galaxy clusters via the state, and the matter power spectrum Karl Menten15 Sunyaev–Zel’dovich Effect (SZE). The normalisation (Carlstrom et al., 2002). Dirk Muders15 receiver contained a focal plane of Florian Pacaud 5 280 superconducting transition-edge The hot ICM is observable at millimetre- Thomas Plagge1 sensor bolometers equipped with a wavelengths via the SZE (Sunyaev & Christian Reichardt1 frequency-domain-multiplexed readout Zel’dovich, 1970). In the SZE, about 1% Paul Richards1 system, and it played a key role in the of cosmic microwave background (CMB) Reinhold Schaaf 5 introduction of these new, robust, and photons incident on the cluster are Peter Schilke16 scalable technologies. With 1-arcminute scattered by free intracluster electrons. Martin Sommer 5,15 resolution, the instrument had a higher The resulting signal distorts the CMB Helmuth Spieler 14 instantaneous sensitivity and covered a blackbody spectrum and is visible as a Carole Tucker 4 larger field of view (22 arcminutes) than temperature decrement (increment) at Axel Weiss15 earlier generations of SZE instruments. frequencies below (above) 217 GHz. The Benjamin Westbrook1 During its period of operation from 2007 SZE surface brightness is proportional Oliver Zahn1 to 2010, APEX–SZ was used to image to the integrated gas pressure along the over 40 clusters and map fields overlap- line of sight through the cluster. Since ping with external datasets. This paper the SZE is a scattering effect seen rela- 1 Department of Physics, University of briefly describes the instrument and tive to the CMB, and the scattered and California, Berkeley, USA data reduction procedure and presents unscattered photons are redshifted 2 ESO a cluster image gallery, as well as results together, the SZE signal is independent 3 Joint ALMA Observatory, Vitacura, for the Bullet cluster, Abell 2204, Abell of the redshift of the scattering cluster. Santiago, Chile 2163, and a power spectrum analysis in In contrast, X-ray and optical surface 4 School of Physics and Astronomy, the XMM-LSS field. brightness dim with increasing redshift. Cardiff University, Wales, United The redshift independence makes the Kingdom SZE a uniquely sensitive method for dis- 5 Argelander Institute for Astronomy, Galaxy clusters covering and observing distant clusters. Bonn University, Germany 6 Center for Astrophysics and Space Galaxy clusters comprise dark matter Astronomy, Department of Astophysical and constituent galaxies with hot, low Instrument overview and Planetary Sciences, University of density ionised gas filling the space Colorado, Boulder, USA between the galaxies. This gas, known APEX–SZ observations were performed at 7 Max-Planck-Institut für extraterres- as the intracluster medium (ICM), domi- 150 GHz (2 mm) from the 12-metre APEX trische Physik, Garching, Germany nates the baryonic mass in clusters. telescope (Güsten et al., 2006). Technical 8 National Institute of Standards and Thus, galaxy clusters are important labo- details of the receiver can be found in Technology, Boulder, USA ratories for studying various astrophysical Schwan et al. (2011). The telescope was The Messenger 147 – March 2012 7 Telescopes and Instrumentation Schwan D. et al., APEX–SZ: The APEX Sunyaev–Zel’dovich Instrument dent on the absorber are measured by the TES mounted at its centre. The TES is a thin superconducting film with a tran­ sition temperature higher than the reser- voir temperature. A constant bias voltage is applied across the TES so that the total applied power (the electrical power dissipated in the TES plus optical power incident on the absorber) raises 1 mm the temperature of the sensor to pre- cisely the transition temperature. An increase (decrease) in the incident optical power produces a cancelling decrease (increase) in electrical power. Since the 25 mm voltage bias is held constant, the change in electrical power is measured as a change in current through the sensor. Figure 1. The TES bolometer array, with inset show- machined from a single aluminium block ing three spiderweb bolometers. Six identical trian- and mounted above the planar focal The bolometers are fabricated using gular sub-arrays are assembled into a single planar array, 133 mm in diameter, containing 330 bolome- surface. Each bolometer was centred standard photolithographic techniques. ters. Each bolometer has two leads which run from behind a cylindrical waveguide in the horn The absorber, the TES and the leads the spiderweb to bonding pads at the edge of its tri- array. There was a small gap between to bias the TES are deposited onto a 1 μm angle, visible as light traces between the bolometers. the horn array and the bolometers to pro- thick, low-stress silicon nitride (LSN) vide thermal isolation. Radiation not spiderweb membrane. The LSN spider- commissioned by the Max-Planck-Institut absorbed by a detector could leak radi- web is suspended by eight LSN legs für Radioastronomie, ESO and the ally to neighbouring detectors or be above a ~ 20 μm vacuum gap etched in Swedish Onsala Space Observatory for reflected back through the horns. This the silicon. The gap thermally isolates use with bolometric and heterodyne leakage resulted in a small, ~ 1%, optical the absorber from the silicon substrate. receivers. It is located near the ALMA site cross-talk between adjacent bolometers. The thermal conductance of the bo- at an altitude of 5107 metres on Llano lom eter is set by a gold thermal link that de Chajnantor in the Atacama desert in The TES bolometers and frequency- runs from the TES to the thermal reser- Chile, which has exceptionally low water domain multiplexed (fMUX) readout sys- voir, parallel to the bias leads. The vapour conditions. In addition, the low tem used in the APEX–SZ focal plane APEX–SZ detector array is assembled latitude of the site (23 degrees south) have low noise, but require a cryogenic from six identical 55-element, triangular allows targeted observation of clusters system for their operation. The receiver subarrays (Figure 1). with rich multi-frequency datasets. Over- interior was cooled by a closed cycle lapping observations at different wave- pulse­tube cooler with cold stages at 60 K The APEX–SZ TES bolometers are read lengths enable a more complete study and 4 K. The mechanical cooler elimi- out with a SQUID (Superconducting and characterisation of clusters. nated the need for open reservoirs of liq- Quantum Interference Device) amplifier uid cryogens, greatly simplifying the fMUX, which allows several detectors to The telescope beam was coupled to the design and construction of the cryostat be read out by a single 4 K SQUID ampli- APEX receiver by a series of reimaging while reducing the cost and difficulty fier connected through a single pair of optics inside the Cassegrain cabin. The of remote observations. The bolometers wires (Lanting et al., 2004). Readout mul- full optical system had 58-arcsecond were further cooled to 280 mK by ti plexing enables the use of large detector FWHM (Full Width at Half Maximum) a three-stage helium sorption fridge. arrays by greatly reducing the thermal beams, diffraction-limited performance, load on the low temperature stage, the over a 22­arcminute field of view (FoV). complexity of cold wiring and the system This large FoV enabled APEX–SZ to map Detectors and readout cost. The APEX–SZ system uses seven the large angular size of low redshift detectors read out by one SQUID ampli- clusters, where the SZE signal can extend Each APEX–SZ bolometer consists of fier in each multiplexer module. beyond a 10-arcminute radius. an optical absorber coupled to a TES that is connected to a thermal reservoir by In the fMUX system, the bolometers are Within the receiver, the APEX–SZ focal a weak thermal link. The absorbing ele- biased with alternating voltages at carrier plane was 133 mm across and consisted ment is a 3 mm diameter gold spiderweb frequencies (0.3–1 MHz) that are much of 330 feedhorn-coupled TES (transition- (Figure 1).