Two Year Cosmology Large Angular Scale Surveyor (CLASS) Observations: Long Timescale Stability Achieved with a Front-End Variable-delay Polarization Modulator at 40 GHz

TL;DR

The paper demonstrates that using a front-end variable-delay polarization modulator in the CLASS telescopes effectively stabilizes the instrument, enabling ground-based observation of large-scale CMB polarization with low noise and leakage over two years.

Contribution

This work develops and applies a demodulation scheme to extract polarization data, showing the stability and low noise performance of the VPM in long-term CMB observations.

Findings

Median knee frequency of 15.12 mHz for polarization data

T→P leakage below 3.8×10⁻⁴ (95% confidence)

Atmospheric PWV contributes significantly to 1/f noise

Abstract

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 ($T\rightarrow P$) 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 $T\rightarrow P$ leakage of $<3.8\times10^{-4}$ (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 $T\rightarrow P$ 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.