In the optical communications frame, the telescopes (emitter and receiver) are used as optical antennas they must have a high optical quality, low central obscuration, low straylight, low weight and low overall dimensions. We are studying, in the SILEX program, the telescope which is an afocal system composed of a Ritchey-Chretien telescope and a dioptric collimator. The parabolic primary mirror (250 mm diameter) is lightened to reduce weight. The mechanical concept is to join the periphery of the secondary mirror spider to the periphery of the primary mirror by a carbon fibers truss which permits high stability. The whole sets of supports are fixed on a sandwich base plate. Thermal environment and operating conditions lead to insure radiative uncoupling between mirrors, truss and outside of telescope and to use thermostated heaters on structural parts. The straylight, mainly due to the sun, can be limited by baffling systems and appropriate choice of materials (studied by MBB). As the telescope optical antenna gain depends on optical quality, optical transmission, obscuration and off-axis effects all these performances must be preserved during the whole flight. We present the study of this telescope whereas a breadboard manufacture and tests are in progress in BERTIN.