In this paper, Hammar et al. demonstrate a new method of addressing low-order optical distortions in off-axis telescopes. The dominant aberration, linear astigmatism, present in two-mirror on-axis telescopes is widely known to be correctable with an additional mirror. This approach is successfully extended for an off-axis design through the selection of optical parameters satisfying the linear-astigmatism-free condition (i.e., identical tilt angles for the tangential and sagittal image planes) in a three-mirror telescope. This freedom allows the use of off-axis elements while maintaining overall image quality. The resulting optical system, similar to its on-axis counterpart, is free of linear astigmatism and provided an ideal starting point for the wide-field telescope implementation explored. The optical layout and baffling opportunities possible in the framework demonstrated suggest an interesting path toward improved telescope capabilities in a variety of optical settings.
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