Introduction

Electromagnetic radiation from astronomical objects encounters different turbulent zones in its way to Earth-based telescopes. Interstellar turbulence, for instance, provokes phase fluctuations of radio waves which are exploited to study the interstellar medium, as presented by several authors in this volume. In the optical domain, wave perturbations occur in the Earth's atmosphere, due to turbulent fluctuations of the refractive-index of air (which is frequently referred to as optical turbulence). This has severe negative effects on astronomical observations as the commonly known “seeing” that strongly limits the achievable angular resolution (Roddier 1981). A number of high angular resolution techniques, like adaptive optics and interferometry, are being developed to overcome this limitation. They owe their good results yet obtained to the knowledge of the optical effects of atmospheric turbulence, and their optimization demands an increasingly high precision of that knowledge.