This paper describes the outcomes of a study funded by the European Space Agency aimed at identifying the technical challenges and trade-offs at the system level, performing preliminary designs of an active correction loop for large deployable telescopes, and defining technological roadmaps for the development of the active correction loop for the selected designs. This study has targeted two very different application cases, one for High Resolution Earth Observation from Geostationary orbit (called GeoHR, with a 4m diameter entrance pupil) and one for Science missions requiring very large telescopes (with a up to 18 m diameter entrance pupil) with high-contrast imaging capabilities for exo-Earth observations and characterization. For both application cases, this paper first summarizes the mission objectives and constraints that have influence on the telescope designs. It then presents the high-level trade-offs that have been led and the optical and mechanical design that have been developed, including the deployable aspects. Finally, the performance assessment is presented, and is the basis for the justification of an active optics correction chain, with a preliminary set of requirements for typical components of the system. The presentation is concluded with proposed technological roadmaps that aim to allow the development of the building blocks on which the next generation instruments will be able to rely on.
Thales Alenia Space is designing and developing space observation instruments since more than 40 years. This paper explains why active optics is needed for next generation of instruments for Earth observation. It describes what kind of solution is preferred and gives an overview of the development status on the associated technologies. Indeed, the future missions will have to deal with better performance, better optical quality while from manufacturing point of view, the total mass, the development schedule and the final cost have to be reduced. These constraints induce a new generation of solutions based on large entrance optics associated to high lightweight ratio which naturally provide solutions sensitive to gravity deformation. In these conditions, the enhancement of the final performance can only be guaranteed by using active optics in flight. A deformable mirror is therefore foreseen to be implemented in future large telescopes in order to correct manufacturing residues and ground/flight evolution, including gravity. Moreover, low mass and low cost require more compact designs which entail solutions more sensitive to misalignment. An active positioning mechanism is then also needed in order to correct the telescope alignment during operation conditions. Thales Alenia Space has been selected by CNES to develop and qualify active optics building blocks and then to test and demonstrate the improvement that new active technologies can bring in a full size instrument representative of the next generation of observation instruments. An overview of the current development status and the achieved performances is given for each building block (Primary Mirror, deformable mirror, 6-dof mechanism, wavefront sensor).
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