Differential image motion in astrometric observations with very large seeing-limited telescopes

TL;DR

This paper models differential image motion in large telescopes, demonstrating how atmospheric turbulence affects astrometric precision and estimating limits for future extremely large telescopes.

Contribution

It introduces a quantitative model linking atmospheric turbulence to differential image motion, accounting for telescope and observation parameters, and estimates astrometric limits for future telescopes.

Findings

Model agrees with observations within 1%

Image motion limits astrometric precision to ~60 micro-arcseconds for current telescopes

Extrapolated limits suggest ~5 micro-arcseconds for extremely large telescopes

Abstract

We investigate how to quantitatively model the observed differential image motion (DIM) in relative astrometric observations. As a test bed we used differential astrometric observations from the FORS2 camera of the Very Large Telescope (VLT) obtained during 2010-2019 under several programs of observations of southern brown dwarfs. The measured image motion was compared to models that decompose atmospheric turbulence in frequency space and translate the vertical turbulence profile into DIM amplitude. This approach accounts for the spatial filtering by the telescope's entrance pupil and the observation parameters (field size, zenith angle, reference star brightness and distribution, and exposure time), and it aggregates that information into a newly defined metric integral term. We demonstrate excellent agreement (within 1%) between the model parameters derived from the DIM variance and determined by the observations. For a 30s exposure of a typical 1 arcmin-radius field close to the Galactic plane, image motion limits astrometric precision to ~60 mu-as when sixth-order transformation polynomial is applicable. We confirm that the measured image motion variance is well described by Kolmogorov-type turbulence with exponent 11/3 dependence on the field size at effective altitudes of 16-18~km, where the best part of the DIM is generated. Extrapolation to observations with extremely large telescopes enables the estimation of the astrometric precision limit for seeing-limited observations of ~5 mu-as, which has a variety of exciting scientific applications.