Towards an overall astrometric error budget with MICADO-MCAO

MICADO is the Multi-AO Imaging Camera for Deep Observations, and it will be one of the first light instruments of the Extremely Large Telescope (ELT). Doing high precision multi-object differential astrometry behind ELT is particularly effective given the increased flux and small diffraction limit. Thanks to its robust design with fixed mirrors and a cryogenic environment, MICADO aims to provide 50 μas absolute differential astrometry (measure star-to-star distances in absolute μas units) over a 53" FoV in the range 1.2-2.5 μm. Tackling high precision astrometry over large FoV requires Multi Conjugate Adaptive Optics (MCAO) and an accurate distortion calibration. The MICADO control scheme relies on the separate calibration of the ELT, MAORY and MICADO systematics and distortions, to ensure the best disentanglement and correction of all the contributions. From a system perspective, we are developing an astrometric error budget supported by optical simulations to assess the impact of the main astrometric errors induced by the telescope and its optical tolerances, the MCAO distortions and the opto-mechanical errors between internal optics of ELT, MAORY and MICADO. The development of an overall astrometric error budget will pave the road to an efficient calibration strategy complementing the design of the MICADO calibration unit. At the focus of this work are a number of opto-mechanical error terms which have particular relevance for MICADO astrometry applications, and interface to the MCAO design.