The ELT/METIS annular groove phase masks

High-contrast imaging instruments use coronagraphs for studying stellar environments by suppressing the intense glare of stars. The Annular Groove Phase Mask (AGPM), a vector vortex coronagraph, has proven to be a valuable tool, offering high-contrast performance at small angular separations. The Mid-infrared ELT Imager and Spectrograph (METIS) project will incorporate multiple AGPMs designed to operate at various wavelengths within the LMN spectral bands. Recently, the METIS project has entered the Manufacture, Assembly, Integration, and Test (MAIT) phase. During MAIT, the AGPMs, referred to as Vortex Phase Masks (VPMs) in the framework of METIS, are subject to an iterative process of manufacturing and testing. In the event of performance discrepancies, a component undergoes a minor re-etching process and is subsequently re-tested until it meets the specified requirements. In this work, we evaluate the performance of the METIS VPMs on two distinct coronagraphic test benches. On one hand, the Vortex Optical Demonstrator for Coronagraphic Applications (VODCA) at the University of Liège, featuring a supercontinuum laser source and a FLIR infrared camera, is employed to assess the METIS L- and M-band VPMs. On the other hand, the performance of the METIS N-band VPMs is assessed using a cryogenic testbed at CEA Paris-Saclay. This second testbed is equipped with a series of lasers spanning the 8 to 12.5μm range, ensuring high wavefront quality with a single-mode output. We present the outcome of our extensive manufacturing and testing campaigns and reveal the measured coronagraphic performance results for all METIS VPMs.