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A two-mirror laser beam expander can be used to provide small amounts of beam steering by moving one mirror. Such a motion would generally introduce aberrations into the output wavefront that would restrict the amount of diffraction-limited beam steering achievable to very small amounts. This paper describes how some or all of the third-order aberrations introduced by moving one mirror can be eliminated. By proper design, substantial amounts of diffraction-limited beam steering can be achieved.

A simple null lens is described that is moved back and forth relative to a point source to provide large amounts of variable wave front aberration. This device can be used to test a wide variety of optical components and systems.

A variety of four-mirror telescopes are described, all corrected for spherical aberration, coma, and astigmatism; all unobscured; and all with spherical surfaces. Performance is discussed with an afocal beam expander and a flat-field telescope as typical design examples.

A type of two-mirror telescope is described in which light leaving the secondary mirror is allowed to reflect off the primary mirror again before coming to the final focus. It is shown that this double bounce off the primary mirror results in a telescope that may be configured to be corrected for spherical aberration, coma, and astigmatism. In fact, there are four distinct families of designs that are anastigmatic, encompassing a wide variety of optical forms. The more interesting designs are described in detail, and some performance numbers are given.

The two-mirror Baker corrector for a parabolic telescope mirror is a well-known, very elegant design. In some variations, both corrector mirrors are spherical. The only thing marring the beauty of this solution to providing anastigmatic performance for large parabolic telescope mirrors is that the system is difficult to baffle. An alternate two-mirror corrector is presented here—based on a different design principle—that is easy to baffle. One of the two corrector mirrors is spherical, and there is a field stop to further aid in baffling. The penalty paid is that the image surface is curved, and the system is longer than the Baker designs.