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Adhesive Free High Stability Optical Mount for Space Laser Applications: Design Optimization For Different CTE Materials Coupling
In the frame of Atmospheric Lidar (ATLID) project, one of the active instruments foreseen to be boarded on the EarthCARE satellite, a high stability adhesive free optical mount has been designed, developed and tested in order to fulfil the tight program requirements. The objective of the EarthCARE mission is to make global observations of clouds, aerosols and radiation. To achieve this challenging objective the satellite carries four scientific instruments: ATLID is a high-spectral resolution lidar able to transmit linearly polarised laser signals at a wavelength of 355nm in the ultraviolet (UV). The use of the ultraviolet has strong benefits over laser wavelengths in the visible range. While the visible range is, in principle, technically easier, the ultraviolet has the advantage that the laser beam diverges much less than at visible wavelengths. This allows the reduction of the laser footprint on the target surface to just a few metres. The main components of ATLID instrument are the transmitter system and the receiver. The transmitter includes the Power Laser Head (PLH) and its electronic driving box: PLH is equipped with a wide range of optical elements able to provide, at the output window, the required UV beam. One of the major project criticality is the opto-mechanical stability of these optical components, strongly needed to keep aligned the laser beam with the receiver, avoiding PLH performance and accuracy degradation. Due to the instrument high sensitivity to Laser Induced Contamination effect (LIC), a deposition mechanism in vacuum environment of volatile molecules, originated by outgassing phenomena, that interact photo-chemically with the laser beam and tend to stick a contamination layer on the closest optics inducing a quick degradation of laser performances, the use of organic materials shall be minimized. Therefore, a fully mechanical mount (adhesive free) based only on the use of inorganic materials, has been designed. A further criticality is the fact that these optical mounts must be equipped, for alignment purpose, with a regulation system able to provide the mirror itself with two rotational degrees of freedom. To maximize the thermo-elastic stability, an adhesive free mounting concept was firstly developed avoiding material transition: it implies a limited mountability just only where the interface material of the supporting structure is the same of the optical mount body. In order to extend the range of applicability of the adhesive free mounting concept into the overall PLH, an optimized design has been studied to couple different CTE materials, fulfilling at the same time the required stability performance. The description of the optimized design developed, manufactured and qualified is presented. A collection of stability results obtained on the optical mount breadboards is also presented, including a description of environmental tests performed, the way to assess the mirror stability after each environmental test and the comparison with the first adhesive free mounting concept, as well as the acceptance criteria derived in order to establish the flight worthiness of the manufactured and assembled hardware.
Mosciarello, Paolo, Di Carmine Emiliano
Paper for Seminar/Symposium/Conference
ECSSMET 2018 - European Conference on Spacecraft Structures, Materials and Environmental Testing (28/05-01/06 2018, c/o ESA-ESTEC, Noordwijk, The Netherlands)
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