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1.INTRODUCTIONThales Alenia Space has been designing and developing space optical instruments for more than 40 years. Thales Alenia Space is now a leader in active optics for space, at all levels: system, building blocks, validation and tests. It has taken one step closer to future systems with deployable elements and active optics with a team of world-leading partners: ONERA (Office National d’Etudes et de Recherche Aérospatiales) is a world leader in High Angular Resolution imaging, in particular in the field of adaptive optics for ground application and active optics for space application. The role of ONERA in this project is to specify the Active optics system for Science case application.; UKATC is the UK’s national laboratory for the design and development of astronomical instrumentation and is a world leader in managing and building space and ground based instrumentation and systems; LAM is one of the three largest space laboratories in France, combining astrophysics with research and development in optics, including active/adaptive optics, for ground-and space-based instruments; the Observatoire de la Côte d’Azur (OCA) is an internationally renowned research center in Earth and Universe sciences, with broad expertise in astronomy and related instrumentation for ground-based and space telescopes. 1.1Scope and purposeIncreasingly demanding space-based applications require a large primary mirror diameter, on which depends the optical instrument resolution. However, mass and volume of a monolithic primary mirror would dramatically increase along with its size, which, in addition to cost and technical issues, is also limited by the launch vehicle fairing available volume. Thus, to reach large primary mirror diameters, development of instrument that will include a deployable, segmented primary mirror and an active correction loop to optimize its optical performance will be of utmost interest. Such technologies will allow addressing highly demanding space-based applications such as high-contrast imaging for Earth-like exoplanets direct detection or high resolution Earth observation from the geostationary orbit. This study aims at defining a technological roadmap of such an active optics correction loop, which will include sensing devices and active correction components. This technological roadmap will be supported by the design of two active correction loop which will be implemented in two preliminary instrument designs dedicated to two different missions:
For both cases, the active optics design has been performed through trade-off analysis of correction strategies and technologies, which will be based on the requested performances evaluation at system level. Such an evaluation will allow identifying the active optics correction chain technical challenges and proposing a development roadmap. 1.2Earth Observation caseThe way to proceed has been to systematically analyze the potential combinations of active optics components, in order to identify the ones that can be studied. The systematic approach is beneficial to ensure that every interesting case is not forgotten and that any case which would not be intuitively considered interesting is also evaluated. The definition of an interesting case is discussed later in this paper, where we propose the criteria chosen to select or dismiss configurations. 1.3Science caseFutur fundamental Science missions like LUVOIR (Large UV/Optica/Infrared Surveryor) shall be with a large international sharing funding and technological developments such as a complete Primary Mirror system. They shall be within a Science Objectives approach and disruptive from the JWST technologies already close to their asymptotic limit and too far from the LUVOIR-A Ultra Stability needs (10pm / 10mn stability). 2.ACTIVE OPTICS IN DEPLOYABLE SYSTEMS FOR EARTH OBSERVATION2.1Summary of requirementsThe active optics correction loop will here be used to guarantee the MTF requirements in PAN and multispectral channels by:
This active optics loop for the EO mission case relies on the following elements:
2.2Trade-Off of System Level ArchitectureBy a systematic approach, we set up a telescope only considering 3 main optical planes and 1 optional optical plane:
The defined parameters are the following:
We end up with 576 possible cases with the 3 main planes and 5184 cases with 4 planes. We used systematic criteria to reduce the cases to a few hundred and then case by case criteria to bring it down to 10. Below the criteria that have been retained, considered interesting for this study:
2.3Optical Design Trade-OffsTo limit the optical computation we selected the following criteria:
Finally 2 solutions were identified as promising:
2.4Performance AssesmentWe have simulated in CodeV the wavefront at the exit pupil level, after correction: Thus, with Matlab we have simulated the WFEoverall obtained, after correction with the deformable mirror, for different numbers of actuators on a perfect DM. The purpose of these simulations is to estimate approximately the minimum number of actuators necessary to correct the WFEoverall. Hereafter a table which resume the versatility of each optical architecture studied above. Each column represents the maximum authorized value of the parameter considered to hold the final specification MTF shall be > 5 points at Nyquist. This translates the margin to relax the parameter considered for each optical architecture. In conclusion we can say that:
Below is presented the first study of the FEM under NASTRAN of the telescope GeoHR, that is to say:
The size of the individual Si3N4 structures, MFD and MLA under the petal are the same than on the 18 petals model. Initial temperature at 20°C, on nodes of the whole FE model of the telescope. DisplacementsDisplacement calculated: The petals nodes are defined in local frames whose X and Y axis projected on the interface plane of the telescope are collinear to each other. The WFE is calculated for each of these frames in the largest inscribed circle in the hexagon. Thanks to Zerodur® used for the segmented primary mirrors, the thermal impact on the WFE is insignificant. 2.5Trade-offs2.6Design DescriptionThe configuration chosen by the TAS team is that (named N°8) with a segmented hexapod(MLA) on the first pupil and a segmented DM(NLA) on the second pupil and with a deployable front cavity. Main advantages:
The architecture for a GeoHR satellite has been retained to be compliant with the main requirement asking a MTF > 5 pts at Nyquist frequency. This architecture is summarized hereafter it consists in:
3.ACTIVE OPTICS IN DEPLOYABLE SYSTEMS FOR SCIENCE3.1The Astrophysical ContextTo be exhaustive, the science objectives of a Space Observatory[1] with diameter of collecting area > 12m are: Exploring the full diversity of exoplanets / Discovering and characterizing exoplanets in the habitable zones of Sun-like stars and searching for biosignatures in their atmospheres / Remote sensing of the planets, moons, and minor bodies of the solar system / Exploring the building blocks of galaxies both in the local universe and their emergence in the distant past, and elucidating the nature of dark matter / Understanding how galaxies form and evolve from active to passive, both by studying their stars and their gaseous fuel across all temperatures and phases / Following the history of stars in the local volume out to tens of megaparsecs to understand how they form and how they depend on their environment / Observing the birth of planets and understanding how the diversity of planetary systems arises. The most demanding one is the search and characterisation of exoplanets by direct imaging to allows their atmosphere characterisation [1]. Since about the early 80ths active optics techniques have became essential in large Earth-based telescopes (E-ELT, VLT, GMT, etc.) mainly for Atmophere correction (eXtrme Adaptive Optics). In addition coronagraphy technics allowed suppressing the stellar light. Static residual supperssion and Post-precessing are finishing isolating the flux from the exoplanet: If the contrast limitation is around 10-6 on ground the expectation in Space is about 10-10 using Apodized Pupil Lyot Coronagraphs (APLC) type with two internal deformable mirrors (DMs) that correct static optical field error to create a “dark hole” and can also correct some wavefront error drift on the order of 0,002 Hz (10 minute update rate) depending on the target star brightness:
A coronagraph dark hole is a region of the detector with low background signal that can support high contrast imaging. The dark hole for an APLC is typically annular and centered on the host star under observation, as in Figure 12. The inner radius of the dark hole is called the “inner working angle” and sets the limit for how close a planet can be to its host star and still be detected. The outer radius of the dark hole is called the “outer working angle” and is limited by the spatial resolution of the DMs (approximately the number of actuators across the telescope pupil divided by 2). The phrase “digging the dark hole” refers to the process by which the deformable mirrors are adjusted to correct for static errors in the telescope to create the annular region. This report is focused on stability – or “maintaining the dark hole” In the last 5 years at least scientists have set a few important points:
3.2Proposed Science case architecture and solutionsA space observatory LUVOIR type for such mission as exoplanet charactisation shall be focused on:
Principle:
Working directly on a picometer displacement & high stability is directly facing the physic laws, for reminder atomic radius ofHydrogen (H1) = 53pm. The use of a huge Sunshield in L2 will guaranty a very high level of thermal stability (~ mK), allowing to use the intrinsic thermal material stability. Smart structural design and characterisation will make it fit to requirements by adjusting temperature compatible with acceptable drift and time correction. Advantages:
Primary Mirror Deployment: A Two Parts OptionLooking forward simplification and efficiency the PM shall be Symetrical to ease optical and postprocessing correction, no-gap articulations for simplified on-ground end-stop calibration and in-orbit motorization control. Hereafter some 3D view of the TAS 2 parts design in stowed and deployed configuration: For the size of alf the PM diameter we can fit all the instrument and minize number of deployment and associated risks. The M2 is first deployed with the 3 tape spring used also as Secondary Mirror pointing mechanism. The central baffle is deployed after the M2. Finaly the two parts of the M1 are deployed and locked: The TAS solution is based on the 3 Tape Springs Deployment technologies with the following advantages:
Future structure will be a compromise of “smart” structure as “auxetic” shape to counterbalance thermoelastic and mechanical deformation during motion of the complete observatory and adjustment capabilities of the active segments (WFE and Piston/Tip/Tilt). CAD model thermo-mechanical sensitivity and WFE impact first results: No issues on both needed corrections and impact of thermal stability on the primary mirror WFE. 4.SYNTHESIS AND CONCLUSIONS4.1On GEO-HR CaseSuch an instrument will have on board correction of micrometric level mirrors (hexapod allowing an overall correction in piston and tip / tilt) but also at the nanometric level (optimization of the mirror surface). There are four technological products essential to the realization in the near future of the first high resolution instrument for monitoring the earth in real time and subjected to two dedicated unit validation: 4.2On Science CaseWay to proceed to reach the extreme requirements of exo-earth direct characterisation on Space Observatory such as LUVOIR-A:
The global approach is simplifed to an intrinsic stability observatory needed for such missions: The strategy: Step 1: Deployment of the M2 then M1, using internal metrology (capacitive, inductive, optical …) for first positioning (under 100nm) Step 2: Set the Coronagraph: Images & ACS tools to adjust the M1 & M2 positionning (pm level) Step 3: Dig the Dark Hole: Using the images to adjust the M1 (pm thermal adjustment) &Coronograph internal loop Step 4: Observation time: M1 correction anticipation (set&forget pm stability) & Coronograph internal loop control
The associated schedule for development and validation of the new technologies: AcknowledgmentsThe authors gratefully acknowledge support for the work provided by the European Space Agency under contract number 4000125154/18/NL/AR/ig “ACTIVE OPTICS IN DEPLOYABLE SYSTEMS FOR FUTURE EO AND SCIENCE MISSIONS”. REFERENCESThe LUVOIR Final Report_2019-08-26_NASA, Google Scholar
Coyle,
(2019). Google Scholar
Pueyo & Norman ApJ,
(2013) https://iopscience.iop.org/article/10.1088/0004-637X/769/2/102 Google Scholar
Ruane,
(2018) https://doi.org/10.1117/1.JATIS.4.1.015004 Google Scholar
ZImmerman,
in SPIE,
(2016). https://doi.org/10.1117/12.2233205 Google Scholar
Iva Laginja,
“Trade-Off analysis and Preliminay Design of the Active Optics System for the Science mision case,”
(2019). Google Scholar
Leboulleux, Sauvage, A. Pueyo,
“Pair-based Analytical model for Segmented Telescopes Imaging from Space for sensitivity analysis,”
https://doi.org/10.1117/1.JATIS.4.3.035002 Google Scholar
System Level Segmented Telescope Design (SLSTD), Google Scholar
Ultra-Stable Telescope Research and Analysis (ULTRA), Google Scholar
EOP-SF/2013-09-1757iss1_GEO-HR, Google Scholar
Astronomical Optics and Elasticity Theory – Active Optics Methods; G. Lemaitre, A&A library, Springer Ed.,
(2008). Google Scholar
Robert A. Gonsalves,
“Phase Retrieval And Diversity In Adaptive Optics,”
Opt. Eng., 21
(5), 215829
(1982). https://doi.org/10.1117/12.7972989 Google Scholar
Arnaud Liotard, Marc Bernot, Mikaël Carlavan, Frédéric Falzon, Thierry Fusco, Vincent Michau, Aurélie Montmerle-Bonnefois, Laurent Mugnier, Céline Engel, Clément Escolle, Marc Ferrari, Emmanuel Hugot, Thierry Bret-Dibat, David Laubier, Projet RASCASSE,
“Wave-front sensing for space active optics: Rascasse project,”
in SPIE 10563,
https://doi.org/10.1117/12.2304111 Google Scholar
Damien Sucher, Guillaume Butel, Guillaume Briche, Jean-François Blanc, Arnaud Liotard, Marc Bernot, Mikaël Carlavan, Aurélien Suau, Nisrine Louh, Lauriane Galtier, Sebastien Guionie, Thierry Viard, Stéphanie Behar-Lafenetre, Fabrice Champandard, Jean-Bernard Ghibaudo, Vincent Costes, Optique active sur démonstrateur,
“Active optics for space telescopes,”
in Proc. SPIE,
1111611
(2019). https://doi.org/10.1117/12.2529103 Google Scholar
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