Small Aperture Telescopes for the Simons Observatory
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Small Aperture Telescopes for the Simons Observatory Aamir M. Ali, et al. [full author details at the end of the article] Received: 21 August 2019 / Accepted: 28 February 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The Simons Observatory (SO) is an upcoming cosmic microwave background (CMB) experiment located on Cerro Toco, Chile, that will map the microwave sky in temperature and polarization in six frequency bands spanning 27 to 285 GHz. SO will consist of one 6-m large aperture telescope fielding ∼ 30,000 detectors and an array of three 0.42-m small aperture telescopes (SATs) fielding an additional 30,000 detectors. This synergy will allow for the extremely sensitive characterization of the CMB over angular scales ranging from an arcmin to tens of degrees, enabling a wide range of scientific output. Here we focus on the SATs targeting degree angular scales with successive dichroic instruments observing at mid-frequency (MF: 93/145 GHz), ultra-high-frequency (UHF: 225/285 GHz), and low-frequency (LF: 27/39 GHz). The three SATs will be able to map ∼ 10% of the sky to a noise level of ∼ 2 μK-arcmin when combining 93 and 145 GHz. The multiple frequency bands will allow the CMB to be separated from galactic foregrounds (primarily synchrotron and dust), with the primary science goal of characterizing the primordial tensor-toscalar ratio, r, at a target level of 𝜎(r) ≈ 0.003. Keywords Small aperture telescope · TES · Refractor · Simons Observatory · Cosmic microwave background · CMB · Inflation · Cosmology
1 Introduction Since its discovery in 1964 [1], the CMB has been a key experimental window into the structure of the universe as a whole, and the early universe in particular. CMB measurements to date are consistent with an inflationary period in the very early universe [2], but a predicted stochastic background of tensor perturbations (inflationary gravitational waves) has yet to be detected. The key observable is a degree-scale signature in the divergence-free ‘B-mode’ component of the CMB polarization [3, 4] parameterized by an overall amplitude given by the tensor-to-scalar ratio r, which probes the energy scale of inflation [5]. A number of experiments have attempted to detect the primordial B-mode signal to determine r. This measurement is challenging. The B-mode signal is much fainter than the curl-free ‘E-mode’ component, and considerably fainter than the polarized galaxy which is everywhere brighter than the primordial B-mode [6], with emission from
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Journal of Low Temperature Physics
synchrotron dominant at lower frequencies and from spinning dust dominant at higher frequencies [7]. The E-mode signal can also be gravitationally deflected by large-scale structure into a lensing B-mode signal which peaks at angular scales of 𝓁 ∼ 1000 [8, 9] ( ∼ arcmins). Although scientifically interesting in their own right, the lensing B-modes act as a foreground to primordial B-modes. The primordial B-modes peak at much larger angular scales of 𝓁 ∼ 90 (
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