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The Simons Observatory (SO) is a cosmic microwave background survey experiment that includes small-aperture telescopes (SATs) observing from an altitude of 5200 m in the Atacama Desert in Chile. The SO SATs will cover six spectral bands between 27 and 280 GHz to search for primordial B-modes to a sensitivity of σ ( r ) = 0.002, with quantified systematic errors well below this value. Each SAT is a self-contained cryogenic telescope with a 35° field of view, 42 cm diameter optical aperture, 40 K half-wave plate, 1 K refractive optics, and 12,000 transition edge sensor detectors. We describe the nominal design of the SATs and present details about the integration and testing for one operating at 93 and 145 GHz.

Improved polarization measurements at frequencies below 70 GHz with degree-level angular resolution are crucial for advancing our understanding of the Galactic synchrotron radiation and the potential polarized anomalous microwave emission and ultimately benefiting the detection of primordial B modes. In this study, we present sensitivity-improved 40 GHz polarization maps obtained by combining the CLASS 40 GHz and Wilkinson Microwave Anisotropy Probe (WMAP) Q -band data through a weighted average in the harmonic domain. The decision to include WMAP Q -band data stems from similarities in the bandpasses. Leveraging the accurate large-scale measurements from the WMAP Q band and the high-sensitivity information from the CLASS 40 GHz band at intermediate scales, the noise level at ℓ ∈ [30, 100] is reduced by a factor of 2–3 in the map space. A pixel domain analysis of the polarized synchrotron spectral index ( β _s ) using the WMAP K band and the combined maps (mean and 16th/84th percentiles across the β _s map: $-{3.08}_{-0.20}^{+0.20}$ ) reveals a stronger preference for spatial variation (probability to exceed for a uniform β _s hypothesis smaller than 0.001) than the results obtained using WMAP K and Ka bands ( $-{3.08}_{-0.14}^{+0.14}$ ). The cross-power spectra of the combined maps follow the same trend as other low-frequency data, and validation through simulations indicates negligible bias introduced by the combination method (subpercent level in the power spectra). The products of this work are publicly available on LAMBDA ( https://lambda.gsfc.nasa.gov/product/class/class_prod_table.html ).

The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background (CMB) over ∼75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the large angular scale CMB polarization to constrain the tensor-to-scalar ratio and the optical depth to last scattering. This paper presents the optical characterization of the 90 GHz telescope. Observations of the Moon establish the pointing while dedicated observations of Jupiter are used for beam calibration. The standard deviations of the pointing error in azimuth, elevation, and boresight angle are 1.′3, 2.′1, and 2.′0, respectively, over the first 3 yr of observations. This corresponds to a pointing uncertainty ∼7% of the beam’s full width at half-maximum (FWHM). The effective azimuthally symmetrized instrument 1D beam estimated at 90 GHz has an FWHM of 0.°620 ± 0.°003 and a solid angle of 138.7 ± 0.6(stats.) ± 1.1(sys.) μ sr integrated to a radius of 4°. The corresponding beam window function drops to ${b}_{{\ell }}^{2}=0.93,0.71,0.14$ at ℓ = 30, 100, 300, respectively. Far-sidelobes are studied using detector-centered intensity maps of the Moon and measured to be at a level of 10 ^−3 or below relative to the peak. The polarization angle of Tau A estimated from preliminary survey maps is 149°.6 ± 0°.2(stats.) in equatorial coordinates. The instrumental temperature-to-polarization ( T → P ) leakage fraction, inferred from per-detector demodulated Jupiter scan data, has a monopole component at the level of 1.7 × 10 ^−3 , a dipole component with an amplitude of 4.3 × 10 ^−3 , with no evidence of quadrupolar leakage.

Measurement of the largest angular scale ( ℓ < 30) features of the cosmic microwave background (CMB) polarization is a powerful way to constrain the optical depth to reionization and search for the signature of inflation through the detection of primordial B -modes. We present an analysis of maps covering 73.6% of the sky made from the 40 GHz channel of the Cosmology Large Angular Scale Surveyor (CLASS) from 2016 August to 2022 May. Taking advantage of the measurement stability enabled by front-end polarization modulation and excellent conditions from the Atacama Desert, we show this channel achieves higher sensitivity than the analogous frequencies from satellite measurements in the range 10 < ℓ < 100. Simulations show the CLASS linear (circular) polarization maps have a white noise level of $125(130)\,\mu {\rm{K}}\,\mathrm{arcmin}$ . We measure the Galaxy-masked EE and BB spectra of diffuse synchrotron radiation and compare to space-based measurements at similar frequencies. In combination with external data, we expand measurements of the spatial variations of the synchrotron spectral energy density (SED) to include new sky regions and measure the diffuse SED in the harmonic domain. We place a new upper limit on a background of circular polarization in the range 5 < ℓ < 125 with the first bin showing D _ℓ < 0.023 $\mu {{\rm{K}}}_{\mathrm{CMB}}^{2}$ at 95% confidence. These results establish a new standard for recovery of the largest-scale CMB polarization from the ground and signal exciting possibilities when the higher sensitivity and higher-frequency CLASS channels are included in the analysis.

The dynamic atmosphere imposes challenges to ground-based cosmic microwave background observation, especially for measurements on large angular scales. The hydrometeors in the atmosphere, mostly in the form of clouds, scatter the ambient thermal radiation and are known to be the main linearly polarized source in the atmosphere. This scattering-induced polarization is significantly enhanced for ice clouds due to the alignment of ice crystals under gravity, which are also the most common clouds seen at the millimeter-astronomy sites at high altitudes. This work presents a multifrequency study of cloud polarization observed by the Cosmology Large Angular Scale Surveyor experiment on Cerro Toco in the Atacama Desert of northern Chile, from 2016–2022, at the frequency bands centered around 40, 90, 150, and 220 GHz. Using a machine-learning-assisted cloud classifier, we made connections between the transient polarized emission found in all four frequencies with the clouds imaged by monitoring cameras at the observing site. The polarization angles of the cloud events are found to be mostly 90° from the local meridian, which is consistent with the presence of horizontally aligned ice crystals. The 90 and 150 GHz polarization data are consistent with a power law with a spectral index of 3.90 ± 0.06, while an excess/deficit of polarization amplitude is found at 40/220 GHz compared with a Rayleigh scattering spectrum. These results are consistent with Rayleigh-scattering-dominated cloud polarization, with possible effects from supercooled water absorption and/or Mie scattering from a population of large cloud particles that contribute to the 220 GHz polarization.

The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background over 75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. This paper describes the CLASS data pipeline and maps for 40 GHz observations conducted from 2016 August to 2022 May. We demonstrate how well the CLASS survey strategy, with rapid (∼10 Hz) front-end modulation, recovers the large-scale Galactic polarization signal from the ground: the mapping transfer function recovers ∼67% (85%) of EE and BB ( VV ) power at ℓ = 20 and ∼35% (47%) at ℓ = 10. We present linear and circular polarization maps over 75% of the sky. Simulations based on the data imply the maps have a white noise level of $110\,\mu {\rm{K}}\,\mathrm{arcmin}$ and correlated noise component rising at low- ℓ as ℓ ^−2.4 . The transfer-function-corrected low- ℓ component is comparable to the white noise at the angular knee frequencies of ℓ ≈ 18 (linear polarization) and ℓ ≈ 12 (circular polarization). Finally, we present simulations of the level at which expected sources of systematic error bias the measurements, finding subpercent bias for the Λ cold dark matter EE power spectra. Bias from E -to- B leakage due to the data reduction pipeline and polarization angle uncertainty approaches the expected level for an r = 0.01 BB power spectrum. Improvements to the instrument calibration and the data pipeline will decrease this bias.

The Earth’s magnetic field induces Zeeman splitting of the magnetic dipole transitions of molecular oxygen in the atmosphere, which produces polarized emission in the millimeter-wave regime. This polarized emission is primarily circularly polarized and manifests as a foreground with a dipole-shaped sky pattern for polarization-sensitive ground-based cosmic microwave background experiments, such as the Cosmology Large Angular Scale Surveyor (CLASS), which is capable of measuring large angular scale circular polarization. Using atmospheric emission theory and radiative transfer formalisms, we model the expected amplitude and spatial distribution of this signal and evaluate the model for the CLASS observing site in the Atacama Desert of northern Chile. Then, using two years of observations at 32.°3 to 43.7 GHz from the CLASS Q-band telescope, we present a detection of this signal and compare the observed signal to that predicted by the model. We recover an angle between magnetic north and true north of −5.°5 ± 0.°6, which is consistent with the expectation of −5.°9 for the CLASS observing site. When comparing dipole sky patterns fit to both simulated and data-derived sky maps, the dipole directions match to within a degree, and the measured amplitudes match to within ∼20%. [ABSTRACT FROM AUTHOR]

The Cosmology Large Angular Scale Surveyor (CLASS) is mapping the polarization of the cosmic microwave background (CMB) at large angular scales (2 < ℓ ≲ 200) in search of a primordial gravitational wave B-mode signal down to a tensor-to-scalar ratio of r ≈ 0.01. The same data set will provide a near sample-variance-limited measurement of the optical depth to reionization. Between 2016 June and 2018 March, CLASS completed the largest ground-based Q-band CMB survey to date, covering over 31,000 square-degrees (75% of the sky), with an instantaneous array noise-equivalent temperature sensitivity of . We demonstrate that the detector optical loading (1.6 pW) and noise-equivalent power (19 ) match the expected noise model dominated by photon bunching noise. We derive a 13.1 ± 0.3 K pW−1 calibration to antenna temperature based on Moon observations, which translates to an optical efficiency of 0.48 ± 0.02 and a 27 K system noise temperature. Finally, we report a Tau A flux density of 308 ± 11 Jy at 38.4 ± 0.2 GHz, consistent with the Wilkinson Microwave Anisotropy Probe Tau A time-dependent spectral flux density model. [ABSTRACT FROM AUTHOR]

We analyze simulated maps of the Cosmology Large Angular Scale Surveyor (CLASS) experiment and recover a nearly cosmic variance limited estimate of the reionization optical depth τ. We use a power spectrum-based likelihood to simultaneously clean foregrounds and estimate cosmological parameters in multipole space. Using software specifically designed to constrain τ, the amplitude of scalar fluctuations As, and the tensor-to-scalar ratio r, we demonstrate that the CLASS experiment will be able to estimate τ within a factor of two of the cosmic variance limit allowed by full-sky cosmic microwave background polarization measurements. Additionally, we discuss the role of CLASS’s τ constraint in conjunction with gravitational lensing of the CMB on obtaining a ≳4σ measurement of the sum of the neutrino masses. [ABSTRACT FROM AUTHOR]