The Simons Observatory is a ground-based cosmic microwave background survey experiment that consists of three 0.5 m small-aperture telescopes and one 6 m large-aperture telescope, sited at an elevation of 5200 m in the Atacama Desert in Chile. SO will deploy 60,000 transition-edge sensor (TES) bolometers in 49 separate focal-plane modules across a suite of four telescopes covering 30/40 GHz low frequency (LF), 90/150 GHz mid frequency (MF), and 220/280 GHz ultra-high frequency (UHF). Each MF and UHF focal-plane module packages 1720 optical detectors spreading across 12 detector bias lines that provide voltage biasing to the detectors. During observation, detectors are subject to varying atmospheric emission and hence need to be re-biased accordingly. The re-biasing process includes measuring the detector properties such as the TES resistance and responsivity in a fast manner. Based on the result, detectors within one bias line then are biased with suitable voltage. Here we describe a technique for re-biasing detectors in the modules using the result from bias-step measurement.
The Simons Observatory (SO) is a ground-based cosmic microwave background survey experiment that consists of three 0.5 m small-aperture telescopes and one 6 m large-aperture telescope, sited at an elevation of 5,200 m in the Atacama Desert in Chile. The SO focal planes will be tiled with 49 universal focal-plane modules (UFMs), in which transition-edge sensor detectors are coupled to microwave SQUID multiplexing readout components. These detector modules contain a stack of silicon wafers and chips, which are encased in an aluminum shield and electrically connected with over 10,000 wire bonds. To ensure the UFMs will maintain their electrical and mechanical integrity throughout their expected lifetime, we have developed a program of robustness testing. This program involves repeated cryogenic cycling to mimic a lifetime of operation in the field. We also describe electrical validation tools that enable the debugging of electrical shorts that can appear during assembly and device screening. As a result of these tests and developments, we expect that the UFMs will maintain operability through in-lab module screening and at least five seasons of observation in Chile.
The Simons Observatory is a suite of instruments sensitive to temperature and polarization of the cosmic microwave background. Five telescopes will host over 60,000 highly multiplexed transition edge sensor (TES) detectors. The universal focal plane modules (UFMs) package multichroic TES detectors with microwave multiplexing electronics compatible with all five receivers. The low-frequency arrays are lenslet-coupled sinuous antennas sensitive to 30 and 40 GHz. The mid-frequency and ultra-high-frequency UFMs are horn-coupled orthomode transducer arrays sensitive to 90/150 GHz and 225/280 GHz, respectively. Here we present the design, assembly details, and initial results of the first UFM.
The Simons Observatory (SO) will observe the cosmic microwave background (CMB) from Cerro Toco in the Atacama Desert of Chile. The observatory consists of three 0.5m Small Aperture Telescopes (SATs) and one 6m Large Aperture Telescope (LAT), covering six frequency bands centering around 30, 40, 90, 150, 230, and 280 GHz. The SO observations will transform our understanding of our universe by characterizing the properties of the early universe, measuring the number of relativistic species and the mass of neutrinos, improving our understanding of galaxy evolution, and constraining the properties of cosmic reionization.1 As a critical instrument, the Large Aperture Telescope Receiver (LATR) is designed to cool ~60,000 transition-edge sensors (TES)2 to <100mK on a 1.7m diameter focal plane. The unprecedented scale of the LATR drives a complex design.3-5 In this paper, We will first provide an overview of the LATR design. Integration and validation of the LATR design is discussed in detail, including mechanical strength, optical alignment, and cryogenic performance of the five cryogenic stages (80 K, 40 K, 4 K, 1 K, and 100 mK). We will also discuss the microwave- multiplexing (μMux) readout system implemented in the LATR and demonstrate operation of dark, prototype TES bolometers. The μMux readout technology enables one coaxial loop to read out Ο(103) TES detectors. Its implementation within the LATR serves as a critical validation for the complex RF chain design. The successful validation of the LATR performance is not only a critical milestone within the Simons Observatory, it also provides a valuable reference for other experiments, e.g. CCAT-prime6 and CMB-S4.7, 8
The Simons Observatory (SO) will perform ground-based observations of the cosmic microwave background (CMB) with several small and large aperture telescopes, each outfitted with thousands to tens of thousands of superconducting aluminum manganese (AlMn) transition-edge sensor bolometers (TESs). In-situ characterization of TES responsivities and effective time constants will be required multiple times each observing-day for calibrating time-streams during CMB map-making. Effective time constants are typically estimated in the field by briefly applying small amplitude square-waves on top of the TES DC biases, and fitting exponential decays in the bolometer response. These so-called “bias step" measurements can be rapidly implemented across entire arrays and therefore are attractive because they take up little observing time. However, individual detector complex impedance measurements, while too slow to implement during observations, can provide a fuller picture of the TES model and a better understanding of its temporal response. Here, we present the results of dark TES characterization of many prototype SO bolometers and compare the effective thermal time constants measured via bias steps to those derived from complex impedance data.
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