The impact of ELT distortions and instabilities on future astrometric observations

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 ∼ 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 ∼ 5 mas comparable to the diffraction-limited PSF size FW H M H = 8.5 mas. The RMS distortions inherent to the ELT design are confined to the 1 st -3 r d order and reduce to an astrometric RMS residual post fit of ∼ 10-20 µas 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 ≤ 50µas, which is one of the core science missions for the ELT / MICADO instrument.

Astrometric planet search around southern ultracool dwarfs

“…We demonstrated that chip array instabilities can seriously degrade high-precision astrometry if left uncorrected. The MICADO near-infrared camera (Davies et al 2010) for the ELT aims at a single-epoch accuracy of ≤50 µas over an FOV of 53 (Rodeghiero et al 2018;Pott et al 2018). The detector of MICADO is a 3 × 3 array, and if the reference field is expanded over the full FoV, the RCM effect can fragment it into 3 × 3 segments.…”

“…The further accuracy improvement beyond 0.1 mas, needed for the detection of Neptune-and lower-mass companions to ultracool dwarfs, is restrained by fundamental limits on the precision of photocenter determination, the atmospheric image motion caused by turbulence in the upper atmospheric layers, and by systematic errors of instrumental origin. It is expected that in the near future the achievable astrometric accuracy will be improved to 0.01-0.05 mas on 30 m class telescopes such as the European Extremely Large Telescope (ELT), primarily due to the use of adaptive optics, better signal-to-noise ratio in the stellar images, and general performance improvements of the cameras (Davies et al 2010;Rodeghiero et al 2018;Pott et al 2018).…”

The MICADO first light imager for ELT: its astrometric performance

“…Since we only seek to estimate three parameters, averaging over more than three stars is quickly giving higher accuracy. We showed this to be an efficient strategy for stellar clusters in our Galaxy (3) . Extragalactic targets will require selfcalibration between epochs using the fact that the global stretch or rotation of the object is negligible.…”

“…For the ELT, the plate scale stability during observations is expected as 10 -4 , so 2mas over the 20arcsec scale length. Thus for the ELT, distortion drift errors σ plate.scale is 20times larger than σ 3rd.order (3) . The drift of larger than third order distortion terms of the opto-mechanics are negligible in the here discussed context.…”

Performance and limitations of using ELT and MCAO for 50 μas astrometry

“…This constraint is typically fulfilled for galactic targets and some extragalactic targets nearby (see discussion in Ref. 13) and is well known from previous relative astrometric mission, both space-and ground-based. In particular, large ground-based astrometry relies on low-order astrometric (plate-scale) calibration via reference stars since atmospheric refraction and telescope alignment are never perfectly stable at the required precision levels.…”