Wide-field Corrector for a Gregory Telescope
  A form of prime focus corrector for the Gregory system is proposed that provides the sub-arcsecond field of view up to 3 degrees in diameter for the spectral range 0.35-0.90 microns. The corrector includes five lenses made of same glass (fused silica is preferable). The distinctive feature of the corrector consists in dissimilar use of the central and edge zones of a front lens disposed in the exit pupil of a two-mirror system.   As an example, the f/1.9 telescope is considered with the 6.5-m aperture and the total length 8.8~m. Its primary and secondary mirrors are pure ellipsoids close to concave paraboloid and concave sphere, respectively. In the basic configuration, all surfaces of the corrector are spherical. The diameter of a star image D_{80} varies from 0''.25 on the optical axis up to 0''.50 at the edge of the 2.3-deg field. Only slightly worse images shows spherical corrector for the 2.4-deg field of view. The fraction of vignetted rays grows on 1.7% from the center of field to its edges. Aspherization of some lens surfaces allows to reach sub-arcsecond images in the field of 3.0 in diameter. 
V.Yu.Terebizh