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• the size of the telescope and the associated complexity of the wavefront control tasks
• the unique scientific capabilities of METIS, including high contrast imaging
• the interaction with the newly established, integrated wavefront control infrastructure of the ELT
• the integration of the near-infrared Pyramid Wavefront Sensor and other key Adaptive Optics (AO) hardware embedded within a large, fully cryogenic instrument.
METIS and it’s AO system have passed the final design review and are now in the manufacturing, assembly, integration and testing phase. The firsts are approached through a compact hard- and software design and an extensive test program to mature METIS SCAO before it is deployed at the telescope. This program includes significant investments in test setups that allow to mimic conditions at the ELT. A dedicated cryo-test facility allows for subsystem testing independent of the METIS infrastructure. A telescope simulator is being set up for end-to-end laboratory tests of the AO control system together with the final SCAO hardware. Specific control algorithm prototypes will be tested on sky. In this contribution, we present the progress of METIS SCAO with an emphasis on the preparation for the test activities foreseen to enable a successful future deployment of METIS SCAO at the ELT.
After the final design reviews of the optics (2021) and the entire system (2022), most hardware procurements have started. In this paper we present an overview of the status of the various ongoing activities. Many hardware components are already in hand, and the manufacturing is in full swing in order to start the assembly and testing of the subsystems in 2024 toward first light at the telescope in 2028/29. This rather brief paper only provides an overview of the project status. For more information, we refer to the detailed instrument paper which will be published soon.
The internal SCAO system designed to maximize the performance for bright targets and has its wavefront sensors (WFSs) build inside the METIS cryostat to minimize the number of warm surfaces towards the science detectors. Although the internal dichroic will reflect all light short wards of 3 micrometers towards the WFS, only the IR light will most likely be used, mainly due to the expected improved performance at longer wavelengths for the WFS. A trade-off has been made between both visible versus infrared wave front sensing as well as Pyramid versus Shack-Hartmann, under various observing conditions and target geometries, taking into account performance, target availability, reliability and technology readiness level. The base line for the SCAO system is to minimize system complexity, thereby ensuring system availability and reliability even under first-light conditions.
Since the SCAO system will require a bright guide star near the science target, it can only be used for a limited number of targets. The LTAO system, consisting of up to 6 LGS and up to 3 low-order NGS WFS and located outside the cryostat, is designed to increase the sky coverage on arbitrary targets to >80%. Investigations are ongoing if the internal SCAO system can be used as either a Low-Order WFS or metrology system.
The mirror with an optical diameter of 200 mm is designed to correct wave front aberrations of up to 2 μm root-mean square (rms). The shape of the wave front is approximated by the first 15 Zernike modes. Finite element analysis of the mirror shows a theoretically possible reduction of the wave front error from 2 μm to 53 nm rms. To produce the desired shapes, the mirror surface is controlled by 19 identical actuator modules at the back of the mirror.
The actuator modules use commercially available Piezo-Knob actuators with a high technology readiness level (TRL). These provide nanometer resolution at cryogenic temperatures combined with high positional stability, and allow for the system to be powered off once the desired shape is obtained. The stiff design provides a high resonance frequency (>200 Hz) to suppress external disturbances.
A full-size demonstrator of the deformable mirror containing 6 actuators and 13 dummy actuators is realized and characterized. Measurement results show that the actuators can provide sufficient stroke to correct the 2 μm rms WFE. The resolution of the actuator influence functions is found to be 0.24 nm rms or better depending on the position of the actuator within the grid. Superposition of the actuator influence functions shows that a 2 μm rms WFE can be accurately corrected with a 38 nm fitting error. Due to the manufacturing method of the demonstrator an artificially large print-through error of 182 nm is observed. The main cause of this print-through error has been identified and will be reduced in future design iterations. After these design changes the system is expected to have a total residual error of less than 70 nm and offer diffraction limited performance (λ14) for wavelengths of 1 μm and above.
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