Extremely Large Telescopes Optical Design and Wavefront Correction

An overview of the state of art within optical design of large astronomical telescopes is given. Recent advances within the field are presented. The importance of new computer-controlled mirror-polishing methods is emphasized. Some important aspects of atmospheric optics are presented together with current compensation methods using adaptive optics, with or without laser guide stars. An overview of wavefront sensing, wave-front reconstruction and techniques for compensation is given. Methods for optical design of large, optical telescopes with fast primary mirrors are presented in a systematic way. Three different approaches are outlined and commented. They include an algebraic method, a method based on optimization through ray tracing and an analytical method based on Fermat’s principle and the Abbe sine condition. Studies of optical designs for large telescopes with fast, spherical primary mirrors are examined and discussed. For two-mirror designs, an analytical method has been derived, using aberration control based on Fermat’s principle. Intrinsic apodization for design of extremely fast primary mirrors is analyzed. For four-mirror designs with spherical primary mirrors, a ray tracing approach has been chosen. A total of fifteen different four-mirror designs have been analyzed and discussed. For the 50 m optical telescope, Euro50, a two-mirror design was chosen. This optical design is presented in detail, together with the optical layout of an adaptive optical system that forms an integrated part of the telescope. An analysis of essential components of the Euro50 is given. Finally, an analytical approach is presented for control of two or more deformable mirrors for adaptive optics in extremely large telescopes for optical wavelengths. The same algorithms are used to evaluate and predict the performance of the adaptive optics for the Euro50.