For space Astronomy as well as Earth Observation from space, more and more focal plane instruments are operating in the near or mid infrared and require optics operating at cryogenic temperature. The challenge here is to design an optical system reaching all optics, mechanics and thermal requirements. Thales SESO is presently under manufacturing of different opto-mechanical sub-assemblies for Meteosat Third Generation program. The imager missions MTG-I will produce images of the Earth simultaneously in different spectral channels, ranging from the visible spectrum to thermal infra-red, in order to fulfill the scientific, governmental & population needs. The sounder mission MTG-S addresses some interferometric measurements in the MWIR and LWIR ranges on the same instrument. The four CO-I objectives and the four one of CO-S work in different infrared ranges, thus, materials and coating have been chosen and designed especially for each spectral group. Each objectives is a stack of lenses bonded into barrels, themselves bonded inside a main barrel. Each objective is optimized based on the opto-mechanical values measured on each component. The objectives need to resist to mechanical and thermal environments in terms of opto-mechanical stability and work under vacuum, thus, with cleanliness specifications. All these requirements need to be taken into account on the design of different part of the cold optics. Through this on-going realization, the main technical issues and corresponding solutions in terms of design, manufacturing and testing of lens assemblies, mirrors and relevant optical coatings will be pointed out.
Regarding the progress of optical design in favor of freeform surfaces, it becomes necessary to scale their feasibility with appropriate criteria in order to get a standard between an optical designer and an optical manufacturer. Two criteria are necessary, one linked to polishing process and one appropriate for measurement limitation. First criterion can be the extension to freeform surfaces (defined here by first Zernike terms) of a previous criterion which was calculated using conical equation. This criterion is representative of surface’s curvatures fluctuations which limit polishing efficiency and can generate high frequency defects. The second criterion should take into account the difficulty to measure the surface as feasibility needs also good knowledge of the correction polishing cycle to be performed. Different solutions exist for an accurate measurement in the range of nm. As demonstrated in the article, slopes versus reference surface is the limiting factor for a majority of measurement solutions. Therefore the criterion will be linked to a slope parameter. The governing principle of these criteria is to remain close to some relevant physical dimensions. In this idea, the polishing feasibility criterion defined in this paper will be comparable to tool diameter of Computer Controlled Polishing which refers to equipment resolution of optical manufacturers. In the same idea, we project to define a dimensioned criterion for measuring feasibility which can be compared to engraving resolution for a given Computer Generated Hologram.
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