In this chapter, instrumental principles will be discussed, with emphasis on system behaviour and without any preconceptions about the wavelength at which one observes. Practical illustrations will inevitably relate to a particular wavelength region (optical and radio, which is where the experience resides). It may therefore be necessary to scan chapter 6 before attempting to understand the present chapter in detail.

Telescopes

The first optical element of an astronomical observing system is always a telescope (disregarding the atmosphere for the present discussion). It is important to realize that, in general, a telescope will modify the polarization of the radiation before the polarimeter measures it. It is equally important to have some general feeling for the conditions under which such modification is likely to be appreciable and how it can be minimized.

The guiding principle is symmetry; any departures from full symmetry will modify the polarization. The considerations below illustrate this, but full understanding will require mathematical treatment by Mueller or Jones calculus, with optical constants applicable to the wavelength of interest.

Oblique incidence on a mirror produces both diattenuation (polarizing action) and retardation (wave plate action). These effects are minimal at near-normal and, somewhat surprisingly, at grazing incidence; the largest effects occur at intermediate angles of incidence, the details depending on the values of the real and imaginary parts of the refractive index (which in their turn depend on the wavelength).