Gregorianoffset reflector
The reflector can be analyzed using the student version of GRASP available from TICRA using a Gaussian beam feed antenna.  The executable GRANETDOF (Linux version) can be downloaded for the geometry parameters. To understand the geometry, see: C. Granet, Designing Classical Offset Cassegrain or Gregorian Dual-Reflector Antennas from Combinations of Prescribed Geometric Parameters, IEEE Antennas and Propagation Magazine, Vol. 44, no. 3, June 2002, pp. 114-123. A set of Fortran subroutines for this geometry is contained in the set: granet_equations.
A step-by-step presentation with audio tracks using student version of GRASP10 can be downloaded Offset_Gregorian10_ audio  with input files:  Offset_Gregorian_GRASP.

dual_offset_greg.gif