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1. Four-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: On-sky Receiver Performance at 40, 90, 150, and 220 GHz Frequency Bands

   https://doi.org/10.3847/1538-4357/ac397c  

   Dahal, Sumit; Appel, John W.; Datta, Rahul; Brewer, Michael K.; Ali, Aamir; Bennett, Charles L.; Bustos, Ricardo; Chan, Manwei; Chuss, David T.; Cleary, Joseph; et al (February 2022, The Astrophysical Journal)

   Abstract 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. 

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2. Cosmology Large Angular Scale Surveyor (CLASS): 90 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance

   https://doi.org/10.3847/1538-4365/ad50a0  

   Datta, Rahul; Brewer, Michael_K; Couto, Jullianna_Denes; Eimer, Joseph; Li_李, Yunyang_云炀; Xu_徐, Zhilei_智磊; Ali, Aamir; Appel, John_W; Bennett, Charles_L; Bustos, Ricardo; et al (July 2024, The Astrophysical Journal Supplement Series)

   Abstract 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.)μsr integrated to a radius of 4°. The corresponding beam window function drops to $b_{\mathit{\ell }}^2=0.93,0.71,0.14$atℓ= 30, 100, 300, respectively. Far-sidelobes are studied using detector-centered intensity maps of the Moon and measured to be at a level of 10⁻³or 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. 

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3. Design and characterization of new 90 GHz detectors for the Cosmology Large Angular Scale Surveyor (CLASS)

   https://doi.org/10.1117/12.2630081  

   Nunez, Carolina; Appel, John W.; Bruno, Sarah Marie; Datta, Rahul; Ali, Aamir; Bennett, Charles L.; Dahal, Sumit; Denes Couto, Jullianna; Denis, Kevin; Eimer, Joseph; et al (August 2022, Proceedings Volume 12190, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI)

   Zmuidzinas, Jonas; Gao, Jian-Rong (Ed.)

   The Cosmology Large Angular Scale Surveyor (CLASS) is a polarization-sensitive telescope array located at an altitude of 5,200 m in the Chilean Atacama Desert. CLASS is designed to measure "E-mode" (even parity) and "B-mode" (odd parity) polarization patterns in the Cosmic Microwave Background (CMB) over large angular scales with the aim of improving our understanding of inflation, reionization, and dark matter. CLASS is currently observing with three telescopes covering four frequency bands: one at 40 GHz (Q); one at 90 GHz (W1); and one dichroic system at 150/220 GHz (G). In these proceedings, we discuss the updated design and in-lab characterization of new 90 GHz detectors. The new detectors include design changes to the transition-edge sensor (TES) bolometer architecture, which aim to improve stability and optical efficiency. We assembled and tested four new detector wafers, to replace four modules of the W1 focal plane. These detectors were installed into the W1 telescope, and will achieve first light in the austral winter of 2022. We present electrothermal parameters and bandpass measurements from in-lab dark and optical testing. From in-lab dark tests, we also measure a median NEP of 12.3 aW√ s across all four wafers about the CLASS signal band, which is below the expected photon NEP of 32 aW√ s from the field. We therefore expect the new detectors to be photon noise limited. 

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4. Improved polarization calibration of the BICEP3 CMB polarimeter at the South Pole

   https://doi.org/10.1117/12.2620212  

   Cornelison, James; Verges, Clara; Ade, P A.; Ahmed, Zeeshan; Amiri, Mandana; Barkats, Denis; Basu Thakur, Ritoban; Beck, Dominic; Bischoff, Colin A.; Bock, James J.; et al (August 2022, Proceedings of the SPIE)

   Zmuidzinas, Jonas; Gao, Jian-Rong (Ed.)

   The BICEP3 Polarimeter is a small aperture, refracting telescope, dedicated to the observation of the Cosmic Microwave Background (CMB) at 95GHz. It is designed to target degree angular scale polarization patterns, in particular the very-much-sought-after primordial B-mode signal, which is a unique signature of cosmic inflation. The polarized signal from the sky is reconstructed by differencing co-localized, orthogonally polarized superconducting Transition Edge Sensor (TES) bolometers. In this work, we present absolute measurements of the polarization response of the detectors for more than approximately 800 functioning detector pairs of the BICEP3 experiment, out of a total of approximately 1000. We use a specifically designed Rotating Polarized Source (RPS) to measure the polarization response at multiple source and telescope boresight rotation angles, to fully map the response over 360 degrees. We present here polarization properties extracted from on-site calibration data taken in January 2022. A similar calibration campaign was performed in 2018, but we found that our constraint was dominated by systematics on the level of approximately 0.5° . After a number of improvements to the calibration set-up, we are now able to report a significantly lower level of systematic contamination. In the future, such precise measurements will be used to constrain physics beyond the standard cosmological model, namely cosmic birefringence. 

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5. Cosmology Large Angular Scale Surveyor (CLASS): pointing stability and beam measurements at 90, 150, and 220 GHz

   https://doi.org/10.1117/12.2630649  

   Datta, Rahul; Brewer, Michael K.; Denes Couto, Jullianna D.; Eimer, Joseph R.; Li, Yunyang; Xu, Zhilei; Appel, John W.; Bustos, Ricardo; Chuss, David; Cleary, Joseph; et al (August 2022, Proc. SPIE 12190, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI)

   Zmuidzinas, Jonas; Gao, Jian-Rong (Ed.)

   The Cosmology Large Angular Scale Surveyor (CLASS) telescope array surveys 75% of the sky from the Atacama desert in Chile at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the largest-angular scale (θ ≳ 1 ° ) CMB polarization with the aim of constraining the tensor-to-scalar ratio, r, measuring the optical depth to reionization, τ , to near the cosmic variance limit, and more. The CLASS Q-band (40 GHz), W-band (90 GHz), and dichroic high frequency (150/220 GHz) telescopes have been observing since June 2016, May 2018, and September 2019, respectively. On-sky optical characterization of the 40 GHz instrument has been published. Here, we present preliminary on-sky measurements of the beams at 90, 150, and 220 GHz, and pointing stability of the 90 and 150/220 GHz telescopes. The average 90, 150, and 220 GHz beams measured from dedicated observations of Jupiter have full width at half maximum (FWHM) of 0.615±0.019° , 0.378±0.005° , and 0.266 ± 0.008° , respectively. Telescope pointing variations are within a few % of the beam FWHM. 

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