The Measurement of Polarization in Radio Astronomy

TL;DR

This paper discusses the complexities of measuring polarization in radio astronomy, emphasizing the importance of calibration, understanding instrumental effects, and standardizing conventions for accurate polarization observations.

Contribution

It clarifies the role of the entire radio telescope system in polarization measurement and provides guidance on calibration and reporting standards.

Findings

Instrumental components can significantly alter polarization signals.

Calibration processes are essential for accurate polarization measurement.

Standardized polarization conventions are necessary for consistent reporting.

Abstract

Modern dual-polarization receivers allow a radio telescope to characterize the full polarization state of incoming insterstellar radio waves. Many astronomers incorrectly consider a polarimeter to be the "backend" of the telescope. We go to lengths to dissuade the reader of this concept: the backend is the least complicated component of the radio telescope when it comes to measuring polarization. The feed, telescope structure, dish surface, coaxial cables, optical fibers, and electronics can each alter the polarization state of the received astronomical signal. We begin with an overview of polarized radiation, introducing Jones and Stokes vectors, and then discuss construction of digitized pseudo-Stokes vectors from the outputs of modern correlators. We describe the measurement and calibration process for polarization observations and illustrate how instrumental polarization can affect a measurement. Finally, we draw attention to the confusion generated by various polarization conventions and highlight the need for observers to state all adopted conventions when reporting polarization results.