Complex Mask Coronagraphs for High-Contrast Imaging

Abstract

Since the discovery of the first exoplanet over two decades ago, humanity has continued to discover more exoplanets with every passing year; this is because the methods of discovery and follow-up observation are maturing at a rapid technological pace, as is the interest spurred by being able to answer the question if other life exists outside of our own. Direct imaging is no exception to this paradigm of rapid technological maturity because it is one of the modes by which astronomers can distinguish if any of the exoplanets exhibit signatures of life. That is, astronomers can perform spectroscopic analyses of directly imaged targets. However, this is a daunting task for earth-analogs, as the requirements are nominally two-fold: an observer must detect a signal from a target exoplanet which is 1e-10 times fainter than the star it orbits, and they must do so extremely close to the star -- inside of its habitable zone where the chance to detect life supporting signatures from an atmosphere is greatest. The Phase-Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC) can accomplish this lofty goal in theory, but the maturity of the complex mask continues to be an open question, as its conversion from a theoretically ideal mathematical quantity to an achromatic, pi-phase shifting optical element in a focal plane has only been demonstrated in laboratory settings at best. Hence we focus on the development of the PIAACMC complex mask for the Subaru Coronagraphic Extreme Adaptive Optics instrument at the Subaru telescope, with the goal in mind of testing its initial on-sky performance. In particular, we demonstrate two separate designs and fabrications of complex masks for the same set of PIAA lenses, paying close attention to the details of the fabrication processes and simulating their effects so as to understand any sensitivities to fabrication errors from design specifications which can ultimately impact coronagraphic performance. We use measurements from fabricated complex masks to develop a simulated polychromatic estimate of raw contrast, and seek to demonstrate how this estimate may be substantially improved, in simulation, by wavefront control. The installation of the complex masks at SCExAO culminates in an on-sky observation of Eta Virginis A and B, a pair of closely separated stars which demonstrate the functionality of PIAACMC for observing companions at small angular separations. Furthermore we employ the technique of lucky imaging with coronagraphic images to qualitatively illustrate how the performance of a coronagraph can nominally be improved from simple frame sub-selection. Armed with this information, we argue in favor of the continued development of the complex mask of a PIAACMC toward delivering competitive raw contrasts in dynamic high-contrast imaging scenes for current extreme AO systems such as MagAO-X, future space missions such as LUVOIR, and ground-based missions such as the future extremely large telescopes.