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.
The achievement of µarcsec relative astrometry with ground-based, near infrared, extremely large telescopes requires a significant endeavour of calibration strategies. In this paper we address the removal of instrument optical distortions coming from the ELT first light instrument MICADO and its adaptive optics system MAORY by means of an astrometric calibration mask. The results of the test campaign on a prototype mask (scale 1:2) has probed the manufacturing precision down to ∼ 50nm/1mm scale, leading to a relative precision δσ ∼ 5e − 5. The assessed manufacturing precision indicates that an astrometric relative precision of δσ ∼ 5e − 5 = 50µas 1ar csec is in principle achievable, disclosing µarcsec near infrared astrometry behind an extremely large telescope. The impact of ∼ 10-100 nm error residuals on the mask pinholes position is tolerable at a calibration level as confirmed by ray tracing simulations of realistic MICADO distortion patterns affected by mid spatial frequencies residuals. We demonstrated that the MICADO astrometric precision of 50 µas is achievable also in presence of a mid spatial frequencies pattern and manufacturing errors of the WAM by fitting the distorted WAM pattern seen through the instrument with a 10 th order Legendre polynomial.
We report on our ongoing efforts to ensure that the MICADO NIR imager reaches differential absolute (often abbreviated: relative) astrometric performance limited by the SNR of typical observations. The exceptional 39m diameter collecting area in combination with a powerful multi-conjugate adaptive optics system (called MAORY) brings the nominal centroiding error, which scales as FWHM/SNR, down to a few 10 µas. Here we show that an exceptional effort is needed to provide a system which delivers adequate and calibrateable astrometric performance over the full field of view (up to 53 arcsec diameter).
Multi-conjugated adaptive optics (MCAO) is essential for performing astrometry with the Extremely Large Telescope (ELT). Unlike most of the 8-m class telescopes, the ELT will be a fully adaptive telescope, and a significant portion of the adaptive optics (AO) dynamic range will be depleted by the correction and stabilization of the telescope aberrations and instabilities. MCAO systems are of particular interest for ground-based astrometry since they stabilize the low-order field distortions and transient plate scale instabilities, which originate from the telescope and in the instrument. All instruments have several optical elements relatively far away from the pupil that can potentially challenge the astrometric precision of the observations with their residual mid-spatial frequencies errors. Using a combined simulation of ray tracing and AO numerical codes, we assess the impact of these systematic errors at different field-ofview (FoV) scales and fitting scenarios. The distortions have been assessed at different sky position angles (PA) and indicate that over large FoVs only small PA ranges (AE1 deg to 3 deg) are accessible with astrometric residuals ≤50 μas. A full compliance with the astrometric requirement, at any PA, is achievable for 2 arc sec 2 FoV patches already with a third-order polynomial. The natural partition of the optical system into three segments, i.e., the ELT, the MAORY MCAO module, and the MICADO instrument, resembles a splitting of the astrometric problem into the three subsystems that are characterized by different distortion amplitudes and calibration strategies. The result is a family portrait of the different optical segments with their specifications, dynamic motions, conjugation height, and AO correctability, leading to tracing their role in the bigger puzzle of the 50-μas as astrometric endeavor. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
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