This paper presents three optical designs based on the work of Maurice Paul. Paul's three-mirror anastigmats produce well-corrected, distortion-free fields of view. His design equations can be solved for a spherical primary mirror with one limitation: the image field is curved. Adding all-spherical refractive field-flattening optics yields well-corrected, flat image-fields of two degrees angular diameter or more. These designs can be scaled to very large telescopes with current technology.
Dan Blanco, Dan Blanco,
"Three wide-field telescopes with spherical primary mirrors", Proc. SPIE 9145, Ground-based and Airborne Telescopes V, 91453P (22 July 2014); doi: 10.1117/12.2056683; https://doi.org/10.1117/12.2056683