Optical design for advanced extremely large telescopes

During last decade, several concepts for the next generation of telescopes have evolved into well-established projects. The majority of the proposed concepts are Extremely Large Telescopes (ELTs) utilizing the advantages of having an AO system as an integrated part of the main optical system. An adaptive secondary mirror (AS) has become a typical prerequisite for the future ground-based optical telescopes. Recent advances in deformable mirror technology used for AS mirrors show a possibility of developing this technology to the mirrors of larger diameters suitable for integration into ELT main optical systems. An idea of fully adaptive telescope has been proposed for the Arcetri 40-m multiconjugate telescope. A four-mirror optical system with a 40 in parabolic segmented primary is considered and optimised for such an advanced ELT. It is shown that the proposed system geometry enables to obtain proper optical conjugation for the tertiary and quaternary mirrors, achieving a diffraction limited image quality over 8-arcmin field. The aspects of internal vignetting and re-imaging of laser guide stars are discussed. If the fabrication of adaptive mirrors of a large size is feasible, the proposed four-mirror system will become highly attractive in terms of system compactness and minimized number of mirrors employed in the ELT multi-conjugate AO system.