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One method of correcting the aberrations of a mirror system is to add refractive corrector elements to the design. These elements provide the degrees of freedom required to achieve a corrected solution. An example of this approach is the Maksutov catadioptric system. This system consists of a primary mirror, in front of which is located a highly meniscus negative element. The meniscus element provides the compensation of the spherical aberration introduced by the spherical mirror. Many systems have been designed and constructed with more than one mirror and with multi-element corrector arrangements. The Maksutov system, consisting of a single spherical mirror and a single refractive element, is an example of the simplest form of correction using meniscus elements. In the fabrication of a well-corrected catadioptric system, which is required to perform near its theoretical limit, it is necessary to test the corrector elements to a high degree of accuracy. This paper describes the design of a special purpose tester for a particular refractive meniscus corrector element, the Strong Shell of the Perkin-Elmer Micralign Model 500 Projection Mask Alignment System. The Model 500 is a catadioptric system used by the semiconductor industry for optical microlithography on silicon wafers. A key factor in the optical design of the special purpose tester was to arrive at a solution that would allow all elements within their constructional tolerances for radius, thickness, and wedge to be evaluated with little further adjustment to the tester. This characteristic greatly accelerates testing and improves test data repeatability. This was accomplished by the selection of the testing conjugate point, a key factor in the overall design of the tester. A brief description of the Model 500 optical design, including the function of the strong shells is provided. Also provided is a description of the tester as well as the methods for alignment and certification.
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