Optimization of METIS High Contrast Imaging Modes

Abstract

The field of exoplanet research has made enormous progress over the last two decades, since the discovery of the first exoplanet around a Sun-like star in 1995 by Mayor and Queloz, with over 4400 confirmed exoplanets to date. The majority of these detections are through indirect detection methods like radial velocity and transit photometry, with a limited number of detections through direct imaging techniques. Direct imaging is particularly difficult due to high contrast and small angular separations between the exoplanets and their host stars. In this regard, the Mid-infrared ELT Imager and Spectrograph (METIS) is one promising instrument that will give access to fainter and closer-in planets along with the possibility of characterizing their atmosphere. METIS will unleash the full potential of the giant adaptive optics-assisted ELT telescope, operating at infrared wavelengths and using advanced coronagraphic techniques like vortex coronagraphs and apodizing phase plates.

In this thesis, we aim to improve the high-contrast imaging (HCI) capabilities of METIS. In the first part, we optimize the three main METIS HCI modes based on focal-plane masks, namely the vortex coronagraph, the ring-apodized vortex coronagraph, and the Lyot coronagraph, to significantly improve their HCI performance. In the second part, we model the vortex center glow effect for the first time, which has proven to be a nuisance in previous mid-infrared observations with the vortex coronagraph. We evaluate its strength and suggest ways to mitigate it. In the last part, we assess the effect of pupil blurring on the METIS thermal background level, which can degrade the HCI performance. We evaluate the level to which this effect can be accounted for based on given constraints.