Analytical design procedure for a flat-field three-mirror afocal telescope

The three-mirror afocal telescope, known as an Offner-afocal telescope, consists of a two-mirror Cassegrain telescope and a paraboloid mirror confocal with it. This configuration produces an aberration-free output beam. This unique characteristic makes it suitable for a wide variety of applications. However, the type still suffers from the Petzval field curvature. This is an important consideration, particularly for applications with the afocal telescope being used as a feeder for a focuser telescope, since a collimated beam having field curvature puts constraints on the design of the focuser telescope. Such a configuration has several unique characteristics that are particularly useful for its application in astronomical telescopes. In this paper, we present an analytical method for the design of an Offner afocal telescope having a flat-field, with only four known mechanical parameters as the starting point of the design. While the theoretical solution is straightforward, our main focus in this paper is to determine the feasibility of the practical implementation of such a concept. The complete design procedure is discussed and analyzed in detail. The approach is then validated using two optical design examples for space telescopes operating in the mid-wavelength to far-IR waveband.