15° circular field unobscured telescope with spherical mirrors

There is a class of optical systems used in IR target simulators that might be called "reflective eyepieces." They are collimating systems with an external pupil. Typically, the light from an IR simulated target is collimated by the reflecting system and is projected into the external pupil. This can then be matched to the pupil, usually internal, of the optical system being tested—usually an IR seeker head on a missile. What is wanted is a reflecting system (for broad spectral coverage) that is unobscured, has a large field of view, a flat image, low distortion, and good image quality. This paper describes a new design that covers a 15° diameter circular unobscured field at f/4.5 It consists of five spherical mirrors and has a spot size of 0.1 mrad over the whole field.