Video mission for Earth Observation will be the next step of optical instrument goals. It will allows to answer to new types of problematic and missions, specifically associated with on board Artificial Intelligence, as it opens the possibility to process a wide amount of data on board. Presently, due to technological limitation on data processing and downlink data rate, there is no instrument on the market designed to collect big amount of data. The VIDEO project will be a new type of instrument designed to be used with next generation of Artificial Intelligence processing capacity on board. It will allow Europe to takes the lead on the EO market within the next years. Due to its specific and innovative technologies and architecture adapted to new generation of data processing, the VIDEO instrument will be the pathfinder of the next instrument generation for Earth Observation.
Silicon nitride is a ceramic material that has very interesting stability properties for space telescopes. Indeed, the objective for telescopes is chiefly stability and with its high strength and stiffness and low CTE, Silicon Nitride is well suited for complex space structures. Thales Alenia Space has demonstrated the ability of Si3N4 to address those needs and requirements for complex designs such as tubes, brackets, brazed supports and bars. All these structures have been tested, qualified and flight proven. In addition, deep characterization of the material strength has been performed. Now that this material has been demonstrated as suitable for telescope structures, several R&D activities are pursued, from production via additive manufacturing to suitability for optical parts. This paper shows an overview of past, on-going and foreseen developments for the Silicon Nitride technology.
Recent Earth Observation instruments require a highly accurate knowledge of their Instrument Spectral Response Function (ISRF). This translates into lengthy and costly characterisation programs during the Assembly, Integration, and Test phase of the instrument. In addition, potential changes of ISRF after launch suggests to use an on-board equipment dedicated to this accurate characterisation in flight. This topic has been studied by Thales Alenia Space and TNO in the frame of a TRP study funded by ESA during years 2019 and 2020. This paper first recalls the potential application cases and the related target performances of the study. Then we identify conceptual solutions, such as operating tuneable laser diodes, Fabry-Perot interferometer, microresonators. Next, we describe the principle of the selected solution: a Fourier Transform Spectrometer (FTS), used as a calibration light source. Indeed, a single FTS can cover a very large spectral range with an extremely high spectral accuracy. But FTS are known as complex and sensitive devices: we have designed a simplified solution, well adapted to space applications. In a second step, we describe the flight hardware designed by Thales Alenia Space, targeting optimised manufacturing and implementation. The main performances, being spectral resolution and signal to noise ratio (SNR), are discussed. Breadboard activities also took place in TNO in order to validate the main hypotheses. Innovative data processing has been tested, that only makes use of interferogram data, while maintaining a high accuracy. Description of the breadboard as well as the first test results are presented.
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).
Silicon nitride is a ceramic material that has very interesting properties for space applications, especially for optical
instruments and telescopes. Indeed, the objective for such structures is chiefly stability to ensure performance, associated
with low mass and high stiffness to sustain launch loads. With its high strength and stiffness and low CTE, Silicon
Nitride is therefore extremely well suited for stable Space structures.
Thales Alenia Space has been using Silicon Nitride for more than a decade, and in that framework has evaluated the
ability of the material to address those needs and requirements for complex designs such as tubes, brackets, brazed
supports, beams and light-weighted plates. All these structures have been tested, qualified and are now flight proven.
In order to improve knowledge and mastering of strength properties, a characterization campaign of the material is under
way in a GSTP funded by CNES and driven by ESA. This paper describes the design of this test campaign, the choices
for the sample types and dimensions, and prediction of the expected results. In addition to the mechanical strength
testing, X-ray tomography has been implemented in order to detect flaws beforehand and to investigate the ability to
predict failure from the extracted information. This will be especially useful since verification (in particular proof tests
that are commonly used on ceramics for Space applications) is based on the relationship between strength and flaws. It is
expected that by improving this knowledge a more straightforward verification process can be derived.
Zerodur® is a well-known glass-ceramic used for optical components because of its unequalled dimensional stability
under thermal environment. In particular it has been used since decades in Thales Alenia Space’s optical payloads for
space telescopes, especially for mirrors. The drawback of Zerodur® is however its quite low strength, but the relatively
small size of mirrors in the past had made it unnecessary to further investigate this aspect, although elementary tests have
always shown higher failure strength. As performance of space telescopes is increasing, the size of mirrors increases
accordingly, and an optimization of the design is necessary, mainly for mass saving. Therefore the question of the
effective strength of Zerodur® has become a real issue.
Thales Alenia Space has investigated the application of the Weibull law and associated size effects on Zerodur® in 2014,
under CNES funding, through a thorough test campaign with a high number of samples (300) of various types. That test
campaign demonstrated a strength in fast fracture higher than 40 MPa ([1], [2]) for the tested surface finish, thus
allowing much more versatility in the designs than the previously accepted strength limit (10 MPa).
Another concern had however been raised: glasses are known to be susceptible to sub-critical crack growth, i.e. slow
propagation of cracks under loads below fracture toughness, thus reducing fast fracture strength capabilities (since failure
is linked to sudden propagation of those cracks). Taking into account data from literature, no effect was expected on
Zerodur® in the conditions of use for space applications, but the very high variability of data made those computations
not reliable enough. A dedicated test campaign was therefore defined in order to assess this effect and its consequences
in conditions as representative as possible of real conditions.
In this paper we show the outcome of this test campaign: the effect of subcritical crack growth is confirmed to be
negligible and the minimum strength of 40 MPa is confirmed. In time, Zerodur® strength seems to even increase but this
phenomenon was not investigated in the study.
Due to more and more stringent requirements for observation missions, diameter of primary mirrors for space telescopes is increasing. Difficulty is then to have a design stiff enough to be able to withstand launch loads and keep a reasonable mass while providing high opto-mechanical performance. Among the possible solutions, Thales Alenia Space France has investigated optimization of ZERODUR mirrors. Indeed this material, although fragile, is very well mastered and its characteristics well known. Moreover, its thermo-elastic properties (almost null CTE) is unequalled yet, in particular at ambient temperature. Finally, this material can be polished down to very low roughness without any coating. Light-weighting can be achieved by two different means : either optimizing manufacturing parameters or optimizing design (or both). Manufacturing parameters such as walls and optical face thickness have been improved and tested on representative breadboards defined on the basis of SAGEM-REOSC and Thales Alenia Space France expertise and realized by SAGEM-REOSC. In the frame of CNES Research and Technology activities, specific mass has been decreased down to 36 kg/m2. Moreover SNAP study dealt with a 2 m diameter primary mirror. Design has been optimized by Thales Alenia Space France while using classical manufacturing parameters – thus ensuring feasibility and costs. Mass was decreased down to 60 kg/m2 for a gravity effect of 52 nm. It is thus demonstrated that high opto-mechanical performance can be guaranteed with large highly lightweighted ZERODUR mirrors.
Space telescopes require large primary mirrors within a demanding thermal environment: observatories at L2 orbit provide a stable environment with a drawback of very low temperature. Besides, it is necessary to limit as far as possible the mirrors mass while withstanding launch loads and keeping image quality within a cryogenic environment.
ZERODUR is a well-known material extensively used for large telescope. Alcatel Alenia Space and Sagem/REOSC have combined their respective skills to go further in the lightweighting ratio of large mirror (36 kg/m2 on 1.5 m2) through a detailed design, performance assessment and technology demonstration with breadboards.
Beyond on a large mirror detailed design supported by analysis, a ZERODUR mock-up has been manufacturing by Sagem/REOSC to demonstrate the achievability of the demanding parameters offering this high lightweighting ratio.
Through the ISO experience on mirror attachments, a detailed design of the mirror fixation has been done as well. A full size mock-up has been manufactured and successfully tested under thermal cycling and static loading.
Eventually, the ZERODUR stability behavior within this large temperature range has been verified through thermal cycling and image quality cryotest on a flat mirror breadboard.
These developments demonstrate that ZERODUR is a good candidate for large space cryogenic mirrors offering outstanding optical performances associated to matured and proven technology and manufacturing process.
Dealing with ceramic materials for more than two decades, Thales Alenia Space – France has identified Silicon Nitride Si3N4 as a high potential material for the manufacturing of stiff, stable and lightweight truss structure for future large telescopes. Indeed, for earth observation or astronomic observation, space mission requires more and more telescopes with high spatial resolution, which leads to the use of large primary mirrors, and a long distance between primary and secondary mirrors. Therefore current and future large space telescopes require a huge truss structure to hold and locate precisely the mirrors. Such large structure requires very strong materials with high specific stiffness and a low coefficient of thermal expansion (CTE). Based on the silicon nitride performances and on the know how of FCT Ingenieurkeramik to manufacture complex parts, Thales Alenia Space (TAS) has engaged, in cooperation with FCT, activities to develop and qualify silicon nitride parts for other applications for space projects.
Dealing with ceramic materials for more than two decades, Thales Alenia Space – France has identified Silicon Nitride Si3N4 as a high potential material for the manufacturing of stiff, stable and lightweight truss structure for future large telescopes. Indeed, for earth observation or astronomic observation, space mission requires more and more telescopes with high spatial resolution, which leads to the use of large primary mirrors, and a long distance between primary and secondary mirrors. Therefore current and future large space telescopes require a huge truss structure to hold and locate precisely the mirrors. Such large structure requires very strong materials with high specific stiffness and a low coefficient of thermal expansion (CTE). Based on the silicon nitride performances and on the know how of FCT Ingenieurkeramik to manufacture complex parts, Thales Alenia Space (TAS) has engaged, in cooperation with FCT, activities to develop and qualify silicon nitride parts for other applications for space projects.
Zerodur® is a well-known glass-ceramic used for optical components because of its unequalled dimensional stability under thermal environment. In particular it has been used since decades in Thales Alenia Space’s optical payloads for space telescopes, especially for mirrors. The drawback of Zerodur® is however its quite low strength, but the relatively small size of mirrors in the past had made it unnecessary to further investigate this aspect, although elementary tests have always shown higher failure strength. As performance of space telescopes is increasing, the size of mirrors increases accordingly, and an optimization of the design is necessary, mainly for mass saving. Therefore the question of the effective strength of Zerodur® has become a real issue.
Thales Alenia Space has investigated the application of the Weibull law and associated size effects on Zerodur® in 2014, under CNES funding, through a thorough test campaign with a high number of samples (300) of various types. The purpose was to accurately determine the parameters of the Weibull law for Zerodur® when machined in the same conditions as mirrors.
The proposed paper will discuss the obtained results, in the light of the Weibull theory. The applicability of the 2-parameter and 3-parameter (with threshold strength) laws will be compared. The expected size effect has not been evidenced therefore some investigations are led to determine the reasons of this result, from the test implementation quality to the data post-processing methodology. However this test campaign has already provided enough data to safely increase the allowable value for mirrors sizing.
Current and future space missions demanding ever more stringent stability and precision requirements are driving the need for (ultra) stable and lightweight structures. Materials best suited to meeting these needs in a passive structural design, centre around ceramic materials or specifically tailored CFRP composite. Ceramic materials have essential properties (very low CTE, high stiffness), but also unfavorable properties (low fracture toughness). Ceramic structures feature in a number of current and planned ESA missions. These missions benefit from the superior stiffness and thermo-elastic stability properties of ceramics, but suffer the penalties inherent to the brittle nature of these materials. Current practice in designing and sizing ceramic structures is to treat ceramic materials in a deterministic manner similar to conventional materials but with larger safety factors and conservatively derived material strength properties. This approach is convenient, but can be penalising in mass and in practice does not arrive at an equivalent structural reliability compared to metallic components. There is also no standardised approach for the design and verification of ceramic structures in Europe. To improve this situation, ESA placed two parallel study contracts with Astrium and Thales Alenia Space with the objective to define design and verification methodology for ceramic structures, with the further goal to establish a common ‘handbook’ for design and verification approach. This paper presents an overview of ceramic structures used in current and future ESA missions and summarises the activities to date in the frame of improving and standardising design and verification methods for ceramic structures.
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