Atmospheric dispersion corrector for 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, which is almost fully funded and whose construction has begun at the time of writing, 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 meters. A spherical aberration corrector (SAC) mounted on a tracker beam at the prime focus delivers a high quality image to the focal plane and enables a celestial object to be followed for twelve degrees across the sky. The telescope is to be optimized for ultraviolet wavelengths so it is especially important that it is equipped with an atomospheric dispersion corrector (ADC). This paper will present a concept for an ADC for SALT using two large prisms whose variable separation can be adjusted for the different zenith distances of the target during observation. This kind of ADC is in use on the ESO VLT and is planned for use on SOAR.