The application of active and adaptive optics allows the construction of large diameter lightweight optical telescopes for observations below the seeing limit of the atmosphere. Active wavefront correction in a Cassegrain/Gregory-type radio telescope can be made with a deformable main reflector or deformable subreflector. Here we suggest the possibility of correcting spatially large-scale wavefront deformations with a small size corrector located near the focus of the telescope. Using representative examples of the Institut de Radioastronomie Millimetrique (IRAM, Spain) 30-meter diameter, millimeter-wavelength telescope, we calculate the improvement expected from the correction of (1) the systematic component in homology deformations, (2) the large-scale residual errors of a reflector adjustment, and (3) the beam degradation experienced in observations with a wobbling subreflector. The improvement in surface/wavefront precision obtained from piston correction with a corrector of some 50 elements is of the order of 30%--40%. We investigate in particular the systematic component of homology deformations, their representation by low-order Zernike polynomials, and their elimination by near-focus correction. We study in detail the homology deformations of the IRAM 30-m reflector. ¿ American Geophysical Union 1996