Large deployable telescopes-also for μm-wavelengths?

New demands for space and earth observations drive the need for large apertures of 10 m and higher in telescopes. This requires new types of alternative lightweight and deployable primary reflector or lens concepts with aerial densities of <3 kg/m². Some syntheses between optical and especially mechanical and material aspects for large planar photon sieves and non-imaging telescopes are discussed. Focus is put on large planar and highly perforated membranes for photon sieves and shell-membrane type deployable (parabolic) reflectors, respectively. The reflecting surface of the latter is composed of specific C-fiber reinforces silicone (CFRS) material. In addition to possible shape control, emphasis is also given to reflective coating of CFRS. Engineering challenges requiring future activities are identified.