Displaced Axis or Ring Focus Dual Reflector
The displaced axis dual reflector has a main reflector with a ring focus where the subreflector fits within this ring so it does not block rays reflected from the main reflector. There are 4 types of ring-focus dual reflectors illustrated in the paper: Samuel Morgan, “Some Examples of Generalized Cassegrainian and Gregorian Antennas,” IEEE Trans. Antennas Propag., vol. AP-12, No. 6, November 1964, pp. 685-691.  The geometry is explained in the article: Christophe Granet, ”A Simple Procedure for the Design of Classical Displaced-Axis Dual-Reflector Antennas Using a Set of Geometric Parameters,” IEEE Antennas and Propagation Magazine, vol. 41, No. 6, December 1999, pp. 64-72. A presentation analyzing the dual reflector using GRASP10 can be downloaded: Displaced_axis_10_audio.ppt or GRASP 9: Displaced_axis_9_audio.ppt.

4 Types with executables for Geometry and GRASP geometry input files

daxsg.jpg
Single Gregorian: daxsg.exe

daxsc.jpg
Single Cassegrain: daxsc.exe

daxdg.jpg
Dual Gregorian: daxdg.exe

daxdc.jpg
Dual Cassegrain: daxdc.exe