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Open access funding provided by Istituto Nazionale di Astrofisica within the CRUI-CARE Agreement. Publisher Copyright: © 2021, The Author(s). Sky survey telescopes and powerful targeted telescopes play complementary roles in astronomy. In order to investigate the nature and characteristics of the motions of very faint objects, a flexibly-pointed instrument capable of high astrometric accuracy is an ideal complement to current astrometric surveys and a unique tool for precision astrophysics. Such a space-based mission will push the frontier of precision astrometry from evidence of Earth-mass habitable worlds around the nearest stars, to distant Milky Way objects, and out to the Local Group of galaxies. As we enter the era of the James Webb Space Telescope and the new ground-based, adaptive-optics-enabled giant telescopes, by obtaining these high precision measurements on key objects that Gaia could not reach, a mission that focuses on high precision astrometry science can consolidate our theoretical understanding of the local Universe, enable extrapolation of physical processes to remote redshifts, and derive a much more consistent picture of cosmological evolution and the likely fate of our cosmos. Already several missions have been proposed to address the science case of faint objects in motion using high precision astrometry missions: NEAT proposed for the ESA M3 opportunity, micro-NEAT for the S1 opportunity, and Theia for the M4 and M5 opportunities. Additional new mission configurations adapted with technological innovations could be envisioned to pursue accurate measurements of these extremely small motions. The goal of this White Paper is to address the fundamental science questions that are at stake when we focus on the motions of faint sky objects and to briefly review instrumentation and mission profiles. publishersversion published

The PicSat mission is based on a 3U Cubesat architecture, with a payload specifically designed for high precision stellar photometry. The satellite is planned to be launched in September 2017. The main objective of the mission is the continuous monitoring of the brightness of Beta Pictoris. The Hill Sphere of planet Beta Pictoris b shall pass in front of its host during this period (from April 2017 to February 2018). To be ready on time for this rendez-vous, our philosophy was to focus our resources on the development of the payload and the flight software, where we have the expertise. We subcontracted the design and the realization of the platform and of the Attitude and Determination Control System. The payload is designed with a 37mm effective aperture and a single pixel avalanche photodiode. A single-mode fiber is used to guide the stellar light from the focal plane to the photodiode. To guarantee photometric precision and payload stabilization, the residual jitter of the three-axis ADCS is complemented with a two-axis piezoelectric actuation system that locks the position of the fiber on the center of the star. The flight software is based on a "L0/L1" dual-level architecture, making use of Gericos, an active object framework developed in-house.

With the present state of exoplanet detection techniques, none of the rocky planets of the Solar System would be discovered, yet their presence is a very strong constraint on the scenarios of formation of planetary systems. Astrometry, by measuring the reflex effect of planets on their central host stars, lead us to the mass of planets and to their orbit determination. This technique is used frequently and is very successful to determine the masses and the orbits of binary stars. From space, it is possible to use differential astrometry around nearby Solar-type stars to detect exoplanets down to one Earth mass in habitable zone, where the sensitivity of the technique is optimal. Finding habitable Earths in the Solar neighborhood would be a major step forward for exoplanet detection and these planets would be prime targets for attempting to find life outside of the Solar System, by searching for bio-markers in their atmospheres.A scientific consortium has formed to promote this kind of astrometric space mission. A mission called NEAT (Nearby Earth Astrometric Telescope) has been proposed to ESA in 2010. A laboratory testbed called NEAT-demo was assembled at IPAG, its main goal is to demonstrate CCD detector calibration to the required accuracy. During my PhD, my activities were related to astrophysical aspects as well as instrumental aspects of the mission. Regarding the scientific case, I compiled a catalog of mission target stars and reference stars (needed for the differential astrometric measurements) and I estimated the scientific return of NEAT-like missions in terms of number of detected exoplanets and their parameter distributions. The second aspect of the PhD is relative to the testbed, which mimics the NEAT telescope configuration. I am going to present the testbed itself, the data analysis methods and the results. An accuracy of 3e-4 pixel was obtained for the relative positions of artificial stars and we have determined that measures of pixel positions by the metrology is currently limited by stray light., Dans l’état actuel des techniques de détection des exoplanètes, aucune planète tellurique du Système Solaire ne pourrait être découverte et pourtant leur présence est une contrainte très forte sur les scénarios de formation des systèmes planétaires. L’astrométrie, en mesurant l’effet réflexe des planètes sur leur étoile centrale, permet de remonter à la masse des planètes et aux paramètres orbitaux. C'est une technique très utilisée pour la détermination des masses et des orbites des étoiles binaires et couronnée de succès. Depuis l'espace, il est possible d'utiliser l'astrométrie différentielle autour des étoiles proches de type solaire pour détecter leurs exoplanètes à partir d'une masse terrestre en zone habitable, là ou la sensibilité de cette technique est optimale. Trouver des Terres habitables au voisinage du système solaire constituerait une avancée majeure en terme de détection d'exoplanètes et ces planètes seraient des cibles de 1er choix pour tenter de détecter de la vie au delà du système solaire, en cherchant des bio-marqueurs dans leur atmosphère.Un consortium scientifique s'est formé pour promouvoir une telle mission spatiale astrométrique. Une mission appelée NEAT (Nearby Earth Astrometric Telescope) a été proposée à l'ESA en 2010. Un banc de test appelé NEAT-demo a été assemblé à l'IPAG, son but principal est de démontrer la calibration du détecteur CCD à la précision requise. Mes activités pendant la thèse se sont portées à la fois sur les aspects astrophysiques et instrumentaux de la mission. En ce qui concerne le cas scientifique, j'ai établi un catalogue des étoiles cibles de la mission et des étoiles de référence (pour la mesure astrométrique différentielle), puis estimé le retour scientifique d'une mission spatiale telle que NEAT en terme de nombre d'exoplanètes détectées et de la distribution de leurs paramètres. Le second volet de la thèse est centré autour du banc de test, qui imite la configuration du télescope spatial NEAT. Je vais présenter le banc lui-même, les méthodes d'analyse des données et les résultats obtenus. Une précision de 3e-4 pixel a été obtenue sur les positions relatives des étoiles artificielles et nous avons déterminé que la mesure de position des pixels par la métrologie est limité par la lumière parasite.