  A 2.5 degree field diameter corrector lens design for the Cassegrain focus of
the VISTA 4 meter telescope is presented. It comprises four single elements of
glasses with high UV transmission, all axi-symmetric for operation at the
zenith. One element is displaced laterally to provide atmospheric dispersion
correction. A key feature, especially beneficial for the VISTA application, is
that the ADC element can be mounted so it is driven simply by gravity; thus its
operation needs no motors, encoders, cabling, or software control. A simple
mechanical design to achieve this and the optical performance details are
described.
