The evolution of an unobscured all-reflective zoom optical system for utilization in the infrared spectrum is presented. The objective is to develop a system which has a flat image surface, wide field- of-view, spatially remote entrance pupil, 2:1 zoom range, and is spatially compact The optical system comprises three aspheric mirrors sharing a common optical axis where the primary mirror is spatially fixed with respect to the entrance pupil, and secondary and tertiary mirrors move during zoom. The field-of-view ranges from 2.2 degs by 2.2 degs to 4.4 degs by 4.4 degs. The focal ratio varies from F/4 to F/8. The inherent characteristics of this type optical system are discussed, as are design methods to control aberrations, distortion and anamorphic error over the zoom range. The baseline design configuration is presented along with MTF performance data. The results of tolerance sensitivity analysis are also discussed.