Many modern radio telescopes employ an observational strategy that involves maximizing the use of their available spaces (cabins), outfitting them with various receivers at different frequencies to detect incoming signals from the sky simultaneously or individually. The Large Latin American Millimeter Array is a joint venture between Argentina and Brazil consisting of the installation and operation of a 12-m aperture Cassegrain telescope. It features three available cabins for instrumentation and plans to install six single-pixel heterodyne receivers, covering different bandwidths in the 30 to 950 GHz window of the electromagnetic spectrum, in its two lateral Nasmyth cabins at different phases of the project. Therefore, it is crucial not only to design a tertiary optical system that couples the antenna beam to those receivers but also to do it in a scalable way. The primary goal for the design is to simultaneously maximize the antenna efficiency while minimizing optical aberrations for all receivers, both fundamental aspects for the optimal functioning of cutting-edge astronomical instruments. We present the entire design process, starting from the quasi-optical approach based on the propagation of a fundamental Gaussian beam mode, continuing with the validation of the design based on physical optics simulations, and ending with a tolerance analysis of the system. As a result of this process, a frequency-independent tertiary optical system has been achieved for almost all the receivers, which is expected to provide high optical performance for the radio telescope.
High-resolution maps of polarization anisotropies of the cosmic microwave background (CMB) are in high demand, since the discovery of primordial B-modes in the polarization patterns would confirm the inflationary phase of the universe that would have taken place before the last scattering of the CMB at the recombination epoch. Transition edge sensors (TES) and microwave kinetic inductance detectors (MKID) are the predominant detector technologies of cryogenic detector array-based CMB instruments that search for primordial B-modes. We propose another type of cryogenic detector to be used for CMB survey: a magnetic microbolometer (MMB) that is based on a paramagnetic temperature sensor. It is an adaption of state-of-the-art metallic magnetic calorimeters (MMCs) that are meanwhile a key technology for high resolution α, β, γ, and x-ray spectroscopy as well as the study of neutrino mass. The effort to adapt MMCs for CMB surveys is triggered by their lack of Johnson noise associated with the detector readout, the possibility of straightforward calibration and higher dynamic range given it possesses a broad and smooth responsivity dependence with temperature, and the absence of Joule dissipation which simplifies the thermal design. A brief proof of concept case study is analyzed, taking into account typical constraints in CMB measurements and reliable microfabrication processes, to assess the suitability of metallic magnetic sensors in CMB experiments. The results show that MMBs provide a promising technology for CMB polarization survey as their sensitivity can be tuned for background limited detection of the sky while simultaneously maintaining a low time response to avoid distortion of the point-source response of the telescope. As the sensor technology and its fabrication techniques are compatible with TES-based bolometric detector arrays, a change of detector technology would even come with very low cost.
QUBIC (Q and U bolometric interferometer for cosmology) is an international ground-based experiment dedicated to the measurement of the polarized fluctuations of the cosmic microwave background (CMB). It is based on bolometric interferometry, an original detection technique which combines the immunity to systematic effects of an interferometer with the sensitivity of low temperature incoherent detectors. QUBIC will be deployed in Argentina, at the Alto Chorrillos mountain site near San Antonio de los Cobres, in the Salta province. The QUBIC detection chain consists of 2048 NbSi transition edge sensors (TESs) cooled to 320 mK. The voltage-biased TESs are read out with time domain multiplexing based on superconducting quantum interference devices (SQUIDs) at 1 K and a novel SiGe application-specific integrated circuit (ASIC) at 60 K allowing an unprecedented multiplexing (MUX) factor equal to 128 to be reached. The current QUBIC version is based on a reduced number of detectors (1/4) in order to validate the detection technique. The QUBIC experiment is currently being validated in the lab in Salta (Argentina) before going to the site for observations. This paper presents the main results of the characterization phase with a focus on the detectors and readout system.
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