Performance of a continuously rotating half-wave plate on the POLARBEAR telescope

Abstract

A continuously rotating half-wave plate (CRHWP) is a promising tool to improve the sensitivity to large angular scales in cosmic microwave background (CMB) polarization measurements. With a CRHWP, single detectors can measure three of the Stokes parameters, I, Q and U , thereby avoiding the set of systematic errors that can be introduced by mismatches in the properties of orthogonal detector pairs. We focus on the implementation of CRHWPs in large aperture telescopes (i.e. the primary mirror is larger than the current maximum half-wave plate diameter of ∼ 0.5 m), where the CRHWP can be placed between the primary mirror and focal plane. In this configuration, one needs to address the intensity to polarization (I→P ) leakage of the optics, which becomes a source of 1/f noise and also causes differential gain systematics that arise from CMB temperature fluctuations. In this paper, we present the performance of a CRHWP installed in the Polarbear experiment, which employs a Gregorian telescope with a 2.5 m primary illumination pattern. The CRHWP is placed near the prime focus between the primary and secondary mirrors. We find that the I→P leakage is larger than the expectation from the physical properties of our primary mirror, resulting in a 1/f knee of 100 mHz. The excess leakage could be due to imperfections in the detector system, i.e. detector non-linearity in the responsivity and time-constant. We demonstrate, however, that by subtracting the leakage correlated with the intensity signal, the 1/f noise knee frequency is reduced to 32 mHz ( ∼ 39 for our scan strategy), which is very promising to probe the primordial B-mode signal. We also discuss methods for further noise subtraction in future projects where the precise temperature control of instrumental components and the leakage reduction will play a key role.

Likelihood Methods for CMB Experiments

“…To reduce the contamination from atmospheric emission, ground-based CMB experiments are usually placed in specific sites, at high altitudes (to reduce the thickness of the atmosphere above the telescope) and dry locations. Residual contamination can be further removed either by detector pair-differencing (see e.g., [11]) or by modulation of the signal (see e.g., [90]). Balloon-borne and especially satellite missions are less or not at all concerned with atmospheric contaminations.…”

Systematic effects induced by half-wave plate precession into measurements of the cosmic microwave background polarization

“…The LSPE collaboration et al (2012); Columbro et al (2019); The EBEX collaboration et al (2018); Thornton et al (2016) in order to modulate the signal at high frequency and it can also rotate step by step (< 1Hz) (Piat et al 2012;Bryan et al 2016) depending on the experiment scanning strategy. Systematic effects like T-P leakage, chromatic HWP behaviour, scanning strategy, are already evaluated in Essinger-Hileman et al (2016); Salatino et al (2011); Takakura et al (2017); Salatino et al (2018), and also measured by Kusaka et al (2014a).…”

A Large-Diameter Cryogenic Rotation Stage for Half-Wave Plate Polarization Modulation on the POLARBEAR-2 Experiment

“…Particularly, the "B-mode" CMB polarization pattern can be used to probe gravitational lensing on arcminute angular scales [1, 2] and primordial gravitational waves on degree angular scales [3][4][5]. In order for a single telescope to characterize small and large scales simultaneously, it must observe with high resolution over a large sky area, requiring both a large primary aperture and good low-frequency (or "1/f") noise performance [6].Rapidly-rotating half-wave plates (HWP) are a common technique to modulate CMB polarization and reduce the impact of low-frequency noise on experiment sensitivity [6][7][8][9][10][11][12].…”

“…Rapidly-rotating half-wave plates (HWP) are a common technique to modulate CMB polarization and reduce the impact of low-frequency noise on experiment sensitivity [6][7][8][9][10][11][12].…”

“…Additionally, a HWP's birefringent substrate rotates the incident polarization vector, allowing a single polarimeter to measure both polarization states, therefore mitigating systematic effects that arise when differencing orthogonal detectors [8,13,14]. HWPs have been adopted as continuous modulators on multiple large-aperture telescopes, including POLARBEAR [6,15], POLARBEAR-2a, [11,16], and the Atacama Cosmology Telescope [10,17].…”