Further space exploration in the far-infrared (FIR) requires larger apertures in order to improve the spatial resolution of captured images. To this purpose, the Thinned Aperture Light Collector (TALC) concept of a deployable annular telescope has been recently developed at CEA, which offers novel perspectives for FIR space missions. The consortium ELICSIR consortium of European institutes and companies has been created to improve the technological readiness level (TRL) of its key systems and components.
TARGETED BREAKTHROUGH, LONG TERM VISION AND OBJECTIVES
Because of the impossibility for astronomers to build experiments to test their hypotheses, astrophysics is likely that part of natural sciences where the largest effort is made on data collection. And since astronomical data essentially mean photons, each domain of the electromagnetic spectrum matters. Some regions of this spectrum are more challenging, among which the mid to far infrared domain (MIR-FIR) is of particular interest. From typically 30μm to 500μm we find a number of unique signatures related to the co-evolution of massive blackholes and their host galaxies, the origin of the initial mass function for stars, or the initial conditions of planetary systems. However, because of our atmosphere, these wavelengths can only be collected from space observatories. This creates a severe technological limitation, as any observing facility needs to be launched into space, imposing strict boundaries in mass and volume to these observatories. This is amplified by the fact that access to space is now driven by recurrent commercial satellites rather than scientific exploration. Those commercial driving needs are not going toward larger launcher capabilities while scientific requirements could be summarized rapidly as a quest for higher angular resolution and sensitivity, and therefore larger observing facilities.
The scientific community has accumulated a string of successes in MIR-FIR astronomy, starting with the IRAS mission in 1983, and culminating with the Herschel Space Observatory in 2009. Europe fared well in this endeavour, with Herschel being internationally recognized as a success. Yet the current approach of monolithic mirrors reached the launcher limits with Herschel. This 3.5 m SiC mirror telescope was an extraordinary achievement of the European community, but in its wavelength range, it delivered an angular resolution no better than Galileo’s telescope in the 17th century. The state of the art facility under construction is the NASA-JWST to be launched in 2018. Its 6.5 m primary mirror design is however still relying on standard mirror fabrication material and process (Beryllium polished mirrors), simple folding topology, and active optics supported by a stiff structure. The cost and development duration of this program revealed clearly the limits of this approach, in terms of technology and processes. As future science needs will require exceeding the capacities of even the JWST, we need to find a path that escapes this deadlock. The mission concept we propose is based on science requirements derived from Herschel’s advances, and an innovative system approach for the mission implementation leading to the Thinned Aperture Light Collector (TALC) concept.
The current design of the TALC telescope features a primary annular mirror of 20 m diameter fitting within mass and volume constraints of the future Ariane 6 fairing.
The TALC concept goes beyond the state of the art by changing the system design approach and relying on:
• An innovative deployable mirror whose topology is based on stacking rather than folding, leading to an optimum ratio of collecting area over volume;
• A tensegrity structure of a ferries wheel to hold the segments of the ring mirror in place with stretched spokes surrounding a central mast;
• A lightweight segmented primary mirror, based on electrodeposited nickel, carbon composite and honeycomb stacks, built with a replica process to control costs and mitigate the industrial risks;
• An innovative active optics control layer in the mirror rear shell allows controlling the shape by in-plane forces instead of the conventional normal-force actuators that require a rigid support structure.
In this paper, we present the progress we have made in the deployment principle that increases its efficiency and robustness, and outline some of the salient points of the roadmap we are building to deliver on each of the three aspects listed above so that these technologies reach a readiness level that is mature enough to allow the astronomical community to prepare a future MIR-FIR large space mission for the post-JWST era (here the term “large” refers to the ESA mission standard, i.e. a mission with a cost of around 1 billion Euros).
CONCEPT OF TALC
To launch effectively large structures at an acceptable cost, their design must rely on elements produced in series, these elements must consist of lightweight material with a low coefficient of thermal expansion, and we must validate efficient, robust and reliable deployable structures. In these three areas our proposal goes significantly beyond the state of the art.
Innovative deployable mirror: One of the key innovative concepts that will be validated is the topology for a deployable, segmented mirror that has been studied since 2012 at CEA. Inceptive work on this concept started in 2011 for the study of a 40-m Sub-millimeter Telescope at Dome C that we re-oriented toward space application due to its high disruptive potential. Three years of mechanical developments, including the realization of a 4-m diameter fully deployable mock-up allowed refining our design. This demonstrates optimum topology for deployed surface to stored volume ratio, and the kinematic of deployment has evolved so that the mirrors are stored parallel inside the fairing (providing higher resistance to launch) and deploy towards a parabolic position (see Durand et al. 2014 SPIE 9143, id91431A).
With a fairing of 4.2 m usable diameter (e.g. an Ariane launcher), 18 segments allow to deploy a 20 m diameter telescope, 24 segments a 25.5 m diameter telescope. We are developing the 20 m telescope with 18 segments, in order to keep the structure easily controllable.
Ultra-light weight replicated mirror: Fabrication of lightweight mirrors for space applications has proven to be an extremely difficult problem and is at the core of research and developments at the main space agencies. Carbon fibre reinforced polymer (CFRP) is an ideal candidate for ultra-light weight mirrors working at cryogenic temperature because of its low areal density, high elastic modulus and coefficient of thermal expansion (CTE) that can be tailored to low or zero. However, the capability to build CFRP mirrors with good optical quality is limited by surface distortion called fiber print through. Our study will consolidate the design of CFRP mirrors identified as a key enabling capability for the TALC telescope through test mirror manufacturing, paying special attention to fibre curing and layering processes that lead to print-through. We will focus on mirrors with high optical quality at 30μm, but the knowledge gained will be scalable toward shorter wavelengths, opening new possibilities in the IR and in the visible for astronomy but also for earth-observation programs. We will aim at validating production of mirrors based on replication, which ensures better control of the costs, a particularly important item for a project of the size of TALC.
Active optics control: Implementation of active optics control is a promising avenue to reach high optical quality on large structures. It reduces the mechanical surface front error requirement by ensuring that the final shape is recovered actively through a control loop. The global image quality needs to be ensured by (1) manufacturing for the smallest cell that cannot be corrected by active control (30-cm in this project), and (2) by active optics correction on larger scales. The active optics layers will be incorporated into the rear side of the mirror, departing radically from standard corrective optics that relies on reaction actuators attached to a rigid reference frame. Our design for a typical 4-m panel would rely on 2 layers of active optics grid for shape correction. The first one, coarse and constant, to correct the first low orders of deformation, the second, meshing the mirror with a typical sub-30-cm cell pattern, to correct for higher spatial frequencies.
SHARE OF TASKS WITHIN ELICSIR COLLABORATION
work package “Deployment and Dynamic Control Studies”
The goal of work package “Deployment and Dynamic Control Studies”, led by ULB and INSA, is to design, manufacture and test at ambient and in cryo-conditions the deployment system on mock-ups of different scales chosen to explore scalability laws. ULB, INSA and CEA will perform numerical studies of the vibration modes of the structure during deployment and after closure, as well as of the control and active damping mechanisms needed to guarantee the integrity of the system. This will be correlated with vibration tests performed at ambient on a complete scale 1/10th mechanical model.
Mechanical tests will also be performed on a scale 1/3rd model of three articulated segments (Figure 1). The three segments will be representative of the actual mirrors in terms of process, interface, mass and thermal behavior. This structure will be equipped with metrology targets to qualify the deployment strategy in terms of accuracy, repeatability, and 3-degrees of freedom correction. The deployment structure will be designed by CEA/INSA/ULB, manufactured by Multiplast, and tested at CEA re-using existing and available large cryo-facilities developed for W7X fusion coils testing (4.5 x 4.0 m, operating at 70 K).
All along the life-cycle of the project, an integrated thermo-mechanical model of the full-scale telescope will be developed and maintained by CEA with elements from the different partners. This model will be correlated with the test results on the various mock-ups at warm and cryo-temperature to ensure scalability toward the 20 m size structure. This Finite Element Model (FEM) will allow for prediction of the final performance of the system. At the end of the study, this FEM of the full system will allow for the derivation of the expected performance of the final system at full scale (20 m) and operational temperature (40 K).
In addition to the ELICSIR program, it is planned to build a motorized model of the full ring mast and spokes telescope at a scale of diameter 1.4m and to test in sequences the reliability of the deployment and stacking back of the telescope during the 20 seconds of a zero G flight.
Lightweight replicated mirrors
The goal of work package “Lightweight Optical-Quality Mirrors”, led by Media-Lario, is to manufacture a 1.2 m parabolic demonstrator of a low-density mirror with optical quality (diffraction limited performance at λ = 30μm). The 1.2 m size requirement derives both from the analysis that this is required to identify processes that will scale up to the 4 m size of TALC’s panels and from the consideration that this size is required to plan future commercial applications. Scalability toward a 20 m structure imposes an aim of an overall density of 10 kg/m2. The material of choice is carbon fiber and honeycomb, but designing how these elements have to be assembled to achieve the optical quality goal is the major challenge. The baseline proposed by CEA is tailored to the objective of relying on replication methods: the layers of the mirror must be grown from the reflective surface upward such that the process propagates the optical quality of the mold surface toward the structural elements.
The process will start from a nickel layer electro-deposited on an optical quality mold, to continue with adding carbon structural elements on the optical Ni layer still on the mold, all at low temperature to preserve the mold quality. Media-Lario, a specialist of electro-deposition, will perform the first step of depositing nickel on optical quality molds. This Ni layer will then have to be bonded with a composite of carbon pitch fibre and honeycomb, providing the rigidity of the structure at low weight. Multiplast and North-TPT will develop a European source of cyanate-ester honeycomb, and of cyanate-ester pre-preg carbon pitch fiber low ply density (<50 gr/m2), to meet the needs of polymerization at low temperature and low coefficient of moisture expansion (CME). Mating the carbon structure to the nickel skin will be a specific challenge of this research task to be tackled both by Media-Lario and Multiplast. The challenge will consist in defining the bonding process, the optimal angles between each fibre layers and the number of layers so that the honeycomb does not print-through. The industrial partners are up to this challenge, combining significant experience in composite mirrors (Media-Lario), heavy investment in R&D on carbon materials (North-TPT), and worldwide leadership in large carbon structure and manufacturing capacities (Multiplast, where we will make use of an automatic plotter capability of 8 m x 2.5 m), while INAF and CEA provide their expertise for qualifying the optical quality at room and cryogenic temperatures.
Active optics control
The goal of the work package “Active Control of Mirrors Segments”, led by Muenster University of Applied Sciences, is to design, integrate and test the performance of a planar piezo- electric control layer at the back of a composite mirror. The work shall characterize the transfer function from the back layer to the optical layer in order to determine the capabilities of actively controlling and correcting the deformations on spatial scales larger than the unitary cell on which surface errors can be controlled by manufacturing for a reasonable cost. A number of 40 cm diameter composite mirrors will be manufactured by Media-Lario for this purpose. These will sample different thickness-to-diameter ratios: (a) mirror thickness homothetically reduced from the 1.2 m segment size in order to probe which modes can be corrected by the active optics system, (b) mirror thickness equivalent to the expected final thickness of the 1.2 m mirror in order to characterize the scalability of the transfer function from the control layer to the optical layer, and (c) a thin mirror to allow testing the option of applying correction at a pupil relay position.
This range of structures will allow testing a number of strategies both for manufacturing the mirror and for implementing the correction layer, thus feeding the trade-off analysis activity. Here as well the mirror size is driven by an analysis of the scale at which the performance becomes scalable to the full scale and at which we will have demonstrated a significant disruption with respect to the state of the art.
The goal of the work package “System Engineering and Trade-Off Analysis”, led by CEA, is to ensure that the three lines of R&D are fully coordinated toward the ultimate goal of ELICSIR.
The design of the large TALC mirror is a sweet spot between stability of the deployed structure, optical surface quality on small scale of composite mirrors, and capacity to correct accurately the full-scale 4 m segments through active optics, all geared toward delivering performance that is requested by science objectives. Therefore, all along the life cycle of the project, the system-engineering group will integrate the latest results into a trade-off analysis in order to rebalance the ultimate requirements between the different actors. This will prevent the study from being locked on local issues that can be solved at system level.
In addition, as the verification approach of such a large system cannot rely on ground testing at full-scale (the JWST is already using the largest facility available on the planet), we will also focus system activities on the correlation of analyses based on mathematical models of the full-scale structure with hardware tests and measurements on physical models (full telescope 1/10th mechanical model for vibration, 3-segment mechanical 1/3rd model for deployment, mirror segments for optical quality tests).
ROADMAD FOR THE FOLLOWING YEARS
TALC is a proposal for a large mission. While a medium mission as Euclid requires 15 years of development, TALC requires about 25 years from concept to launch. As a European flagship, TALC shall include high innovation content both for the telescope and the instrumentation part.
Period 2013 – 2020: Initial technology development shall be initiated with focus on:
• TRL5 must be demonstrated at beginning of implementation phase
• Some technological developments are accepted for L-class within the 2-3 years following adoption.
On the scientific field, the roadmad of the Talc team is to prepare Far-IR mission is to gather a scientific community from multinational contributors that have experience and interest in the post – Herschel/ALMA/JWST era.
On the technical field, the roadmap is to consolidate the System Engineering at Mission levels through trade off analysis and preliminary design of Mission (Telescope/Instrument/AOCS/Concept of Operation and Ground Segment) to support a future Mission concept costing and proposal.
The first action was to identify the key critical components and take actions to raise them to acceptable level in order to match Cosmic Vision TRL requirements. This is the purpose of the TALC H2020 proposal supported by the ELICSIR collaboration.
When the key components will be validated at a level of TRL3, the next action will be use funding from ESA within TRP, CTP or GSTP programs to raise the programs to TRL5.
The concept of a deployable ring mirror whose structure uses the tensegrity of a wheel is a rupture innovative concept. It gives access to large collecting area, high stiffness and lightweight. The maturity of the key components will allow to build telescopes for telecom as well as for shorter wavelength.
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