Almost every issue of the leading astronomical journals includes some polarimetry, either directly or indirectly. Polarimetry as a working tool has clearly come of age. Optical and radio techniques are most advanced, but infrared, sub-millimetre and ultraviolet are following on rapidly, while X-ray techniques are being developed also. There is no technical reason why astronomers should not use polarimetry when it suits their astronomical purposes; polarimetry often yields information that other methods of observation cannot give, and this is the main reason why all astronomers, and today's students in particular, should understand the basic ideas behind polarimetry.

Within the astronomical context, the degree of polarization is often low; a few per cent is typical, though both higher and (much) lower values occur. A polarimetric measurement is basically that of the ratio of the small difference between two signals to their sum. Difference and ratio methods have been devised to measure this small difference without systematic bias or drift errors, but photometric noise (detector noise or photon noise of the signal itself) is always present. To reduce this noise to the low level required for sufficiently accurate polarimetry, considerable observing time on a large telescope is generally needed. Polarimetry should therefore not be used indiscriminately, but only when it provides insight which other methods cannot give. Such judgment also requires a grasp of polarimetric basics.

This book aims to create an awareness of what polarimetry can do and at what price (in observing time, in complexity of equipment and of procedures).