  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 $\mu$as absolute differential astrometry (measure
star-to-star distances in absolute $\mu$as units) over a 53" FoV in the range
1.2-2.5 $\mu$m. 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.
