No 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.

Adaptive Optics (AO) systems in large telescopes do not only correct atmospheric phase disturbances, but they also telescope structure vibrations induced by wind or telescope motions. Often the additional wavefront error due to mirror vibrations can dominate the disturbance power and contribute significantly to the total tip-tilt Zernike mode error budget. Presently, these vibrations are compensated for by common feedback control laws. However, when observing faint natural guide stars (NGS) at reduced control bandwidth, high-frequency vibrations (>5 Hz) cannot be fully compensated for by feedback control. In this paper, we present an additional accelerometer-based disturbance feedforward control (DFF), which is independent of the NGS wavefront sensor exposure time to enlarge the "effective servo bandwidth". The DFF is studied in a realistic AO end-to-end simulation and compared with commonly used suppression concepts. For the observation in the faint (>13 mag) NGS regime, we obtain a Strehl ratio by a factor of two to four larger in comparison with a classical feedback control. The simulation realism is verified with real measurement data from the Large Binocular Telescope (LBT); the application for on-sky testing at the LBT and an implementation at the E-ELT in the MICADO instrument is discussed.

MICADO will equip the E-ELT with a first light capability for diffraction limited imaging at near-infrared wavelengths. The instrument's observing modes focus on various flavours of imaging, including astrometric, high contrast, and time resolved. There is also a single object spectroscopic mode optimised for wavelength coverage at moderately high resolution. This contribution provides an overview of the key functionality of the instrument, outlining the scientific rationale for its observing modes. The interface between MICADO and the adaptive optics system MAORY that feeds it is summarised. The design of the instrument is discussed, focussing on the optics and mechanisms inside the cryostat, together with a brief overview of the other key sub-systems.