This book---unique in the literature---presents the basic elements of third-order aberrations of optical telescopes, along with the necessary mathematical background, to design almost all optical combinations used in astronomy. The book familiarizes readers with the mathematical aspects of optical design, without the need for tedious computer calculations. It also includes programs written using the Mathematica software package, which allow readers to obtain the third-order design of several optical configurations of astronomical telescopes in a few seconds. Replete with workout examples and exercises, Geometric Optics is an excellent reference for advanced graduate students, researchers, and practitioners in applied mathematics, engineering, astronomy, and astronomical optics. The work may be used as a text for a graduate-level course in astronomical optics, optical design, optical engineering, programming with Mathematica, or geometric optics.