Textbook in PDF format Anyone who has looked through binoculars, a telescope or microscope has used an eyepiece. Telescope Eyepieces: Optical Theory and Design explores the wide range of eyepiece designs. It introduces optics theory progressively to build understanding of how lenses control light in an optical system, both generally and in eyepieces specifically, linking optics fundamentals, design evolution and the implications for image quality. This book presents a logical narrative starting with Snell’s law of refraction at a flat surface, progressing to paraxial and real rays at spherical and aspherical surfaces, lenses and thin-lens systems. It demystifies the origin of aberrations by considering wavefront deviations, all brought to life in the context of the familiar eyepiece. Principles are explored both descriptively and mathematically, and carefully interpreted so the reader is not swamped by a sea of equations. This book contains many diagrams of ray tracing results to illustrate optical principles and the consequences of design choices, enabling the reader to visualise their impact on image quality and to cut through the puffery that is sometimes found in the marketing of telescope eyepieces. It bridges the gap between introductory books and expert-level optical design texts, written jointly for an astronomy readership and for physics and optical-design students