title: Four-mirror unobscured anastigmatic telescopes with all-spherical surfaces.
abstract: A variety of four-mirror telescopes are described, all corrected for spherical aberration, coma, and astigmatism; all unobscured; and all with spherical surfaces. Performance is discussed with an afocal beam expander and a flat-field telescope as typical design examples.
authors.forename: D R
authors.initials: DR
authors.lastname: Shafer