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

Abstract

Context. The measure of the primordial B-mode signal in the Cosmic Microwave Background (CMB) represents the smoking gun of the cosmic inflation and it is the main goal of current experimental effort. The most accessible method to measure polarization features of the CMB radiation is by means of a Stokes Polarimeter based on the rotation of an Half Wave Plate. Aims. The current observational cosmology is starting to be limited by the presence of systematic effects. The Stokes polarimeter with a rotating Half Wave Plate (HWP) has the advantage of mitigating a long list of potential systematics, by modulation of the linearly polarized component of the radiation, but the presence of the rotating HWP can by itself introduce new systematic effects, which must be under control, representing one of the most critical part in the design of a B-Modes experiment. It is therefore mandatory to take into account all the systematic effects the instrumentation can induce. In this paper we present, simulate and analyse the spurious signal arising from the precession of a rotating HWP. Methods. We first find an analytical formula for the impact of the systematic effect induced by the HWP precession on the propagating radiation, using the 3D generalization of the Müller formalism. We then perform several numerical simulations, showing the effect induced on the Stokes parameters by this systematic. We also derive and discuss the impact into B-modes measured by a satellite experiment. Results. We find the analytical formula for the Stokes parameters from a Stokes polarimeter where the HWP follows a precessional motion with an angle θ 0 . We show the result depending on the HWP inertia tensor, spinning speed and on θ 0 . The result of numerical simulations is reported as a simple timeline of the electric fields. Finally, assuming to observe all the sky with a satellite mission, we analyze the effect on B-modes measurements.Conclusions. The effect is not negligible giving the current B-modes experiments sensitivity, therefore it is a systematic which needs to be carefully considered for future experiments.

Instrumental systematics biases in CMB lensing reconstruction: A simulation-based assessment

“…Consistent with the design of SO LAT, our instrument model does not simulate the effects of a half-wave plate (HWP). Although a HWP could help to mitigate instrumental systematics for polarization [30,31], especially if operated at cryogenic temperatures, the large-aperture telescopes typically used for lensing surveys do not normally use one as it is challenging to produce the large-sized plates required, and the HWP could also produce large unwanted systematics of its own [32].…”

QUBIC VI: cryogenic half wave plate rotator, design and performances

“…The details are reported in the following subsection. The HWP orientation with respect to the optical axis has been measured during the rotation with a Mitutoyo micrometer position sensor, in order to avoid mechanical wobbling of the system and the systematic effect that goes with it [26]. The planarity of the HWP support has been measured as well to separate the two effects.…”

A numerical study of observational systematic errors in lensing analysis of CMB polarization

“…gain error, angle error, and pointing error, the corrections of which still require conventional calibration procedures. Intensity (temperature) to polarization leakage, which was one of the most significant systematic errors, is avoided in principle by measurement with a polarization modulator [56] though other types of systematic errors associated with the apparatus itself may arise instead [57,58,59]. Apart from them, also beam related mis-measurements such as elliptic deformation or sidelobe elongation are major sources of systematic errors.…”