Straylight analyses and mitigation strategies for the ELT METIS instrument

The Mid-infrared ELT Imager and Spectrograph (METIS) is one of the three first-generation instruments on the Extremely Large Telescope (ELT). It will provide 20 instrument configurations for direct and high-contrast imaging, medium and high resolution spectroscopy in the wavelength range of 3 - 13μ. The straylight will affect the image contrast and objects recognition thus influencing the final instrument performance. For this reason it should be taken into account and accurately modeled at the design stage. In the present study we consider straylight from the following sources: surface roughness and defects of the optical surfaces, multiple reflections and diffraction, which will all influence the instrument performance. We estimate their influence using a bottom-up modelling approach at the system level and derive the requirements for some critical parameters. Using empirical and analytical models and performing non-sequential raytracing we demonstrate that the target straylight level can be reached in the current design with reasonable specifications on the optical components.