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7.1 Obscured Systems
7.1.1 Korea Advanced Institute of Science and Technology
A 2:1 axially symmetric zoom reflective system with all spherical surfaces was designed by S. Y. Rah and S. S. Lee to satisfy the requirements listed in Table 7.1. In a four-mirror zoom system, for a given set of mirror curvatures, there are three independent parameters: the distances between the mirrors. A condition imposed upon the design is that the system continually satisfies the aplanatic condition while it is zooming. In an aplanatic system spherical aberration and coma are zero. The optical designers made use of the concept in third-order aberration theory that for an aplanatic system the shift of the aperture stop does not affect the aberration coefficients for spherical aberration, coma, and astigmatism. The stop was placed at the last surface of the system for ease of control of illumination of the image point. Although several reflective configurations were examined, the Cassegrain-Cassegrain combination was selected as the most appropriate for this system since it was found to have the smallest image spot size throughout the zoom range. The optical schematic is shown in Fig. 7.1. Astigmatism limits the field angle of the system to less than 1.0 deg. A conic surface such as an ellipsoid for the primary surface has been found to be effective in reducing astigmatism throughout the zoom range.
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