<title>Diffraction-limited 8- to 20-m telescope with an active and adaptive tertiary</title>

Abstract

Active Optics Methods are extremely performing to obtain highly aspherical mirrors. The development of these methods is underlined with the presently proposed 4 to 5 mirror large telescope in the 8 - 20 m class. In this design, a particular emphasis has been placed to achieve the following main features: diffraction limited images over a restricted 1 or 2 arcmin field of view at f/30 for separate use with 16 Mpx detectors, diffraction limited images over a 2 to 5 arcsec field of view at the interferometric Mersenne focus, fast primary mirror of spherical shape, next mirrors the smallest as possible, low asphericity secondary and a tertiary spherically polished. It was found from optimization process with, for instance a 8 m primary at f/1.75, that, of the four mirrors required to achieve a diffraction limited afocal beam to a Mersenne focus, the spherical tertiary is the only mirror to be actively aspherized. Thus, due to a small aperture of this mirror (0.7 m), the vase form already developed from elasticity analysis would allow accurate aspherization by active optics. Similarly to our previous proposal TEMOS, the tertiary figure at f/6 can be achieved in situ, from a spherical polishing, by air depressure inside the mirror. The Sphe3 deformation sag of this mirror is quite large since is congruent to 1.3 mm ptv. Further, slight variations of the loading intensity allow a full optimization of the system as a function of the selected spectral range. The telescope design includes a pupil transfer on the tertiary realized by a doublet-lens. Thus, an adaptive optics system can be co- added to the same mirror. Finally, a concept for an active- adaptive tertiary is proposed.