This thesis investigates a new approach for dealing with the adverse effects of large-scale deformations in the main reflector of large Cassegrain antennas. In this method, the incident aperture distribution is imaged onto a tertiary focal plane. This is accomplished by using an optical imaging system consisting of a lens mounted behind the Cassegrain focus of the antenna. The lens forms a real image of the product of the incident aperture distribution and the pupil function of the antenna. The pupil function describes the profile of the main reflector of the antenna. If the incident aperture distribution is a plane wave, a real image of the pupil function of the main reflector will be produced at the focal plane of the image lens. Any imperfections in the main reflector will be imaged onto the tertiary focal plane but over a smaller area as defined by the magnification of the system. In principle, an active correcting element placed into the tertiary focal plane could compensate for these errors, thus preserving the maximum efficiency of the antenna. Experimental verification of this principle was carried out in the lab using a dielectric lens 152.4mm in diameter. Phase perturbations were simulated by placing dielectric shims in the incident aperture plane. The phase of these shims in most cases was measured to within 10 degrees in the image plane. This degree of accuracy is found to be quite adequate for correcting large-scale errors in the main reflector of the antenna.