TALC: a new deployable concept for a 20m far-infrared space telescope

TALC, Thin Aperture Light Collector is a 20 m space observatory project exploring some unconventional optical solutions (between the single dish and the interferometer) allowing the resolving power of a classical 27 m telescope. With TALC, the principle is to remove the central part of the prime mirror dish, cut the remaining ring into 24 sectors and store them on top of one-another. The aim of this far infrared telescope is to explore the 600 μm to 100 μm region. With this approach we have shown that we can store a ring-telescope of outer diameter 20m and ring thickness of 3m inside the fairing of Ariane 5 or Ariane 6. The general structure is the one of a bicycle wheel, whereas the inner sides of the segments are in compression to each other and play the rule of a rim. The segments are linked to each other using a pantograph scissor system that let the segments extend from a pile of dishes to a parabolic ring keeping high stiffness at all time during the deployment. The inner corners of the segments are linked to a central axis using spokes as in a bicycle wheel. The secondary mirror and the instrument box are built as a solid unit fixed at the extremity of the main axis. The tensegrity analysis of this structure shows a very high stiffness to mass ratio, resulting into 3 Hz Eigen frequency. The segments will consist of two composite skins and honeycomb CFRP structure build by replica process. Solid segments will be compared to deformable segments using the controlled shear of the rear surface. The adjustment of the length of the spikes and the relative position of the side of neighbor segments let control the phasing of the entire primary mirror. The telescope is cooled by natural radiation. It is protected from sun radiation by a large inflatable solar screen, loosely linked to the telescope. The orientation is performed by inertia-wheels. This telescope carries a wide field bolometer camera using cryocooler at 0.3K as one of the main instruments. This telescope may be launched with an Ariane 6 rocket up to 800 km altitude, and use a plasma stage to reach the Lagrange 2 point within 18 month. The plasma propulsion stage is a serial unit also used in commercial telecommunication satellites. When the plasma launch is completed, the solar panels will be used to provide the power for communication, orientation and power the cryo-coolers for the instruments. The guide-line for development of this telescope is to use similar techniques and serial subsystems developed for the satellite industry. This is the only way to design and manufacture a large telescope at a reasonable cost.