The design and performance of the wheel systems in the common foreoptics of the ELT METIS

The Mid-infrared ELT Imager and Spectrograph (METIS) is one of the first light instruments for the Extremely Large Telescope (ELT) and will cover the thermal- and mid-infrared (3-13 µm).With the single conjugate adaptive optics (SCAO) system, it will enable high contrast imaging and integral field unit (IFU) spectroscopy (R ~ 100 000) at the diffraction limit of the ELT.Inside the METIS cryostat, it has a modular design and is composed of the common fore optics (CFO), the imager (IMG), the SCAO, and the L and M band integral field spectrograph (LMS).The components are cooled down to around 60K, or lower for the detectors, during the operation to reduce the background.In the CFO of METIS, four wheels are inserted in the optical path including the atmospheric dispersion corrector (ADC) wheel, the pupil plane one (PP1) wheel, the focal plane two (FP2) wheel and the LMS pickoff wheel.The PP1 wheel and the ADC wheel are located near the first pupil plane while the other two wheels are at the second focal plane.These wheels accommodate the optics, slits, masks and filters for various operation modes of METIS.In each wheel, common cartridges are designed to hold the optical elements to facilitate an easy exchange between them.High positioning repeatability is required for some of the optics.We will report the design and the initial integration and tests of the wheels in this paper.