The Spitzer Space Telescope, which has operated very successfully since 2003 in its unique Earth-trailing solar orbit, is NASA's Great Observatory for infrared astronomy. We provide a quick overview of the optical characteristics of Spitzer and review the observatory design. The main emphasis is on two unique on-orbit activities used to optimize the scientific return from Spitzer: 1. an unusual approach to focusing the telescope that minimized the use of the cryogenic focus mechanism, and 2. a methodology for extending the cryogenic lifetime of Spitzer by actively controlling the telescope temperature.