Optical design for an adaptive anastigmatic five-mirror extremely large telescope

Observations in extremely large telescopes will rely on adaptive corrections. To reduce the size and complexity of the instruments observing over large fields, the telescope should correct not only fast telescope misalignments and guiding errors, but also the bulk of the errors introduced by the atmosphere. This paper describes an optical design with five mirrors, three of them powered, which fulfils this requirement. The optical system is effectively diffraction limited over a field of 10 arcmin, also delivering very good image quality of laser guide stars. In addition, it offers the possibility of installing a large field atmospheric dispersion corrector and the freedom to switch between a large number of foci by changing the focal length. A weak field curvature concentric to the exit pupil eases the design of instrumentation.