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The Cosmology Large Angular Scale Surveyor (CLASS) is a four-telescope arrayobserving the largest angular scales ($2 \lesssim \ell \lesssim 200$) of thecosmic microwave background (CMB) polarization. These scales encode informationabout reionization and inflation during the early universe. The instrumentstability necessary to observe these angular scales from the ground is achievedthrough the use of a variable-delay polarization modulator (VPM) as the firstoptical element in each of the CLASS telescopes. Here we develop a demodulationscheme used to extract the polarization timestreams from the CLASS data andapply this method to selected data from the first two years of observations bythe 40 GHz CLASS telescope. These timestreams are used to measure the $1/f$noise and temperature-to-polarization ($T\rightarrow P$) leakage present in theCLASS data. We find a median knee frequency for the pair-differenceddemodulated 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 thesources of $1/f$ noise present in the data and find the component of $1/f$ dueto 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 focalplane. The low level of $T\rightarrow P$ leakage and $1/f$ noise achievedthrough the use of a front-end polarization modulator enables the observationof the largest scales of the CMB polarization from the ground by the CLASStelescopes.

astrophysical journal/the astrophysical journal

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