The warm calibration unit of METIS: Laboratory tests and proof-of-concept

METIS, the mid-infrared imager and spectrograph for the wavelength range 2.9-14 µm (astronomical L-, M- and N bands), will be equipped with a calibration unit, developed at the University of Cologne, which task is to deliver simulated sources for the test and calibration of the main imaging and spectral functionalities of METIS. Our subsystem, as the full METIS instrument, is currently in the Phase C of the project, which leads to the Final Design Review expected by the end of 2021. In this contribution, we first briefly introduce the general concepts chosen for the Warm Calibration Unit (WCU) and then detail the laboratory work that is undertaken in Cologne to validate most of the concepts presented at the Preliminary Design Review. A core unit of the WCU is the integrating sphere combined with the black body, which is the hub delivering the calibration functionalities. We first report the measured spatial uniformity of the output port of the integrating sphere when fed with the black body source radiation. The measurement made using our uncooled thermal camera, evidences a spatial uniformity below 1% RMS. Longer integration times will further improve the final accuracy on this important parameter. We also take a closer look at the black body source and report on its flux temporal stability, which is found to be better than 1% over a 2h duration. We characterize time windows for different settings of the main WCU light source, which is the black body and stability and repeatability of the detected signal. Through different experiments we investigated the best options to manufacture the aperture mask that will be used to generate artificial point sources.