Two forms of reflective zoom systems have been described in the literature. Rah & Lee (1989) and Johnson et alia (1990) show rotationally symmetric systems comprising a Cassegrain imager, followed by an inverse Cassegrain relay. This form suffers from large values of central obscuration at some zoom positions and uses independent motion of all four mirrors to achieve a stationary image. These systems employ spherical and simple conic surfaces to achieve appropriate aberration correction. Earlier Woehl (1981) described an all reflective zoom relay system, using multiple moving off-axis mirrors for use in optical beam shaping applications. The systems described here are combinations of the two types, having a fixed conventional Cassegrain imager, used at a small off-set field angle, to allow the re-imaging ray beams to pass without further obscuration through a set of off-axis zooming mirrors, to a final image. The relay mirror curvatures and separations are chosen to achieve the required zoom ratio and fixed image location, and appropriate tilts and decentrations may be impressed upon the relay elements to permit unhindered ray passage.
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