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The achievement of $\mu$arcsec relative astrometry with ground-based, nearinfrared, extremely large telescopes requires a significant endeavour ofcalibration strategies. In this paper we address the removal of instrumentoptical distortions coming from the ELT first light instrument MICADO and itsadaptive optics system MAORY by means of an astrometric calibration mask. Theresults of the test campaign on a prototype mask (scale 1:2) has probed themanufacturing precision down to $\sim$ 50nm/1mm scale, leading to a relativeprecision $\delta\sigma \sim 5e-5$. The assessed manufacturing precisionindicates that an astrometric relative precision of $\delta\sigma \sim 5e-5 =\frac{50\mu as}{1 arcsec}$ is in principle achievable, disclosing $\mu$arcsecnear infrared astrometry behind an extremely large telescope. The impact of$\sim$ 10-100 nm error residuals on the mask pinholes position is tolerable ata calibration level as confirmed by ray tracing simulations of realistic MICADOdistortion patterns affected by mid spatial frequencies residuals. Wedemonstrated that the MICADO astrometric precision of 50 $\mu$as is achievablealso in presence of a mid spatial frequencies pattern and manufacturing errorsof the WAM by fitting the distorted WAM pattern seen through the instrumentwith a 10$^{th}$ order Legendre polynomial.

Development of the Warm Astrometric Mask for MICADO astrometry calibration