Detector Development and Polarization Analyses for the Atacama Cosmology Telescope

Abstract

We are currently in an era of precision cosmology. Study of the Cosmic Microwave Background (CMB) has been central to our understanding of the universe ever since its discovery in 1965. Recent results from the Planck satellite have provided the best constraints on the 6-parameter $\Lambda\mathrm{CDM}$ model, the ``standard model'' of cosmology. In order to improve these constraints we must further the development of the technologies used to measure the CMB as well as our understanding of their associated systematics. The Atacama Cosmology Telescope Polarimeter (ACTPol) and its successor instrument, known as Advanced ACTPol (AdvACT), are polarization sensitive upgrades to the Atacama Cosmology Telescope (ACT). ACT is an off-axis Gregorian telescope with a 6m primary reflector and a 2m secondary reflector. The ACTPol and AdvACT instruments utilize kilo-pixel scale arrays of superconducting transition edge sensor (TES) bolometers to measure the CMB anisotropies at frequencies ranging from 27-220 GHz. The increased spectral coverage of AdvACT will enable a wide range of CMB science, such as improving constraints on dark energy, the sum of the neutrino masses, and the existence of primordial gravitational waves. Precise polarization calibration can also enable improved constraints on cosmic polarization rotation. In this thesis we present an overview of the AdvACT instrument before detailing the development of the AdvACT TES device geometries, which are tuned specifically for each of AdvACT's five frequency bands (27, 39, 90, 150, and 220 GHz). Each detector couples to a single linear polarization. The angle of this coupling projected onto the sky is a critical calibration step in making maps of the CMB polarization. Any offset in this calibration angle can introduce a spurious B-mode polarization signal, resulting in non-zero EB and TB cross-correlation power spectra. ACTPol uses a unique optical modeling based approach to this calibration. We present this procedure, as well as a method for directly measuring the relative angles of the detectors by use of a rapidly rotating polarizer. We also present maps of the polarized source Tau A, from the first three seasons of ACTPol at 90 and 150 GHz and discuss how these maps are affected by the polarization calibration. Finally, we discuss the telescope control, computer systems, and remote observations team which keep the telescope running, and briefly conclude with a summary that motivates improving calibration techniques for future CMB experiments.