COSMO (COSmic Monopole Observer) is an experiment aimed at the searching for spectral distortions in the CMB (Cosmic Microwave Background) between 120 and 300 GHz. COSMO will be operated from Concordia Station on the Antarctic plateau. The experiment is based on a cryogenic Martin-Puplett interferometer whose superconductive detectors are KIDs (Kinetic Inductance Detectors). The interferometer produces interferograms proportional to the difference between the sky and an internal reference black body. The sky signal has a fast modulation to compensate for the atmospheric fluctuations. A key requirement of the readout is an ultra-fast rate to track the signal modulation and also for detector diagnostic. The readout architecture is based on an IQ transceiver generating a comb of test tones tuned to each detector. We developed a modular readout based on commercial components for reliability and fast prototyping. We were able to reach and sustain a readout rate higher than 60 kHz for 18 detectors. In this contribution a general description of the architecture, together with the main performances in terms of amplitude and phase noise are given.
In this work, we present the design and manufacturing of the two multi-mode antenna arrays of the COSMO experiment and the preliminary beam pattern measurements of their fundamental mode compared with simulations.
COSMO is a cryogenic Martin-Puplett Fourier Transform Spectrometer that aims at measuring the isotropic y-type spectral distortion of the Cosmic Microwave Background from Antarctica, by performing differential measurements between the sky and an internal, cryogenic reference blackbody. To reduce the atmospheric contribution, a spinning wedge mirror performs fast sky-dips at varying elevations while fast, low-noise Kinetic Inductance detectors scan the interferogram.
Two arrays of antennas couple the radiation to the detectors. Each array consists of nine smooth-walled multi-mode feed-horns, operating in the 120−180 GHz and 210−300 GHz range, respectively. The multi-mode propagation helps increase the instrumental sensitivity without employing large focal planes with hundreds of detectors. The two arrays have a step-linear and a linear profile, respectively, and are obtained by superimposing aluminum plates made with CNC milling. The simulated multi-mode beam pattern has a ~ 20° − 26° FWHM for the low-frequency array and ~16° FWHM for the high-frequency one. The side lobes are below −15 dB.
To characterize the antenna response, we measured the beam pattern of the fundamental mode using a Vector Network Analyzer, in far-field conditions inside an anechoic chamber at room temperature. We completed the measurements of the low-frequency array and found a good agreement with the simulations. We also identified a few non-idealities that we attribute to the measuring setup and will further investigate. A comprehensive multi-mode measurement will be feasible at cryogenic temperature once the full receiver is integrated.
In this paper we present the European Low Frequency Survey (ELFS), a project that will enable foregrounds-free measurements of the primordial B-mode polarization and a detection of the tensor-to-scalar ratio, r, to a level σ(r) = 0:001 by measuring the Galactic and extra-galactic emissions in the 5–120 GHz frequency window. Indeed, the main difficulty in measuring the B-mode polarization comes from the fact that many other processes in the Universe also emit polarized microwaves, which obscure the faint Cosmic Microwave Background (CMB) signal. The first stage of this project is being carried out in synergy with the Simons Array (SA) collaboration, installing a 5.5–11GHz (X-band) coherent receiver at the focus of one of the three 3.5m SA telescopes in Atacama, Chile, followed by the installation of the QUIJOTE-MFI2 in the 10–20 GHz range. We designate this initial iteration of the ELFS program as ELFS-SA. The receivers are equipped with a fully digital back-end that will provide a frequency resolution of 1MHz across the band, allowing us to clean the scientific signal from unwanted radio frequency interference, particularly from low-Earth orbit satellite mega constellations. This paper reviews the scientific motivation for ELFS and its instrumental characteristics, and provides an update on the development of ELFS-SA.
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