  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.
