Abstract
The European Extremely Large Telescope (ELT) is a 39m large, ground-based optical and near- to mid-infrared telescope under construction in the Chilean Atacama desert. Operation is planned to start around the middle of the next decade. All first light instruments will come with wavefront sensing devices that allow control of the ELT’s intrinsic M4 and M5 wavefront correction units, thus building an adaptive optics (AO) system. To take advantage of the ELT’s optical performance, full diffraction-limited operation is required and only a high performance AO system can deliver this. Further technically challenging requirements for the AO come from the exoplanet research field, where the task to resolve the very small angular separations between host star and planet, has also to take into account the high-contrast ratio between the two objects. We present in detail the results of our simulations and their impact on high-contrast imaging in order to find the optimal wavefront sensing device for the METIS instrument. METIS is the mid-infrared imager and spectrograph for the ELT with specialised high-contrast, coronagraphic imaging capabilities, whose performance strongly depends on the AO residual wavefront errors. We examined the sky and target sample coverage of a generic wavefront sensor in two spectral regimes, visible and near-infrared, to pre-select the spectral range for the more detailed wavefront sensor type analysis. We find that the near-infrared regime is the most suitable for METIS. We then analysed the performance of Shack-Hartmann and pyramid wavefront sensors under realistic conditions at the ELT, did a balancing with our scientific requirements, and concluded that a pyramid wavefront sensor is the best choice for METIS. For this choice we additionally examined the impact of non-common path aberrations, of vibrations, and the long-term stability of the SCAO system including high-contrast imaging performance.
Highlights
The mid-infrared ELT imager and spectrograph METIS is one of three first instruments on the European Extremely Large Telescope (ELT) [1]
Non-common path aberrations (NCPAs) are typically all optical aberrations that may appear in the instrument after the position where the light of the telescope is split into the science channel and wavefront sensor channel
The goal of the work presented in this paper was to find the most suitable natural guide star wavefront sensor for the METIS instrument to be installed at the ELT
Summary
The mid-infrared ELT imager and spectrograph METIS is one of three first instruments on the European Extremely Large Telescope (ELT) [1]. The other 2 first light instruments are MICADO [2], a near-infrared, 0.8–2.4 μm, imager and spectrograph, and HARMONI [3], an integral field spectrograph sensitive in the 0.47–2.45 μm regime. The compact imaging field of view of ∼ 10 × 10 together with a much larger isoplanatic angle of about 20 for the shortest science wavelength and median atmospheric conditions (Table 4), clearly indicated the use of a single conjugate adaptive optics (SCAO) system to achieve diffraction limited performance [8].
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