Compare the small CCD, distorted image area of a coma-free Thin Paraboloid (left), with the full sized distortion free, CCD image area of a coma-free Thick True Parabolic Mirror (right).  Both are 14" in diameter with a 48" focal length and a 2" Secondary Mirror giving us a pair of 12" Clear Aperture f/4 Newtonian Telescopes.