In earlier chapters we treated light as either a pencil ray (for the purpose of geometrical raytrace and aberration analysis) or as surface area for the passage of radiating power (radiometry). In this chapter we discuss another property of light: its polarization state. Electromagnetic theory shows that a more complete description of light is provided when we consider light as an electromagnetic wave. This representation is necessary to describe, in detail, the phenomena of interference, diffraction, and image formation. Two representations of the electromagnetic wave are common: scalar and vector. In Chapter 9, we will discuss the propagation of scalar wavefronts. In this chapter we examine the vector nature of light and describe how an optical telescope and instrument system interact with a partially polarized beam.
Many astronomical optical sources are broadband thermal radiators; therefore, these radiation sources have no preferential polarization. Important physical processes within stars, planetary nebulae, interstellar matter, galaxies, the sun, planetary atmospheres, and planetary surfaces, to just name a few, are revealed only by using a measurement of the polarized-light content. Some examples are: (1) the strong electromagnetic fields present in the Crab Nebula that polarize radiation at its source, (2) the magnetic fields in the solar photosphere that are measured using pairs of spectral lines whose separation reveals magnetic field strength (the Zeeman effect), and (3) measuring the polarization content of distant stars to reveal the nature of magnetic fields in the interstellar medium.
All telescopes and instruments modify the state of polarization of the incoming radiation before measurement. Optical devices such as mirrors and lenses partially polarize the radiation as it traverses the optical system to the focal plane. If accurate radiometric intensities as a function of wavelength are required, we cannot ignore the polarization calibration of the instrument simply because the source is unpolarized. In many cases, the transmittivity of a telescope and instrument system are polarization dependent in addition to being wavelength dependent and need calibration before reporting astronomical measurements.