Optical design concept of the 4-m visible and infrared survey telescope for astronomy

This paper describes the optical design of VISTA (Visible and Infrared Survey Telescope for Astronomy) in the infrared and visible configurations. The design is based on a fast quasi Ritchey-Chretien Telescope with an f/1 primary and an f/3 secondary. The large field of views available: 1.65 degrees in the IR and 2.1 degrees in the visible, makes use of the latest technology in optical materials, active optics and large arrays of detectors. The residual third order spherical aberration for on-axis images introduced in the two-mirror design is used to compensate the residual spherical aberration in the field corrector lenses. The infrared camera is included in the telescope optimization, letting the radii of curvature and conic constants of the two mirrors in the telescope vary in order to get the best performance across the entire IR detector array. It also contains an innovative cold baffle with a special black coating. The visible camera contains an ADC incorporated in the field corrector lenses. The acquisition, guiding and wavefront sensing of this telescope is integrated in the instruments.