A Wide-Field Corrector at the Prime Focus of a Ritchey-Chretien   Telescope
  We propose a form of a lens corrector at the prime focus of a hyperboloidal mirror that provides a flat field of view up to 3 deg diameter at image quality D_{80} < 0.8 arcsec in integrated (0.32-1.10 mcm) light. The corrector consists of five lenses made of fused silica. Only spherical surfaces are used, so the system is capable of achieving better images, if necessary, by aspherizing the surfaces. The optical system of the corrector is stable in the sense that its principal features are retained when optimized after significant perturbations of its parameters. As an example, three versions of the corrector are designed for the V.M.Blanco 4-m telescope at Cerro Tololo Inter-American Observatory with 2.12 deg, 2.4 deg, and 3.0 deg fields of view. 
V.Yu.Terebizh