Although the optical telescope is the most venerated instrument in astronomy, it developed relatively little between the time of Galileo and Newton and the beginning of the twentieth century. In contrast to the microscope, which enjoyed considerable conceptual development during the same period from the application of physical optics, telescopes suffered from atmospheric disturbances, and therefore physical optics was considered irrelevant to their design. The realization that wave interference could be employed to overcome the atmospheric resolution limit was first recorded by Fizeau and put into practice by Michelson around 1900, but his experience then lay dormant until the 1950s. Since then, first in radio astronomy and later in optical and infrared astronomy, interferometric methods have improved in leaps and bounds. Today, many optical interferometric observatories around the world are adding daily to our knowledge about the cosmos.

The aim of this book is to build on a basic knowledge of physical optics to describe the ideas behind the various interferometric techniques, the way in which they are being put into practice in the visible and the infrared regions of the spectrum, and how they can be projected into the future. Some techniques consist of optical additions to existing large telescopes; others require complete observatories which have been built specially for interferometry. Today all these are being used to make accurate measurements of stellar angular positions, to discern features on stellar surfaces and to study the structure of clusters and galaxies.