The impact of ELT distortions and instabilities on future astrometric observations

The paper discusses an assessment study about the impact of the distortions on the astrometric observations with the Extremely Large Telescope originated from the optics positioning errors and telescope instabilities. Optical simulations combined with Monte Carlo approach reproducing typical inferred opto-mechanical and dynamical instabilities show rms distortions between similar to 0.1 and 5 mas over 1 arcmin field of view (FoV). Over minutes time-scales the plate scale variations from ELT-M2 caused by wind disturbances and gravity flexures and the field rotation from ELT-M4-M5 induce distortions and PSF jitter at the edge of 1 arcmin FoV (radius 35 arcsec) up to similar to 5 mas comparable to the diffraction-limited PSF size FWHMH = 8.5 mas. The rms distortions inherent to the ELT design are confined to the first to thrid order and reduce to an astrometric rms residual post fit of similar to 10-20 mu as for higher order terms. In this paper, we study which calibration effort has to be undertaken to reach an astrometric stability close to this level of higher order residuals. The amplitude and time-scales of the assumed telescope tolerances indicate the need for frequent on-sky calibrations and MCAO stabilization of the plate scale to enable astrometric observations with ELT at the level of