An ultra-fast focal reducer design originally intended for use with spherical primary mirrors (published in Optical Engineering, February, 2003), has been adapted for use as a retro-fit for existing telescopes of ~2m pupil diameter that have aspheric mirrors. The focal reducer has only one aspheric surface, located at the aperture stop, and can provide speeds up to f/0.75 while maintaining a wide field with pixel-limited resolution and a broad spectral passband. Examples have been computed for Ritchie-Chretien, Dall-Kirkham and Cassegrain systems, and for use at the prime focus of mirrors with non-zero conic constants.
"Ultrafast focal reducer design for ~2-m-class telescopes", Proc. SPIE 5523, Current Developments in Lens Design and Optical Engineering V, (14 October 2004); doi: 10.1117/12.558945; https://doi.org/10.1117/12.558945