In preparation for the Astro 2020 Decadal Survey NASA has commissioned the study four flagship missions spanning to a wide range of observable wavelengths: the Origins Space Telescope (OST, formerly the Far-Infrared Surveyor), Lynx (formerly the X-ray Surveyor), the Large UV/Optical/Infrared Surveyor (LUVOIR) and the Habitable Exoplanet Imager (HabEx). One of the key scientific objectives of the latter two is the detection and characterization of the earth-like planets around nearby stars using the direct imaging technique (along with a broad range of investigations regarding the architecture of and atmospheric composition exoplanetary systems using this technique). As a consequence dedicated exoplanet instruments are being studied for these mission concepts. This paper discusses the design of the coronagraph instrument for the architecture “A” (15 meters aperture) of LUVOIR. The material presented in this paper is aimed at providing an overview of the LUVOIR coronagraph instrument. It is the result of four months of discussions with various community stakeholders (scientists and technologists) regarding the instrument’s basic parameters followed by meticulous design work by the the GSFC Instrument Design Laboratory team. In the first section we review the main science drivers, presents the overall parameters of the instrument (general architecture and backend instrument) and delve into the details of the currently envisioned coronagraph masks along with a description of the wavefront control architecture. Throughout the manuscript we describe the trades we made during the design process. Because the vocation of this study is to provide a baseline design for the most ambitious earth-like finding instrument that could be possibly launched into the 2030’s, we have designed an complex system privileged that meets the ambitious science goals out team was chartered by the LUVOIR STDT exoplanet Working Group. However in an effort to minimize technological risk we tried to maximize the number of technologies that will be matured by the WFIRST coronagraph instruments.
Obstructions due to large secondary mirrors, primary mirror segmentation, and secondary mirror support struts all introduce diffraction artifacts that limit the performance offered by coronagraphs. However, just as vortex coronagraphs provides theoretically ideal cancellation of on-axis starlight for clear apertures, the Polynomial Apodized Vortex Coronagraph (PAVC) completely blocks on-axis light for apertures with central obscurations, and delivers off-axis throughput that improves as the topological charge of the vortex increases. We examine the sensitivity of PAVC designs to tip/tilt aberrations and stellar angular size, and discuss methods for mitigating these effects. By imposing additional constraints on the pupil plane apodization, we decrease the sensitivity of the PAVC to the small positional shifts of the on-axis source induced by either tip/tilt or stellar angular size; providing a route to overcoming an important hurdle facing the performance of vortex coronagraphs on telescopes with complicated pupils.
Imaging planets requires instruments capable of dealing with extreme contrast ratios and that have a high resolution. Coronagraphs that can reach a contrast ratio in a neighborhood of 109 will be capable of observing Jupiter-like planets, while those with a contrast greater than a benchmark number of 1010 to within a few λ/D of an on-axis star will render possible imaging of Earth-like planets. Plans for achieving this feat have been developed for use on telescopes with unobscured, circularly symmetric apertures. However; given that the next generation of large telescopes are on-axis designs with support structures in the telescope aperture and a central obstruction due to the secondary mirror, it has proven necessary to develop coronagraphic techniques that compensate for obstructions. Pueyo and Norman (2012) present a possible solution to the problem of the support structures using ACAD. In this paper, we present a coronagraphic design that uses a vector vortex and pupil apodization to compensate for the secondary mirror that could possibly be used in conjunction with a wavefront control system and/or ACAD. This coronagraph is capable of achieving a contrast ratio of at least 1010 in a working angle of (1:5 - 30) λ/D in conjunction with an on-axis telescope. We can construct our pupil using a classical transmissive apodizer or pupil remapping. We find that the mirror shapes required are relatively simple (requiring ≤ 40 degrees of freedom to describe) and we expect they will be feasible to manufacture, and potentially even to implement with deformable mirrors. By combining existing high-contrast imaging techniques, we demonstrate that a relatively simple design may be used to image exo-Earths.