The new generation of 8-10m telescopes is opening up important possibilities for polarimetry of astrophysically interesting sources, mainly because the large collecting area is particularly advantageous in this technique, which requires high S/N ratio. This course starts by emphasizing the importance of polarimetry in astronomy and giving some examples of polarizing phenomena in everyday life. Then an introduction to the Stokes parameters and to Mueller calculus is given, with examples on how to describe the most common polarizing optical components, and the main mechanisms producing polarized light in astrophysics are reviewed. The section devoted to instruments starts with a brief overview of the classical photopolarimeter, follows with a description of an imaging polarimeter, with examples of data obtained and an analysis of the sources of errors, and ends with a discussion of modern spectropolarimetry. The following section is devoted to an analysis of the gains of large 8–10 m telescopes for polarimetry and to a review of the polarimeters planned for them. The course ends with a discussion of polarimetry of AGN, as an example of a field of research, where polarimetry has provided important results, by disentangling unresolved geometries and mixed spectral components.

The beauty of polarimetry

Astronomy is an observational science, not an experimental one in the usual sense, since for the understanding of the objects in the Universe we cannot perform controlled experiments, but have to relay on observations of what these objects do, independently of us.