The optical and thermal design of the 1.5 m solar telescope GREGOR is presented. The three first main mirrors of GREGOR will be made from Cesic, a silicon carbide material. One major constraint of large solar telescopes is the thermal load of the structure and the mirrors. The mirrors are heated by the solar radiation and introduce potentially harmful mirror seeing. GREGOR will use an active mirror cooling system and an open telescope structure to reduce these negative effects. A thermal analysis shows that the equilibrium temperature of the Cesic Mirror without active cooling is 6° above ambient temperature. Additional cooling will reduce the temperature difference of the optical surface and ambient air to below 0.1° K. With tempered airflow (about 2.5 m3/s per square meter mirror surface) the temperature gradient on the surface of the face sheet is less than 0.1°K. The telescope will have an open structure and a complete retractable dome to support mirror and structure cooling by wind.