[IEEE 2007 Joint 32nd International Conference on Infrared and Millimeter Waves and the 15th International Conference on Terahertz Electronics (IRMMW-THz) - Cardiff (2007.9.2-2007.9.9)] 2007 Joint 32nd International Conference on Infrared and Millimeter Waves and the 15th International Conference on Terahertz Electronics - Alignment tolerances of antenna optics for KVN 21-m shaped cassegrain antenna