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Register a new userTwo Year Cosmology Large Angular Scale Surveyor (CLASS) Observations: Long Timescale Stability Achieved with a Front-End Variable-delay Polarization Modulator at 40 GHz
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Kathleen Harrington
Publication date
2020
fields
Physics
and research's language is
English
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The Cosmology Large Angular Scale Surveyor (CLASS) is a four-telescope array observing the largest angular scales ($2 lesssim ell lesssim 200$) 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 ($Trightarrow P$) 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 $Trightarrow P$ leakage of $<3.8times10^{-4}$ (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 $203 pm 12 mathrm{mu K_{RJ}sqrt{s}}$ for 1 mm of PWV when evaluated at 10 mHz; accounting for $sim32%$ of the $1/f$ noise in the central pixels of the focal plane. The low level of $Trightarrow P$ 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.
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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 $1^circ lesssim theta leq 90^circ$, corresponding to a multipole range of $2 leq ell lesssim 200$. 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 $124pm4,mathrm{mu K}$ when observed at an elevation of $45^circ$, and discuss its potential impact as a foreground to CMB experiments. Filtering the atmospheric component, CLASS places a 95% C.L. upper limit of $0.4,mathrm{mu K}^2$ to $13.5,mathrm{mu K}^2$ on $ell(ell+1)C_ell^{VV}/(2pi)$ between $1 leq ell leq 120$, representing a two-orders-of-magnitude improvement over previous limits.
The Cosmology Large Angular Scale Surveyor (CLASS) instrument will measure the polarization of the cosmic microwave background at 40, 90, and 150 GHz from Cerro Toco in the Atacama desert of northern Chile. In this paper, we describe the optical design of the 40 GHz telescope system. The telescope is a diffraction limited catadioptric design consisting of a front-end Variable-delay Polarization Modulator (VPM), two ambient temperature mirrors, two cryogenic dielectric lenses, thermal blocking filters, and an array of 36 smooth-wall scalar feedhorn antennas. The feed horns guide the signal to antenna-coupled transition-edge sensor (TES) bolometers. Polarization diplexing and bandpass definition are handled on the same microchip as the TES. The feed horn beams are truncated with 10 dB edge taper by a 4 K Lyot-stop to limit detector loading from stray light and control the edge illumination of the front-end VPM. The field-of-view is 19deg x 14deg with a resolution for each beam on the sky of 1.5deg FWHM.
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 ($1^circlesssimthetaleqslant 90^circ$) CMB polarization to constrain the tensor-to-scalar ratio at the $rsim0.01$ 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 2016 September to 2018 February. High signal-to-noise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is $<0.023^circ$ ($<1.6$% of the beams 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 an FWHM of $1.579^circpm.001^circ$ and a solid angle of $838 pm 6 mu{rm sr}$, consistent with physical optics simulations. The corresponding beam window function has a sub-percent error per multipole at $ell < 200$. An extended $90^circ$ 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 CLASSs CMB survey data. In particular, Tau A measurements establish degree-level precision for instrument polarization angles.
The Cosmology Large Angular Scale Surveyor (CLASS) observes the polarized cosmic microwave background (CMB) over the angular scales of 1$^circ lesssim theta leq$ 90$^circ$ 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 $mathrm{mu K}_mathrm{cmb}sqrt{mathrm{s}}$ 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.
The Earths 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 GHz 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 $(-5.5 pm 0.6)^circ$, which is consistent with the expectation of $-5.9^circ$ 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 ${sim}20%$.
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