Instrumentation compacte pour l’astronomie : intégrer la spectroscopie et la polarisation au niveau du détecteur

This thesis assesses the potential of gathering several instrumental functions into a bolometer array for sub-millimetric astronomical observations. This possibility is now conceivable thanks to the recent progress made in micro-technologies.First, we optimized the design of polarimetric pixels invented for the B-BOP instrument of the future space observatory SPICA. This work enabled the quantification of the cross-polarization (1/1000) and to rethink the geometry of the pixels in order to obtain detectors better matched to incident radiation.This thesis has also been an opportunity to deal with the integration of spectroscopy within the focal plane, independently from the polarimetry aspect. We accordingly focused on Fabry-Pérot (FP) interferometry, as an FP in a collimated beam can well-suited for imaging. Nonetheless, we chose to integrate the spectrometer closer to the detector (and thus in the convergent beam). We also initiated the development of an FP made from high-resistivity silicon in order to lower the losses due to metallic mirrors, generally used in this range of wavelengths. With the objective to enhance the spectral capabilities of the FP, mirrors are built as a stack of silicon layers, separated by vacuum (Bragg mirrors). This increases the reflectivity of the mirrors while keeping the complexity to a reasonable level : a finesse of 215, for instance, is expected at 320 µm for a FP using Si/vacuum/Si mirrors. As a next step, we studied the detector/FP coupling which is enhanced by the resonance of two optical cavities formed by the whole system. Eventually, calculations showed that an FP with a moderate finesse (150) put in the focal plane barely deteriorates imaging or spectroscopy.By the end of this thesis, several FP etalons have been built and have already demonstrated favorable properties: we obtained a spectral resolution of 180. Moreover, measurements showed that silicon has a negligible absorption at a temperature of 77 K.Cette thèse étudie la possibilité d'intégrer plusieurs fonctions instrumentales au sein de la matrice de bolomètres pour les observations dans le domaine du sub-millimétrique. Ceci est désormais envisageable grâce aux progrès des micro-technologies.Dans un premier temps, nous avons optimisé le design des pixels polarimétriques inventés pour l'instrument B-BOP du futur observatoire spatial SPICA. Ce travail a notamment permis de quantifier la cross-polarisation (1/1000) et de repenser la géométrie des pixels afin d'obtenir des détecteurs mieux adaptés au rayonnement incident.Cette thèse a également été l'occasion, et ce de manière indépendante à la polarimétrie, de réfléchir à l'intégration de la spectroscopie au sein du plan focal. Pour cela, nous nous sommes orientés vers de l'interférométrie de type Fabry Pérot (FP). Un FP dans un faisceau collimaté présente l'avantage d'être très facilement compatible avec l'imagerie. Toutefois, nous avons choisi d'intégrer le spectromètre au plus près du détecteur (et donc dans le faisceau convergent). Nous avons également initié le développement de FP tout silicium (Si) à haute résistivité afin de réduire les pertes dûes aux miroirs métalliques, conventionnellement utilisés dans nos gammes de longueurs d'onde. Afin d'améliorer les performances spectrales du FP, les miroirs sont fabriqués via un empilement de couches de silicium interposés de vide tels des miroirs de Bragg. Cela permet d'augmenter rapidement le coefficient de réflexion des miroirs sans toutefois en augmenter trop la complexité: une finesse de 215 est, par exemple, attendue à 320 µm pour un FP utilisant des miroirs Si/vide/Si. Ensuite, nous avons étudié le couplage détecteur/FP qui se voit renforcé par la résonance des deux cavités optiques formées par le système complet. Enfin, des calculs ont montré qu'un FP avec une finesse raisonnable (150) mis dans le plan focal ne dégrade que très peu l'imagerie et la spectroscopie.A la fin de cette thèse, plusieurs étalons FP ont été réalisés et ont déjà démontré des propriétés intéressantes: une résolution spectrale de 180 a notamment été obtenue. En plus de cela, les mesures ont montré que le silicium avait une absorption négligeable à 77 K

Dissecting Star-forming regions with the GeMS MCAO instrument: lessons learned for optimal post-processing of WFAO data

The advent of a new generation of Wide Field AO (WFAO) systems marks the beginning of a new era in high spatial resolution imaging. By using multiple Laser Guide Stars, WFAO significantly increases the field of view of the AOcorrected images, and the fraction of the sky that can benefit from such correction. The newly commissioned Gemini South Multi-Conjugate Adaptive Optics System (GeMS) combined with the infrared camera GSAOI is delivering almost di raction-limited images over a field of   2 arc-minutes across. In this paper, we first present recent observations of the young star-forming region N159W located in the Large Magellanic Cloud. We obtained deep JHKs images from the GeMS/ GSAOI instrument and developed reduction tools, in order to photometrically study the properties of the stellar members of the cluster and to bring new elements to our understanding of the process of massive star formation. However, despite the excellent performance of the GeMS/ GSAOI system, some variable residues are still limiting the correction quality over the field. In particular, GSAOI is severely a ected by distortion that can strongly degrade the resolution when combining multiple frames and can consequently reduce the sensitivity. The accuracy of the distortion correction of an instrument is critical for its use for high- precision astrometry and photometry. In a second part of this paper, we investigate an optimal way to correct for the distortion following an inverse problem approach. The formalism as well as first simulation results are presented.

On-chip spectroscopic solutions for polarimetric bolometer arrays in submillimeter astronomy

International audienceTwo technologies of all-silicon on-chip spectrometers based on the Fabry-Perot interferometer principle are studied and under development for a target wavelength of 158µm ([CII]). We are developing these spectroscopic capabilities with the objective of including them in polarimetric detector arrays cooled at 50mK. The first solution is a tunable cavity Fabry-Perot with silicon mirrors driven by cryogenic piezoelectric motors with a sub-micron step size. Each mirror is a dielectric Bragg structure made of quarter-wave layers of silicon and air providing a high reflectivity without metal losses. The theoretical performance of a Fabry-Perot resonator with such Bragg mirrors has been confirmed by measurement in a low temperature FTS: the finesse of this interferometer is more than twice that of a traditional Fabry-Perot. The second solution is a fixed Fabry-Perot array with a silicon microstructured cavity, which allows having different optical indices in different areas. The cavity is made of deep-etched silicon microstructures whose section is adapted to obtain the adequate optical index. Therefore, multiple wavelengths around 158µm, distributed on the array, are transmitted by this Fabry-Perot. The mirrors of this spectrometer are metallic capacitive grids designed to be highly reflective at the targeted wavelength, easy to manufacture with reduced metal losses. The simulations show high performances in resolution, close to the Bragg mirrors Fabry-Perot. The first prototypes of this solution have been manufactured by the CEA/LETI and will be soon measured in the cryogenic facilities in Saclay

Development of Tropical Cyclones from African Easterly Waves : comparative study of three African Easterly Wave events during the CADDIWA campaign

During the CADDIWA (Cloud-Atmospheric Dynamics-Dust Interaction in West Africa) airborne campaign that took place in September 2021, three tropical disturbances initiated from African Easterly Waves were sampled. Two of them reached the Tropical Storm state and were named Rose and Peter by the National Hurricane Center. The last one, later named Pierre-Henri, failed to develop. After a validation of reanalysis data for september 2021 against the data collected during the campaign, a comparative study of the three events will be conducted using ERA5, satellite and campaign data, with a focus on energy budgets. Several processes that may be catalysts or inhibiters for tropical cyclogenesis (AEW-Monsoon -, AEW-AEW -, dust-dynamics interaction ...) will especially be discussed

The optical bread-board models of the LiteBIRD Medium and High Frequency Telescope

International audienceIn this paper we discuss the modeling, development and testing of the optical bread-board models of the Medium and High Frequency Telescope (MHFT) onboard the LiteBIRD satellite. The future JAXA mission LiteBIRD will search for the signature of primordial gravitational waves through the measurement of the “B-modes” of the cosmic microwave background polarization. MHFT will observe the polarized microwave sky between 89 and 448 GHz by means of two refractive telescopes. The accurate knowledge of their optical properties is fundamental to assess the impact of systematic effects (e.g. beam deformation, side lobes and intensity to polarization leakage) on the future observations. To gain early experience with our test approach, and to provide hints of possible criticalities in the design and characterization of a MHFT-like refractive system, we developed two optical bread-board models. The BB1, a single dielectric lens coupled to a fully characterized W-band corrugated horn, allows us to assess the accuracy and potential limitations of different measurement methods and to verify the reliability of optical simulators in predicting refractive elements and systems, as compared to the precision required by LiteBIRD. The BB2, a 1⁄2-scaled version of the Medium Frequency Telescope, focuses on the modelling and issues of dual-lens coupling, while providing a test-bed to finalize the MHFT optical calibration plan for its higher levels of integration

BRAHMS: polarimetric bolometer arrays for the SPICA observatory camera (Conference Presentation)

International audienceIn the last decades, a very large effort has been made to measure, with high sensitivity, the intensity and spectral contents of millimetric (mm) and submillimetric (submm) light from the Universe. Today this picture is in the way to be routinely completed by polarization measurements that give access to previously hidden processes, for example the traces of primordial gravitational waves in the case of CMB (mainly mm), or the effect of magnetic field for star formation mechanisms (submm and mm optical ranges). The classical way to measure the light polarization is to split the two components by a polarizer grid and record intensities with two conjugated detection setups. This approach implies the deployment of a complex instrument system, very sensitive to external constraints (vibrations, alinement, thermal expansion…), or internal ones: determine low degrees of polarization implies a large increase in sensitivity when compared with intensity measurements. The need of detector arrays, with in pixel polarization measurement capabilities, has been well understood for years: all the complexity being reported at the focal plane level. Subsequently, the instrument integration, verification and tests procedure is considerately alleviated, specially for space applications.All silicon bolometer arrays using the same micromachining techniques than the Herschel PACS modules are well suited for this type of development. New thermometers doped for 50 mK operations permit to achieve, with a new design, sensitivities close to the aW/√Hz. It is based on all-legs bolometers (ALB), where the absorbing, insulating and thermometric functions are made by the same suspended silicon structure. This ALB structure, with in this case a spiral design, permits to separate the absorption of the two electromagnetic components of the light polarization. Each pixel consists of four bolometer divided in two pairs, each sensitive to one direction of polarization. This permits to combine the bolometer bridges in a fully differential global structure with a Wheatstone bridge arrangement. Total intensity and polarization unbalance are available directly at the detector level, thanks to a cold readout circuit integrated in the detector structure. This combination of functions is achieved by above IC manufacture techniques (IC for Integrated Circuit).All these developments take place in the prospect of the joint JAXA-ESA SPICA project, to equip a 1344 pixels polarimetric and imaging camera covering three spectral bands (100, 200 and 350 µm)

Thinice : campagne de mesure aéroportée dédiée à l'étude des dépressions arctiques et des interactions avec les nuages et la banquise

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The optical design of the Litebird Medium and High Frequency Telescope

International audienceLiteBIRD is a JAXA strategic L-class mission devoted to the measurement of polarization of the Cosmic Microwave Background, searching for the signature of primordial gravitational waves in the B-modes pattern of the polarization. The onboard instrumentation includes a Middle and High Frequency Telescope (MHFT), based on a pair of cryogenically cooled refractive telescopes covering, respectively, the 89-224 GHz and the 166-448 GHz bands. Given the high target sensitivity and the careful systematics control needed to achieve the scientific goals of the mission, optical modeling and characterization are performed with the aim to capture most of the physical effects potentially affecting the real performance of the two refractors. We describe the main features of the MHFT, its design drivers and the major challenges in system optimization and characterization. We provide the current status of the development of the optical system and we describe the current plan of activities related to optical performance simulation and validation

Cyclogenesis in the Tropical Atlantic: First Scientific Highlights from the Clouds–Atmospheric Dynamics–Dust Interactions in West Africa (CADDIWA) Field Campaign

International audienceDuring the boreal summer, mesoscale convective systems generated over West Africa propagate westward and interact with African easterly waves, and dust plumes transported from the Sahel and Sahara by the African easterly jet. Once off West Africa, the vortices in the wake of these mesoscale convective systems evolve in a complex environment sometimes leading to the development of tropical storms and hurricanes, especially in September when sea surface temperatures are high. Numerical weather predictions of cyclogenesis downstream of West Africa remains a key challenge due to the incomplete understanding of the clouds–atmospheric dynamics–dust interactions that limit predictability. The primary objective of the Clouds–Atmospheric Dynamics–Dust Interactions in West Africa (CADDIWA) project is to improve our understanding of the relative contributions of the direct, semidirect, and indirect radiative effects of dust on the dynamics of tropical waves as well as the intensification of vortices in the wake of offshore mesoscale convective systems and their evolution into tropical storms over the North Atlantic. Airborne observations relevant to the assessment of such interactions (active remote sensing, in situ microphysics probes, among others) were made from 8 to 21 September 2021 in the tropical environment of Sal Island, Cape Verde. The environments of several tropical cyclones, including Tropical Storm Rose, were monitored and probed. The airborne measurements also serve the purpose of regional model evaluation and the validation of spaceborne wind, aerosol and cloud products pertaining to satellite missions of the European Space Agency and EUMETSAT (including the Aeolus, EarthCARE, and IASI missions)

The THINICE field campaign: Interactions between Arctic cyclones, tropopause polar vortices, clouds and sea ice in summer

The THINICE field campaign, based from Svalbard in August 2022, provided unique observations of summertime Arctic cyclones, their coupling with cloud cover, and interactions with tropopause polar vortices and sea ice conditions. THINICE was motivated by the need to advance our understanding of these processes and to improve coupled models used to forecast weather and sea ice, as well as long-term projections of climate change in the Arctic. Two research aircraft were deployed with complementary instrumentation. The Safire ATR42 aircraft, equipped with the RALI (RAdar-LIdar) remote sensing instrumentation and in-situ cloud microphysics probes, flew in the mid-troposphere to observe the wind and multi-phase cloud structure of Arctic cyclones. The British Antarctic Survey MASIN aircraft flew at low levels measuring sea-ice properties, including surface brightness temperature, albedo and roughness, and the turbulent fluxes that mediate exchange of heat and momentum between the atmosphere and the surface. Long duration instrumented balloons, operated by WindBorne Systems, sampled meteorological conditions within both cyclones and tropospheric polar vortices across the Arctic. Several novel findings are highlighted. Intense, shallow low-level jets along warm fronts were observed within three Arctic cyclones using the Doppler radar and turbulence probes. A detailed depiction of the interweaving layers of ice crystals and supercooled liquid water in mixed-phase clouds is revealed through the synergistic combination of the Doppler radar, the lidar and in-situ microphysical probes. Measurements of near-surface turbulent fluxes combined with remote sensing measurements of sea ice properties are being used to characterize atmosphere-sea ice interactions in the marginal ice zone