METIS is a sophisticated mid-infrared imager and spectrograph covering wavelengths from 2.9 to 13.5 μm (astronomical L, M, and N bands), stands as one of the three primary science instruments at the Extremely Large Telescope (ELT). METIS will provide diffraction-limited imaging, coronagraphy, high-resolution integral field spectroscopy, and low/medium-resolution slit spectroscopy. Within the international METIS consortium, the University of Cologne is responsible for designing, manufacturing, integrating, and testing the Warm Calibration Unit (WCU). Among the other calibration functionalities, the WCU also facilitates alignment checks during Assembly, Integration, and Verification (AIT & AIV) through a visible light channel utilizing a CMOS camera, which can capture images of internal focal planes, pupil planes of the WCU, and various METIS sub-systems. This paper details the final optical design, preliminary opto-mechanical design, and analysis of alignment optics in the WCU. A laboratory test setup has been discussed, showcasing the estimation of SNR for the AIV mode of the WCU using an off-the-shelf CMOS camera and interating sphere. This paper also introduces the novel design concept of aluminum spherical mirrors employed in the main optical train of the WCU. We also present a brief overview of alignment verification procedures of the offner relay optics of the WCU sub-system. Furthermore, a detailed overview of the thermal analysis of the whole WCU will be discussed, which ensures the temperature difference between the WCU and the ELT dome will be within specified limits.
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