The evolution of an unobscured all-reflective zoom optical system for utilization in the infrared spectrum is presented. The system is based upon earlier investigations by one of the authors with the objective of developing 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 degrees by 2.2 degrees to 4.4 degrees by 4.4 degrees. 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 resultant design and optical performance of the zoom optical system is presented and discussed.
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