The primary distinction between an ordinary optical system and one which is both unobscured and reflective is that the elements of the latter must be tilted or decentered with respect to one another. In general, this results in an optical system which has no axis of rotational symmetry, and therefore the classical aberration theory of symmetric systems is no longer applicable. Furthermore, the image becomes anamorphic and keystone distorted, due to the relative tilt between the object and the optical surfaces. The first part of this work is the development of a semi-analytic treatment of the properties (through third order) of systems possessing large tilts and decentrations. The Gaussian properties of both the image and pupil are described in terms of tilt, decentration, magnification, keystone distortion, and anamorphic distortion parameters. In computing these parameters, it is important to take into account the transferred components of the parameters, which are due to the Gaussian properties of the preceding surfaces. The third order properties are computed using the fact that each optical surface, together with its associated entrance pupil, form an optical subsystem which possesses an axis of approximate symmetry. About this axis, the aberration contributions of that surface may be described in the classical wave aberration expansion. The coefficients in this expansion may be corrected for the non-circular appearance of both the object and pupil, using the parametric description of their Gaussian form. the aberration fields due to the various surface contributions are then combined vectorally to yield the resultant aberration field in the image plane. The vector theory is applied to the analysis of several optical systems, and the accuracy of the theory verified by comparison with raytrace data. As a demonstration of the usefulness of the theory to an optical designer, a three mirror unobscured system was designed using a methodology derived from the theory. At each step, the design options are explained, and the probable results are discussed. The residual aberrations of the resulting system are studied, and the selection of another design starting point is discussed from the point of view of the theory.
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