Long-timescale stability in CMB observations at multiple frequencies using front-end polarization modulation

Cleary, J. W.; Datta, Rahul

doi:10.1117/12.2629723

Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI 2022

DOI: 10.1117/12.2629723

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Long-timescale stability in CMB observations at multiple frequencies using front-end polarization modulation

J. W. Cleary¹,

Rahul Datta²

Abstract: The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array observing the Cosmic Microwave Background (CMB) at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the CMB polarization on the largest angular scales to constrain the inflationary tensor-to-scalar ratio and the optical depth due to reionization. To achieve the long time-scale stability necessary for this measurement from the ground, CLASS utilizes a front-end, variable-delay polarization modulator on each telescope. … Show more

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Cited by 3 publications

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CLASS Data Pipeline and Maps for 40 GHz Observations through 2022

Li 李,

Eimer,

Osumi

et al. 2023

ApJ

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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 μ K 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.

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CLASS Data Pipeline and Maps for 40 GHz Observations through 2022

Li 李,

Eimer,

Osumi

et al. 2023

ApJ

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Sensitivity-improved Polarization Maps at 40 GHz with CLASS and WMAP Data

Shi 时,

Appel,

Bennett

et al. 2024

ApJ

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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).

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

Datta,

Brewer,

Couto

et al. 2024

ApJS

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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 ℓ 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.

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Long-timescale stability in CMB observations at multiple frequencies using front-end polarization modulation

Cited by 3 publications

References 9 publications

CLASS Data Pipeline and Maps for 40 GHz Observations through 2022

CLASS Data Pipeline and Maps for 40 GHz Observations through 2022

Sensitivity-improved Polarization Maps at 40 GHz with CLASS and WMAP Data

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

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