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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 primordialB-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$125(130)\mu \mathrm{K}\mathrm{arcmin}$. We measure the Galaxy-maskedEEandBBspectra 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 showingD_ℓ< 0.023$\mu {\mathrm{K}}_{\mathrm{CMB}}^2$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.

 

We report on the disk-averaged absolute brightness temperatures of Venus measured at four microwave frequency bands with the Cosmology Large Angular Scale Surveyor. We measure temperatures of 432.3 ± 2.8, 355.6 ± 1.3, 317.9 ± 1.7, and 294.7 ± 1.9 K for frequency bands centered at 38.8, 93.7, 147.9, and 217.5 GHz, respectively. We do not observe any dependence of the measured brightness temperatures on solar illumination for all four frequency bands. A joint analysis of our measurements with lower-frequency Very Large Array observations suggests relatively warmer (∼7 K higher) mean atmospheric temperatures and lower abundances of microwave continuum absorbers than those inferred from prior radio occultation measurements.

 

Abstract The current and future cosmic microwave background (CMB) experiments fielding kilopixel arrays of transition-edge sensor (TES) bolometers require accurate and robust gain calibration methods. We simplify and refactor the standard TES model to directly relate the detector responsivity calibration and optical time constant to the measured TES current I and the applied bias current I b . The calibration method developed for the Cosmology Large Angular Scale Surveyor (CLASS) TES bolometer arrays relies on current versus voltage ( I – V ) measurements acquired daily prior to CMB observations. By binning Q -band (40 GHz) I – V measurements by optical loading, we find that the gain calibration median standard error within a bin is 0.3%. We test the accuracy of this I – V bin detector calibration method by using the Moon as a photometric standard. The ratio of measured Moon amplitudes between the detector pairs sharing the same feedhorn indicates a TES calibration error of 0.5%. We also find that, for the CLASS Q -band TES array, calibrating the response of individual detectors based solely on the applied TES bias current accurately corrects TES gain variations across time but introduces a bias in the TES calibration from data counts to power units. Since the TES current bias value is set and recorded before every observation, this calibration method can always be applied to the raw TES data and is not subject to I – V data quality or processing errors. 

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%) ofEEandBB(VV) power atℓ= 20 and ∼35% (47%) atℓ= 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$110\mu \mathrm{K}\mathrm{arcmin}$and correlated noise component rising at low-ℓasℓ^−2.4. The transfer-function-corrected low-ℓcomponent is comparable to the white noise at the angular knee frequencies ofℓ≈ 18 (linear polarization) andℓ≈ 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 matterEEpower spectra. Bias fromE-to-Bleakage due to the data reduction pipeline and polarization angle uncertainty approaches the expected level for anr= 0.01BBpower spectrum. Improvements to the instrument calibration and the data pipeline will decrease this bias.

 

Abstract The Cosmology Large Angular Scale Surveyor (CLASS) is a four-telescope array observing the largest angular scales (2≲ ℓ ≲ 200) of the cosmic microwave background (CMB) polarization. These scales encode information about reionization and inflation during the early universe. The instrument stability necessary to observe these angular scales from the ground is achieved through the use of a variable-delay polarization modulator as the first optical element in each of the CLASS telescopes. Here, we develop a demodulation scheme used to extract the polarization timestreams from the CLASS data and apply this method to selected data from the first 2 yr of observations by the 40 GHz CLASS telescope. These timestreams are used to measure the 1/ f noise and temperature-to-polarization ( T → P ) leakage present in the CLASS data. We find a median knee frequency for the pair-differenced demodulated linear polarization of 15.12 mHz and a T → P leakage of <3.8 × 10 −4 (95% confidence) across the focal plane. We examine the sources of 1/ f noise present in the data and find the component of 1/ f due to atmospheric precipitable water vapor (PWV) has an amplitude of 203 ± 12 μ K RJ s for 1 mm of PWV when evaluated at 10 mHz; accounting for ∼17% of the 1/ f noise in the central pixels of the focal plane. The low levels of T → P leakage and 1/ f noise achieved through the use of a front-end polarization modulator are requirements for observing of the largest angular scales of the CMB polarization by the CLASS telescopes. 

The Cosmology Large Angular Scale Surveyor (CLASS) observes the polarized cosmic microwave background (CMB) over the angular scales of 1° ≲θ≤ 90° with the aim of characterizing primordial gravitational waves and cosmic reionization. We report on the on-sky performance of the CLASSQ-band (40 GHz),W-band (90 GHz), and dichroicG-band (150/220 GHz) receivers that have been operational at the CLASS site in the Atacama desert since 2016 June, 2018 May, and 2019 September, respectively. We show that the noise-equivalent power measured by the detectors matches the expected noise model based on on-sky optical loading and lab-measured detector parameters. Using Moon, Venus, and Jupiter observations, we obtain power to antenna temperature calibrations and optical efficiencies for the telescopes. From the CMB survey data, we compute instantaneous array noise-equivalent-temperature sensitivities of 22, 19, 23, and 71$\mu {\mathrm{K}}_{\mathrm{cmb}}\sqrt{\mathrm{s}}$for the 40, 90, 150, and 220 GHz frequency bands, respectively. These noise temperatures refer to white noise amplitudes, which contribute to sky maps at all angular scales. Future papers will assess additional noise sources impacting larger angular scales.

 

Using the Cosmology Large Angular Scale Surveyor, we measure the disk-averaged absolute Venus brightness temperature to be 432.3 ± 2.8 K and 355.6 ± 1.3 K in theQandWfrequency bands centered at 38.8 and 93.7 GHz, respectively. At both frequency bands, these are the most precise measurements to date. Furthermore, we observe no phase dependence of the measured temperature in either band. Our measurements are consistent with a CO₂-dominant atmospheric model that includes trace amounts of additional absorbers like SO₂and H₂SO₄.