The impact of ELT distortions and instabilities on future astrometric observations

TL;DR

This study evaluates how optical distortions and instabilities in the ELT affect high-precision astrometric measurements, emphasizing the need for frequent calibration to achieve 50 microarcsecond accuracy.

Contribution

It provides a detailed simulation-based assessment of ELT distortions and proposes calibration strategies to attain the desired astrometric precision.

Findings

RMS distortions range from 0.1 to 5 mas over 1 arcmin field.

Higher order residuals can be reduced to 10-20 μas with proper calibration.

Frequent on-sky calibrations and MCAO stabilization are necessary for 50 μas astrometric accuracy.

Abstract

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 $\sim$ 0.1-5 mas over 1 arcmin field of view. Over minutes timescales 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 $\sim$ 5 mas comparable to the diffraction-limited PSF size $FWHM_H = 8.5$ mas. The RMS distortions inherent to the ELT design are confined to the 1$^{st}$-3$^{rd}$ order and reduce to an astrometric RMS residual post fit of $\sim$ 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 timescales 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 $\leq 50 \mu as$, which is one of the core science missions for the ELT / MICADO instrument.