Diffraction

Abstract

This chapter introduces the subject of diffraction—the key mechanism that determines how light propagates through the atmosphere and comes to a final focus in the telescope image plane. The origin and basis of the Fresnel-Kirchhoff diffraction formula is described; this formula derives directly from Maxwell’s equations. Solutions to the formula are given in three domains, all of which are used in the subsequent light propagation analysis: the geometrical optics region, the near-field Fresnel region and the far-field Fraunhofer region. The distribution of light energy in the Fraunhofer region describes the final image formed by the telescope. Optical system terminologies used to describe optical systems—telescopes in particular—are introduced (e.g., optical axis, telescope objective, central obstruction and telescope pupil function). Ray terminologies are also introduced (e.g., principal ray, marginal ray). The amplitude and intensity point-spread functions of telescopes are defined. Linear superposition, convolution, isoplanaticity, and coherence are described, for use in dealing with extended objects.