Inverse Cassegrain reflector
The reflector can be analyzed using the student version of GRASP9 available from TICRA ( using a Gaussian beam feed antenna.  A step-by-step presentation can be downloaded: inv_Cass_ppt_audio with an audio track or inv_cass10_ppt_audio using GRASP10 along with a set of GRASP files: inv_Cass_GRASP.  To understand the geometry, see: C. Granet, Designing Classical Offset Inverse-Cassegrain Dual-Reflector Antennas from Combinations of Prescribed Geometric Parameters, IEEE Antennas and Propagation Magazine, Vol. 45, no. 3, June 2003, pp. 102-108. A set of Fortran subroutines for this geometry is contained in the set: granet_equations.