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.
Sensing starlight rejected from a coronagraph is essential in stabilizing the telescope pointing and wavefront drift, but performance is degraded for dim stars. Laser Metrology (MET) provides a different, complementary sensing method, one that can be used to measure changes in the alignment of the optics at high bandwidth, independent of the magnitude of the host star. Laser metrology measures changes in the separation of optical fiducial pairs, which can be separated by many meters. The principle of operations is similar to the laser metrology system used in LISA-Pathfinder to measure the in-orbit displacement between two test masses to a precision of ~10 picometers. In closed loop with actuators, MET actively maintains rigid body alignment of the front-end optics, thereby eliminating the dominant source of wavefront drift. Because MET is not photon starved, it can operate at high bandwidth and feed-forward secondary-mirror jitter to a fast-steering mirror, correcting line-of-sight errors. In the case of a segmented, active primary mirror, MET provides six degrees of freedom sensing, replacing edge sensors. MET maintains wavefront control even during attitude maneuvers, such as slews between target stars, thereby avoiding the need to repeat time-consuming speckle suppression. These features can significantly improve the performance and observational efficiency of future large-aperture space telescopes equipped with internal coronagraphs. We evaluate MET trusses for various proposed monolithic and segmented spacebased coronagraphs and present the performance requirements necessary to maintain contrast drift below 10-11.
The detection of molecular species in the atmospheres of earth-like exoplanets orbiting nearby stars requires an optical system that suppresses starlight and maximizes the sensitivity to the weak planet signals at small angular separations. Achieving sufficient contrast performance on a segmented aperture space telescope is particularly challenging due to unwanted diffraction within the telescope from amplitude and phase discontinuities in the pupil. Apodized vortex coronagraphs are a promising solution that theoretically meet the performance needs for high contrast imaging with future segmented space telescopes. We investigate the sensitivity of apodized vortex coronagraphs to the expected aberrations, including segment co-phasing errors in piston and tip/tilt as well as other low-order and mid-spatial frequency aberrations. Coronagraph designs and their associated telescope requirements are identified for conceptual HabEx and LUVOIR telescope designs.
Current state-of-the-art high contrast imaging instruments take advantage of a number of elegant coronagraph designs to suppress starlight and image nearby faint objects, such as exoplanets and circumstellar disks. The ideal performance and complexity of the optical systems depends strongly on the shape of the telescope aperture. Unfortunately, large primary mirrors tend to be segmented and have various obstructions, which limit the performance of most conventional coronagraph designs. We present a new family of vortex coronagraphs with numerically-optimized gray-scale apodizers that provide the sensitivity needed to directly image faint exoplanets with large, segmented aperture telescopes, including the Thirty Meter Telescope (TMT) as well as potential next-generation space telescopes.