Experimental tests of the calibration of high precision differential astrometry for exoplanets

High precision differential Astrometry is the branch of astronomy that evaluates the relative position, distance and motion of celestial objects with respect to the stars present in the field of view. A mission called Theia has been submitted in 2022 for ESA's M7 call for missions, using a diffraction-limited telescope about 1m in diameter and with a field of view of 0.5 degrees, capable of achieving sub-micro-arcsecond angular accuracy, corresponding to 1e-5 pixel on the detector. Such precision makes it possible to study the nature of dark matter in our galaxy and to reveal the architecture of exoplanetary systems close to the Sun, down to the mass of the Earth. The aim of the experimental tests presented in this poster is to improve the TRL of 2 specific aspects: the calibration of new CMOS detectors with very large number of pixels and the calibration of the telescope aberrations.First, a key element of such a space telescope is the focal plane, which must be calibrated spatially with an extreme precision down to the 1e-5 pixel level. Previous work has shown that this is possible with small detector matrices (80x80 px) [1]. The goal is now to check the performances and validate this method with the new very large detectors. Pyxalis, a company based near Grenoble, is developing very large detectors (8000x5000 px) that have a low noise level and high sensitivity. The aim is to characterize and validate this type of detectors in a laboratory demonstration (see poster Pancher et al.), to ensure that the performance achieved meets the required specifications. We present the results of these characterization in this contribution.The telescope stability is also a sensitive issue. Recent work [2] has shown that the reference stars in the field of the telescope can be used as actual metrology sources in order to compute the field distortion function. Our simulations allow to model the optical aberrations with bivariate polynoms. The effects on the calibration accuracy of the degrees of the polynoms, the number of reference stars and the tilt perturbation of the M2 mirror are investigated. This poster will present the latest results obtained on a test bed developed to experimentally study the performances of this new field calibration method.