The correction of spherical aberration in the Southern African Large Telescope (SALT)

The Southern African Large Telescope (SALT) is a 10-m class telescope for optical/infrared astronomy to be sited at Sutherland, the observing station of the South African Astronomical Observatory. This telescope will be based on the principle of the Hobby Eberly Telescope (HET) at McDonald Observatory, Texas: a cost-effective design involving a tilted-Arecibo concept with a segmented spherical primary of diameter 11 metres. A spherical aberration corrector mounted on a tracker beam at the prime focus enables a celestial object to be followed for twelve degrees across the sky. This paper will discuss the correction of the spherical aberration for SALT, including presenting optical designs similar do the HET corrector, but with substantially better performance expected. Although the discussion will concentrate on the specific case of SALT, the scope is wide enough to be of interest for spherical aberration correction of similar telescopes (spherical primary feeding a tracking prime focus) which may be amongst the next generation of very Large aperture instruments.