On-sky performance of the CLASS Q-band telescope

The Cosmology Large Angular Scale Surveyor (CLASS) is mapping the polarization of the Cosmic Microwave Background (CMB) at large angular scales ($2<\mathrm{\ell }\lesssim 200$) in search of a primordial gravitational wave B-mode signal down to a tensor-to-scalar ratio of $r\approx 0.01$. The same data set will provide a near sample-variance-limited measurement of the optical depth to reionization. Between June 2016 and March 2018, CLASS completed the largest ground-based Q-band CMB survey to date, covering over 31 000~square-degrees (75% of the sky), with an instantaneous array noise-equivalent temperature (NET) sensitivity of $32\mu K_{cmb}\sqrt{s}$. We demonstrate that the detector optical loading ($1.6pW$) and noise-equivalent power ($19aW\sqrt{s}$) match the expected noise model dominated by photon bunching noise. We derive a $13.1\pm 0.3K\mathrm{/}pW$ calibration to antenna temperature based on Moon observations, which translates to an optical efficiency of $0.48\pm 0.04$ and a $27K$ system noise temperature. Finally, we report a Tau A flux density of $308\pm 11Jy$ at $38.4\pm 0.2GHz$, consistent with the WMAP Tau A time-dependent spectral flux density model.