High-Efficiency and Low-Noise Detectors for the Upgraded CLASS 90 GHz Focal Plane
Carolina Núñez, John W. Appel, Rahul Datta, Charles L. Bennett, Michael K. Brewer, Sarah Marie Bruno, Ricardo Bustos, David T. Chuss, Nick Costen, Jullianna Denes Couto, Sumit Dahal, Kevin L. Denis, Joseph R. Eimer, Thomas Essinger-Hileman, Jeffrey Iuliano, Yunyang Li, Tobias A. Marriage, Jennette Mateo, Matthew A. Petroff, Rui Shi, Karwan Rostem, Deniz A. N. Valle, Duncan Watts, Edward J. Wollack, Lingzhen Zeng
We present the in-lab and on-sky performance for the upgraded 90 GHz focal plane of the Cosmology Large Angular Scale Surveyor (CLASS), which had four of its seven detector wafers updated during the austral winter of 2022. The update aimed to improve the transition-edge-sensor (TES) stability and bias range and to realize the high optical efficiency of the sensor design. Modifications included revised circuit terminations, electrical contact between the TES superconductor and the normal metal providing the bulk of the bolometer’s heat capacity, and additional filtering on the TES bias lines. The upgrade was successful: 94% of detectors are stable down to 15% of the normal resistance, providing a wide overlapping range of bias voltages for all TESs on a wafer. The median telescope efficiency improved from 0.42+0.15−0.22 to 0.60+0.10−0.32 (68% quantiles). For the four upgraded wafers alone, median telescope efficiency increased to 0.65+0.06−0.06. Given our efficiency estimate for the receiver optics, this telescope efficiency implies a detector efficiency exceeding 0.90. The overall noise-equivalent temperature of the 90 GHz focal plane improved from 19 μKs√ to 11.3 μKs√.

Sensitivity-Improved Polarization Maps at 40 GHz with CLASS and WMAP data
Rui Shi, John W. Appel, Charles L. Bennett, Ricardo Bustos, David T. Chuss, Sumit Dahal, Jullianna Denes Couto, Joseph R. Eimer, Thomas Essinger-Hileman, Kathleen Harrington, Jeffrey Iuliano, Yunyang Li, Tobias A. Marriage, Matthew A. Petroff, Karwan Rostem, Zeya Song, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
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 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 WMAP Q band and the high-sensitivity information from 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 (βs) using WMAP K band and the combined maps (mean and 16/84th percentile across the βs map: −3.08+0.20−0.20) reveals a stronger preference for spatial variation (PTE for a uniform βs 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 (sub-percent level in the power spectra). The products of this work are publicly available on 𝙻𝙰𝙼𝙱𝙳𝙰.

CLASS Observations of Atmospheric Cloud Polarization at Millimeter Wavelengths
Yunyang Li, John W. Appel, Charles L. Bennett, Ricardo Bustos, David T. Chuss, Joseph Cleary, Jullianna Denes Couto, Sumit Dahal, Rahul Datta, Rolando Dünner, Joseph R. Eimer, Thomas Essinger-Hileman, Kathleen Harrington, Jeffrey Iuliano, Tobias A. Marriage, Matthew A. Petroff, Rodrigo A. Reeves, Karwan Rostem, Rui Shi, Deniz A. N. Valle, Duncan J. Watts, Oliver F. Wolff, Edward J. Wollack, Zhilei Xu
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 (CLASS) experiment on Cerro Toco in the Atacama Desert of northern Chile, from 2016 to 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.

CLASS Angular Power Spectra and Map-Component Analysis for 40 GHz Observations through 2022
Joseph R. Eimer, Yunyang Li, Michael K. Brewer, Rui Shi, Aamir Ali, John W. Appel, Charles L. Bennett, Ricardo Bustos, David T. Chuss, Joseph Cleary, Sumit Dahal, Rahul Datta, Jullianna Denes Couto, Kevin L. Denis, Rolando Dünner, Thomas Essinger-Hileman, Pedro Fluxá, Johannes Hubmayer, Kathleen Harrington, Jeffrey Iuliano, John Karakla, Tobias A. Marriage, Carolina Núñez, Lucas Parker, Matthew A. Petroff, Rodrigo A. Reeves, Karwan Rostem, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu, Lingzhen Zeng
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, τ, and search for the signature of inflation through the detection of primordial B-modes. We present an analysis of maps covering nearly 75% of the sky made from the ground-based 40GHz channel of the Cosmology Large Angular Scale Surveyor (CLASS) from August 2016 to May 2022. Using fast front-end polarization modulation from the Atacama Desert in Chile, we show this channel achieves higher sensitivity than the analogous frequencies from satellite measurements in the range 10<ℓ<100. After a final calibration adjustment, noise simulations show the CLASS linear (circular) polarization maps have a white noise level of 125(130)μKarcmin. 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 regions of the sky and measure the faint 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 Dℓ<0.023 μK2CMB 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.

Cosmology Large Angular Scale Surveyor (CLASS): 90 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
Rahul Datta, Michael K. Brewer, Jullianna Denes Couto, Joseph Eimer, Yunyang Li, Zhilei Xu, Aamir Ali, John W. Appel, Charles L. Bennett, Ricardo Bustos, David T. Chuss, Joseph Cleary, Sumit Dahal, Francisco Espinoza, Thomas Essinger-Hileman, Pedro Fluxá, Kathleen Harrington, Kyle Helson, Jeffrey Iuliano, John Karakla, Tobias A. Marriage, Sasha Novack, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Karwan Rostem, Rui Shi, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Lingzhen Zeng
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 90GHz telescope, which has been observing since July 2018. 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 arcminutes, respectively, over the first 3 years of observations. This corresponds to a pointing uncertainty ~7% of the beam’s full width at half maximum (FWHM). The effective azimuthally-symmetrized 1D beam estimated at 90 GHz from per detector intensity beam maps has a FWHM of 0.614+/-0.003 deg and a solid angle of 136.3+/-0.6(stats.)+/-1.1(sys.) usr integrated to a radius of 4 deg. The corresponding beam window function drops to b_ell^2 = 0.92, 0.70, 0.14 at ell = 30, 100, 300, respectively, with relative uncertainties < 2% for ell < 200. 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.) deg in equatorial coordinates consistent with prior measurements. Instrumental temperature-to-polarization (T-to-P) leakage is measured at a 95% confidence upper limit of (1.7+/-0.1) x 10^-3 in single detector demodulated data using observations of Jupiter and the Moon. Using pair-differenced demodulated data, a 95% confidence upper limit of 3.6 x 10^-4 is obtained on the T-to-P leakage.

CLASS Data Pipeline and Maps for 40 GHz Observations through 2022
Yunyang Li, Joseph Eimer, Keisuke Osumi, John Appel, Michael Brewer, Aamir Ali, Charles Bennett, Sarah Marie Bruno, Ricardo Bustos, David Chuss, Joseph Cleary, Jullianna Couto, Sumit Dahal, Rahul Datta, Kevin Denis, Rolando Dunner, Francisco Raul Espinoza Inostroza, Thomas Essinger-Hileman, Pedro Fluxa, Kathleen Harrington, Jeffrey Iuliano, John Karakla, Tobias Marriage, Nathan Miller, Sasha Novack, Carolina Núñez, Matthew Petroff, Rodrigo Reeves, Karwan Rostem, Rui Shi, Deniz Valle, Duncan Watts, J. Weiland, Edward Wollack, Zhilei Xu, Lingzhen Zeng
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 August 2016 to May 2022. We demonstrate how well the CLASS survey strategy, with rapid (10Hz) front-end modulation, recovers the large-scale Galactic polarization signal from the ground: the mapping transfer function recovers 75\% of EE, BB, and VV power at ℓ=20 and 45\% 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 110μKarcmin and correlated noise component rising at low-ℓ as ℓ−2.2. The transfer-function-corrected low-ℓ component is comparable to the white noise at the angular knee frequencies of ℓ≈16 (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 sub-percent bias for the ΛCDM 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.

Microwave Observations of Venus with CLASS
Sumit Dahal, Michael K. Brewer, Alex B. Akins, John W. Appel, Charles L. Bennett, Ricardo Bustos, Joseph Cleary, Jullianna D. Couto, Thomas Essinger-Hileman, Jeffrey Iuliano, Yunyang Li, Tobias A. Marriage, Carolina Núñez, Matthew A. Petroff, Rodrigo Reeves, Karwan Rostem, Rui Shi, Deniz A. N. Valle, Duncan J. Watts, Edward J. Wollack, Zhilei Xu
We report on the disk-averaged absolute brightness temperatures of Venus measured at four microwave frequency bands with the Cosmology Large Angular Scale Surveyor (CLASS). We measure temperatures of 432.3 ± 2.8 K, 355.6 ± 1.3 K, 317.9 ± 1.7 K, 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 (VLA) 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.

Testing CMB Anomalies in E-mode Polarization with Current and Future Data
Rui Shi, Tobias A. Marriage, John W. Appel, Charles L. Bennett, David T. Chuss, Joseph Cleary, Joseph Eimer, Sumit Dahal, Rahul Datta, Francisco Espinoza, Yunyang Li, Nathan J. Miller, Carolina Núñez, Ivan L. Padilla, Matthew A. Petroff, Deniz A. N. Valle, Edward J. Wollack, Zhilei Xu
In this paper, we explore the power of the cosmic microwave background (CMB) polarization (E-mode) data to corroborate four potential anomalies in CMB temperature data: the lack of large angular-scale correlations, the alignment of the quadrupole and octupole (Q-O), the point-parity asymmetry, and the hemispherical power asymmetry. We use CMB simulations with noise representative of three experiments — the Planck satellite, the Cosmology Large Angular Scale Surveyor (CLASS), and the LiteBIRD satellite — to test how current and future data constrain the anomalies. We find the correlation coefficients ρ between temperature and E-mode estimators to be less than 0.1, except for the point-parity asymmetry (ρ=0.17 for cosmic-variance-limited simulations), confirming that E-modes provide a check on the anomalies that is largely independent of temperature data. Compared to Planck component-separated CMB data (SMICA), the putative LiteBIRD survey would reduce errors on E-mode anomaly estimators by factors of ∼3 for hemispherical power asymmetry and point-parity asymmetry, and by ∼26 for lack of large-scale correlation. The improvement in Q-O alignment is not obvious due to large cosmic variance, but we found the ability to pin down the estimator value will be improved by a factor ≳100. Improvements with CLASS are intermediate to these.

Calibration of TES bolometer arrays with application to CLASS
John W. Appel, Charles L. Bennett, Michael K. Brewer, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna D. Couto, Sumit Dahal, Rahul Datta, Kevin Denis, Joseph Eimer, Thomas Essinger-Hileman, Kathleen Harrington, Jeffrey Iuliano, Yunyang Li, Tobias A. Marriage, Carolina Núñez, Keisuke Osumi, Ivan L. Padilla, Matthew A. Petroff, Karwan Rostem, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
Current and future cosmic microwave background (CMB) experiments fielding kilo-pixel 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 Ib. 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 (40GHz) 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 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 raw TES data and is not subject to I–V data quality or processing errors.

Four-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: On-sky Receiver Performance at 40, 90, 150, and 220 GHz Frequency Bands
Sumit Dahal, John W. Appel, Rahul Datta, Michael K. Brewer, Aamir Ali, Charles L. Bennett, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna D. Couto, Kevin L. Denis, Rolando Dünner, Joseph Eimer, Francisco Espinoza, Thomas Essinger-Hileman, Joseph E. Golec, Kathleen Harrington, Kyle Helson, Jeffrey Iuliano, John Karakla, Yunyang Li, Tobias A. Marriage, Jeffrey J. McMahon, Nathan J. Miller, Sasha Novack, Carolina Núñez, Keisuke Osumi, Ivan L. Padilla, Gonzalo A. Palma, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Gary Rhoades, Karwan Rostem, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
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 CLASS Q-band (40 GHz), W-band (90 GHz), and dichroic G-band (150/220 GHz) receivers that have been operational at the CLASS site in the Atacama desert since June 2016, May 2018, and September 2019, 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, 24, and 56 μKcmbs√ 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.

Two Year Cosmology Large Angular Scale Surveyor (CLASS) Observations: Long Timescale Stability Achieved with a Front-End Variable-delay Polarization Modulator at 40 GHz
Kathleen Harrington, Rahul Datta, Keisuke Osumi, Aamir Ali, John W. Appel, Charles L. Bennett, Michael K. Brewer, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Denes Couto, Sumit Dahal, Rolando Dünner, Joseph R. Eimer, Thomas Essinger-Hileman, Johannes Hubmayr, Francisco Raul Espinoza Inostroza, Jeffrey Iuliano, John Karakla, Yunyang Li, Tobias A. Marriage, Nathan J. Miller, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Matthew A. Petroff, Bastian Pradenas Márquez, Rodrigo Reeves, Pedro Fluxá Rojas, Karwan Rostem, Deniz Augusto Nunes Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
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 (VPM) 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 two years 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±12μKRJs√ for 1 mm of PWV when evaluated at 10 mHz; accounting for ∼32% of the 1/f noise in the central pixels of the focal plane. The low level of T→P leakage and 1/f noise achieved through the use of a front-end polarization modulator enables the observation of the largest scales of the CMB polarization from the ground by the CLASS telescopes.

Venus Observations at 40 and 90 GHz with CLASS
Sumit Dahal, Michael K. Brewer, John W. Appel, Aamir Ali, Charles L. Bennett, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna D. Couto, Rahul Datta, Kevin L. Denis, Joseph Eimer, Francisco Espinoza, Thomas Essinger-Hileman, Dominik Gothe, Kathleen Harrington, Jeffrey Iuliano, John Karakla, Tobias A. Marriage, Sasha Novack, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Gary Rhoades, Karwan Rostem, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
Using the Cosmology Large Angular Scale Surveyor (CLASS), we measure the disk-averaged absolute Venus brightness temperature to be 432.3 ± 2.8 K and 355.6 ± 1.3 K in the Q and W frequency 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 SO₂ and H₂SO₄.

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
Zhilei Xu, Michael K. Brewer, Pedro Fluxa, Yunyang Li, Keisuke Osumi, Bastian Pradenas, Aamir Ali, John W. Appel, Charles L. Bennett, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Couto, Sumit Dahal, Rahul Datta, Kevin Denis, Rolando Dunner, Joseph Eimer, Thomas Essinger-Hileman, Kathleen Harrington, Jeffrey Iuliano, Tobias A. Marriage, Nathan Miller, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Karwan Rostem, Duncan J. Watts, Janet Weiland, Edward J. Wollack
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 (1∘≲θ⩽90∘) CMB polarization to constrain the tensor-to-scalar ratio at the r∼0.01 level and the optical depth to last scattering to the sample variance limit. This paper presents the optical characterization of the 40 GHz telescope during its first observation era, from September 2016 to February 2018. High signal-to-noise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is <0.023∘ (<1.6% of the beam’s full width at half maximum (FWHM)). We estimate beam parameters per detector and in aggregate, as in the CMB survey maps. The aggregate beam has a FWHM of 1.579∘±.001∘ and a solid angle of 838±6 μsr, consistent with physical optics simulations. The corresponding beam window function has sub-percent error per multipole at ℓ<200. An extended 90∘ beam map reveals no significant far sidelobes. The observed Moon polarization shows that the instrument polarization angles are consistent with the optical model and that the temperature-to-polarization leakage fraction is <10−4 (95% C.L.). We find that the Moon-based results are consistent with measurements of M42, RCW 38, and Tau A from CLASS’s CMB survey data. In particular, Tau A measurements establish degree-level precision for instrument polarization angles.

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A First Detection of Atmospheric Circular Polarization at Q Band
Matthew A. Petroff, Joseph R. Eimer, Kathleen Harrington, Aamir Ali, John W. Appel, Charles L. Bennett, Michael K. Brewer, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Denes Couto, Sumit Dahal, Rolando Dünner, Thomas Essinger-Hileman, Pedro Fluxá Rojas, Jeffrey Iuliano, Tobias A. Marriage, Nathan J. Miller, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Rodrigo Reeves, Karwan Rostem, Deniz Augusto Nunes Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
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 (CMB) experiments, such as the Cosmology Large Angular Scale Surveyor (CLASS), which is uniquely 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 near 40 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%.

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz
Ivan L. Padilla, Joseph R. Eimer, Yunyang Li, Graeme E. Addison, Aamir Ali, John W. Appel, Charles L. Bennett, Ricardo Bustos, Michael K. Brewer, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Couto, Sumit Dahal, Kevin Denis, Rolando Dünner, Thomas Essinger-Hileman, Pedro Fluxá, Saianeesh K. Haridas, Kathleen Harrington, Jeffrey Iuliano, John Karakla, Tobias A. Marriage, Nathan J. Miller, Carolina Núñez, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Karwan Rostem, Robert W. Stevens, Deniz Augusto Nunes Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
We report circular polarization measurements from the first two years of observation with the 40 GHz polarimeter of the Cosmology Large Angular Scale Surveyor (CLASS). CLASS is conducting a multi-frequency survey covering 75% of the sky from the Atacama Desert designed to measure the cosmic microwave background (CMB) linear E and B polarization on angular scales 1∘≲θ≤90∘, corresponding to a multipole range of 2≤ℓ≲200. The modulation technology enabling measurements of linear polarization at the largest angular scales from the ground, the Variable-delay Polarization Modulator, is uniquely designed to provide explicit sensitivity to circular polarization (Stokes V). We present a first detection of circularly polarized atmospheric emission at 40 GHz that is well described by a dipole with an amplitude of 124±4μK when observed at an elevation of 45∘, and discuss its potential impact as a foreground to CMB experiments. Filtering the atmospheric component, CLASS places a 95% C.L. upper limit of 0.4μK2 to 13.5μK2 on ℓ(ℓ+1)CVVℓ/(2π) between 1≤ℓ≤120, representing a two-orders-of-magnitude improvement over previous limits.

The CLASS 150/220 GHz Polarimeter Array: Design, Assembly, and Characterization
Sumit Dahal, Mandana Amiri, John W. Appel, Charles L. Bennett, Lance Corbett, Rahul Datta, Kevin Denis, Thomas Essinger-Hileman, Mark Halpern, Kyle Helson, Gene Hilton, Johannes Hubmayr, Benjamin Keller, Tobias Marriage, Carolina Nunez, Matthew Petroff, Carl Reintsema, Karwan Rostem, Kongpop U-Yen, Edward Wollack
Video Link
We report on the development of a polarization-sensitive dichroic (150/220 GHz) detector array for the Cosmology Large Angular Scale Surveyor (CLASS) delivered to the telescope site in June 2019. In concert with existing 40 and 90 GHz telescopes, the 150/220 GHz telescope will make observations of the cosmic microwave background over large angular scales aimed at measuring the primordial B-mode signal, the optical depth to reionization, and other fundamental physics and cosmology. The 150/220 GHz focal plane array consists of three detector modules with 1020 transition edge sensor (TES) bolometers in total. Each dual-polarization pixel on the focal plane contains four bolometers to measure the two linear polarization states at 150 and 220 GHz. Light is coupled through a planar orthomode transducer (OMT) fed by a smooth-walled feedhorn array made from an aluminum-silicon alloy (CE7). In this work, we discuss the design, assembly, and in-lab characterization of the 150/220 GHz detector array. The detectors are photon-noise limited, and we estimate the total array noise-equivalent power (NEP) to be 2.5 and 4 aWs√ for 150 and 220 GHz arrays, respectively.
arXiv: 1908.00480

On-sky performance of the CLASS Q-band telescope
Appel, John W.; Xu, Zhilei; Padilla, Ivan L.; Harrington, Kathleen; Pradenas Marquez, Bastián et al.
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 dataset will provide a near sample-variance-limited measurement of the optical depth to reionization. Between June 2016 and March 2018, 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 (NET) sensitivity of 32 μKcmbs√. We demonstrate that the detector optical loading (1.6 pW) and noise-equivalent power (19 aWs√) match the expected noise model dominated by photon bunching noise. We derive a 13.1±0.3 K/pW calibration to antenna temperature based on Moon observations, which translates to an optical efficiency of 0.48±0.04 and a 27 K system noise temperature. Finally, we report a Tau A flux density of 315±19 Jy at 38.8±1.0 GHz, consistent with the WMAP Tau A time-dependent spectral flux density model.
arXiv:1811.08287

A 3D-printed broadband millimeter wave absorber  
Petroff, Matthew; Appel, John; Rostem, Karwan et al.
We present the design, manufacturing technique, and characterization of a 3D-printed broadband graded index millimeter wave absorber. The absorber is additively manufactured using a fused filament fabrication (FFF) 3D printer out of a carbon-loaded high impact polystyrene (HIPS) filament and is designed using a space-filling curve to optimize manufacturability using said process. The absorber’s reflectivity is measured from 63 GHz to 115 GHz and from 140 GHz to 215 GHz and is compared to electromagnetic simulations. The intended application is for terminating stray light in Cosmic Microwave Background (CMB) telescopes, and the absorber has been shown to survive cryogenic thermal cycling.
arXiv:1808.00820

A Projected Estimate of the Reionization Optical Depth Using the CLASS Experiment’s Sample-Variance Limited E-Mode Measurement
Watts, Duncan J.; Wang, Bingjie et al.
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 A s , 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.
arXiv:1801.01481

Cosmology Large Angular Scale Surveyor (CLASS) Focal Plane Development
Chuss, D. T. et al.
The Cosmology Large Angular Scale Surveyor (CLASS) will measure the polarization of the Cosmic Microwave Background to search for and characterize the polarized signature of inflation. CLASS will operate from the Atacama Desert and observe ~70% of the sky. A variable-delay polarization modulator provides modulation of the polarization at ~10 Hz to suppress the 1/ f noise of the atmosphere and enable the measurement of the large angular scale polarization modes. The measurement of the inflationary signal across angular scales that spans both the recombination and reionization features allows a test of the predicted shape of the polarized angular power spectra in addition to a measurement of the energy scale of inflation. CLASS is an array of telescopes covering frequencies of 38, 93, 148, and 217 GHz. These frequencies straddle the foreground minimum and thus allow the extraction of foregrounds from the primordial signal. Each focal plane contains feedhorn-coupled transition-edge sensors that simultaneously detect two orthogonal linear polarizations. The use of single-crystal silicon as the dielectric for the on-chip transmission lines enables both high efficiency and uniformity in fabrication. Integrated band definition has been implemented that both controls the bandpass of the single-mode transmission on the chip and prevents stray light from coupling to the detectors.
arXiv151104414C [astro-ph.CO] Accepted by the Journal of Low Temperature Physics (2015).

Fabrication of Feedhorn-Coupled Transition Edge Sensor Arrays for Measurement of the Cosmic Microwave Background Polarization
Kevin Denis et al.
Characterization of the minute cosmic microwave background polarization signature requires multi-frequency, high-throughput precision instrument systems. We have previously described the detector fabrication of a 40 GHz focal plane and now describe the fabrication of detector modules for measurement of the CMB at 90 GHz. The 90 GHz detectors are a scaled version of the 40 GHz architecture where, due to smaller size detectors, we have implemented a modular (wafer level) rather than the chip-level architecture. The new fabrication process utilizes the same design rules with the added challenge of increased wiring density to the 74 TES’s as well as a new wafer level hybridization procedure. The hexagonally shaped modules are tile-able, and as such can be used to form the large focal planes required for a space-based CMB polarimeter. The detectors described here will be deployed in two focal planes with seven modules each in the Johns Hopkins University led ground-based Cosmology Large Angular Scale Surveyor (CLASS) telescope.
arXiv151105036D [astro-ph.CO] Presented at Low Temperature Detectors Conference Grenoble France (2015).

Recovery of Large Angular Scale CMB Polarization for Instruments Employing Variable-delay Polarization Modulators
Nathan J. Miller; David T. Chuss; Tobias A. Marriage; Edward J. Wollack et al.
Variable-delay Polarization Modulators (VPMs) are currently being implemented in experiments designed to measure the polarization of the cosmic microwave background on large angular scales because of their capability for providing rapid, front-end polarization modulation and control over systematic errors. Despite the advantages provided by the VPM, it is important to identify and mitigate any time-varying effects that leak into the synchronously modulated component of the signal. In this paper, the effect of emission from a 300 K VPM on the system performance is considered and addressed. Though instrument design can greatly reduce the influence of modulated VPM emission, some residual modulated signal is expected. VPM emission is treated in the presence of rotational misalignments and temperature variation. Simulations of time-ordered data are used to evaluate the effect of these residual errors on the power spectrum. The analysis and modeling in this paper guides experimentalists on the critical aspects of observations using VPMs as front-end modulators. By implementing the characterizations and controls as described, front-end VPM modulation can be very powerful for mitigating 1/f noise in large angular scale polarimetric surveys. None of the systematic errors studied fundamentally limit the detection and characterization of B-modes on large scales for a tensor-to-scalar ratio of r = 0.01. Indeed, r < 0.01 is achievable with commensurately improved characterizations and controls.
arXiv1509.04628 [astro-ph.CO] Submitted to ApJ (2015).

Measuring the Largest Angular Scale CMB B-mode Polarization with Galactic Foregrounds on a Cut Sky
Duncan J. Watts, David Larson, Tobias A. Marriage et al.
We consider the effectiveness of foreground cleaning in the recovery of Cosmic Microwave Background (CMB) polarization sourced by gravitational waves for tensor-to-scalar ratios in the range 0<r<0.1. Using the planned survey area, frequency bands, and sensitivity of the Cosmology Large Angular Scale Surveyor (CLASS), we simulate maps of Stokes Q and U parameters at 40, 90, 150, and 220 GHz, including realistic models of the CMB, diffuse Galactic thermal dust and synchrotron foregrounds, and Gaussian white noise. We use linear combinations (LCs) of the simulated multifrequency data to obtain maximum likelihood estimates of r, the relative scalar amplitude s, and LC coefficients. We find that for 10,000 simulations of a CLASS-like experiment using only measurements of the reionization peak (ℓ≤23), there is a 95% C.L. upper limit of r<0.017 in the case of no primordial gravitational waves. For simulations with r=0.01, we recover at 68% C.L. r=0.012+0.011−0.006. The reionization peak corresponds to a fraction of the multipole moments probed by CLASS, and simulations including 30≤ℓ≤100 further improve our upper limits to r<0.008 at 95% C.L. (r=0.01+0.004−0.004 for primordial gravitational waves with r=0.01). In addition to decreasing the current upper bound on r by an order of magnitude, these foreground-cleaned low multipole data will achieve a cosmic variance limited measurement of the E-mode polarization’s reionization peak.
Astrophysical Journal, Vol. 814, 103 (2015).

Fabrication of a Silicon Backshort Assembly for Waveguide-Coupled Superconducting Detectors
Erik J. Crowe et al.
The Cosmology Large Angular Scale Surveyor (CLASS) is a ground-based instrument that will measure the polarization of the cosmic microwave background to search for evidence for gravitational waves from a posited epoch of inflation early in the Universe’s history. This measurement will require integration of superconducting transition-edge sensors with microwave waveguide inputs with excellent control of systematic errors, such as unwanted coupling to stray signals at frequencies outside of a precisely defined microwave band. To address these needs, we present work on the fabrication of micromachined silicon, producing conductive quarter-wave backshort assemblies for the CLASS 40 GHz focal plane. Each 40 GHz backshort assembly consists of three degeneratively doped silicon wafers. Two spacer wafers are micromachined with through-wafer vias to provide a 2.04-mm-long square waveguide delay section. The third wafer terminates the waveguide delay in a short. The three wafers are bonded at the wafer level by Au-Au thermal compression bonding then aligned and flip chip bonded to the CLASS detector at the chip level. The micromachining techniques used have been optimized to create high aspect ratio waveguides, silicon pillars, and relief trenches with the goal of providing improved out of band signal rejection. We will discuss the fabrication of integrated CLASS superconducting detector chips with the quarter-wave backshort assemblies.
IEEE Transactions on Applied Superconductivity, vol. 23 , Issue, 3 (2013).

Properties of a variable-delay polarization modulator
Chuss, David T.; Wollack, Edward J. et al.
We investigate the polarization modulation properties of a variable-delay polarization modulator (VPM). The VPM modulates polarization via a variable separation between a polarizing grid and a parallel mirror. We find that in the limit where the wavelength is much larger than the diameter of the metal wires that comprise the grid, the phase delay derived from the geometric separation between the mirror and the grid is sufficient to characterize the device. However, outside of this range, additional parameters describing the polarizing grid geometry must be included to fully characterize the modulator response. In this paper, we report test results of a VPM at wavelengths of 350 microns and 3 mm. Electromagnetic simulations of wire grid polarizers were performed and are summarized using a simple circuit model that incorporates the loss and polarization properties of the device.
Applied Optics, vol. 51, Issue 2, p. 197 (2012).

Electromagnetic Design of Feedhorn-Coupled Transition-Edge Sensors for Cosmic Microwave Background Polarimetry
Chuss, D. T. et al.
Observations of the cosmic microwave background (CMB) provide a powerful tool for probing the evolution of the early universe. Specifically, precision measurement of the polarization of the CMB enables a direct test for cosmic inflation. A key technological element on the path to the measurement of this faint signal is the capability to produce large format arrays of background-limited detectors. We describe the electromagnetic design of feedhorn-coupled, TES-based sensors. Each linear orthogonal polarization from the feedhorn is coupled to a superconducting microstrip line via a symmetric planar orthomode transducer (OMT). The symmetric OMT design allows for highly-symmetric beams with low cross-polarization over a wide bandwidth. In addition, this architecture enables a single microstrip filter to define the passband for each polarization. Care has been taken in the design to eliminate stray coupling paths to the absorbers. These detectors will be fielded in the Cosmology Large Angular Scale Surveyor (CLASS).
Journal of Low Temperature Physics, Volume 167, Issue 5-6, pp. 923-928 (2012).

Vector reflectometry in a beam waveguide
Eimer, J. R. et al.
We present a one-port calibration technique for characterization of beam waveguide components with a vector network analyzer. This technique involves using a set of known delays to separate the responses of the instrument and the device under test. We demonstrate this technique by measuring the reflected performance of a millimeter-wave variable-delay polarization modulator.
Review of Scientific Instruments, Volume 82, Issue 8, pp. 086101-086101-3 (2011).

A Low Cross-Polarization Smooth-Walled Horn With Improved Bandwidth
Zeng, Lingzhen et al.
Corrugated feed horns offer excellent beam symmetry, main beam efficiency, and cross-polar response over wide bandwidths, but can be challenging to fabricate. An easier-to-manufacture smooth-walled feed is explored that approximates these properties over a finite bandwidth. The design, optimization and measurement of a monotonically-profiled, smooth-walled scalar feedhorn with a diffraction-limited ~14° FWHM beam is presented. The feed was demonstrated to have low cross polarization (< -30 dB) across the frequency range 33-45 GHz (30% fractional bandwidth). A power reflection below -28 dB was measured across the band.
IEEE Transactions on Antennas and Propagation, vol. 58, Issue 4, pp. 1383-1387 (2010).