Pancake ELT: a practical design for an Extremely Large Telescope

In this paper we present optical designs for an entirely new approach to extremely large telescopes and telescopes with spherical primary mirrors. The key feature is a novel optical system referred to as the improved spherical aberration corrector (ISAC). This corrector works exceptionally well for post prime focus applications, as well as for Cassegrain and Couder/Schwarzschild-like optical systems with both spherical primary and spherical secondary mirrors. The Couder/Schwarzschild configuration also adapts to a pancake configuration where the telescope is physically shorter than its aperture.