Design, manufacturing and prototype testing of the METIS ring apodizer

The Mid-Infrared ELT Imager and Spectrograph (METIS) is one of the three first-generation instruments of the Extremely Large Telescope (ELT). One of the high-contrast imaging modes of METIS is implemented with a vortex coronagraph. However, the complex pupil of the ELT with its large central obscuration limits the performance of a classical vortex coronagraph. Using a grayscale ring apodizer in a pupil plane upstream of the vortex phase mask can correct for the effect of the central obscuration and partly restore the coronagraphic performance of the vortex for the ELT pupil. Here, we present the ring apodizer for the METIS instrument based on a Chromium microdots technology. We describe the design process of the microdot patterns and perform simulations with various geometric parameters, describe the manufacturing and testing of three prototypes and of the final ring apodizer for METIS.