The perimeter of a sunflower-like starshade has hundreds of meters of sharp edges that are directly exposed to sunlight. The sunlight diffracts and reflects from the edge resulting in a dual-lobed glint pattern that can be brighter than an exoplanet. We present estimates of the glint brightness distribution for the Starshade Rendezvous Mission and the HabEx Starshade Mission concepts based on measurements of flight-like, environmentally tested, uncoated metallic edges using custom-built scatterometers. A companion paper addresses the performance for edges coated with a thin anti-reflection coating.
Starshades are designed to enable the direct observation of an exoplanet by blocking the light of the planet’s star from reaching the telescope. As discussed in our companion paper [S. Shaklan et al., “Solar glint from uncoated starshade optical edges,” J. Astron. Telesc. Instrum. Syst.7(2), 021204 (2021)], diffraction and reflection of sunlight incident on the starshade’s razor-sharp uncoated edges will appear as glint that may be brighter than the feeble light of the exoplanet. We report on the measurement and modeling of thin, conformal, multilayer antireflection coatings that reduce solar glint by more than an order of magnitude when applied to uncoated edges. We used the Lumerical finite-difference time-domain simulation software suite to determine the performance of coatings designed to work on a flat surface when applied to a sharp, curved edge. Laboratory measurements of coated edges, including a 50-cm long segment, confirm the glint reduction predicted by these models. We consider two coating approaches and compare their performance: a line-of-sight coating and a coating that uniformly covers the entire terminal edge. Starting with a wide range of coating designs emphasizing different angles of incidence and bandpass characteristics, we use Lumerical to account for edge diffraction and reflection, and we optimize the designs for the Starshade Rendezvous Mission and the HabEx mission concept.
A starshade is a large flower-shaped screen designed to enable the direct imaging of exoplanets with a space telescope. The starshade perimeter is composed of sharp, precisely shaped edges to minimize the glint of sunlight into the telescope. Past work has focused on bare edges to minimize the terminal radius. This paper describes the broadband, wide-angle performance of edges coated with a thin multi-layer anti-reflection coating. This coating uses a combination of interference and absorption to reduce the surface reflectivity and to avoid the negative effects associated with a large cross-sectional area. A custom scattered light testbed has been developed to quantify the amount of light scattered from sample edges and to validate Finite-Difference Time-Domain (FDTD) models of the optical scatter. We show that optical edge samples with this coating significantly reduce the solar glint pattern compared to similar uncoated optical edges.
A sunflower-like starshade positioned between an exoplanet host star and a telescope forms a deep shadow at the telescope enabling the faint exoplanet to be viewed without being overwhelmed by veiling glare from the star. The starshade perimeter has hundreds of meters of sharp edge that are directly exposed to sunlight. The sunlight diffracts and reflects from the edge resulting in a glint pattern that can be brighter than the exoplanet. We have developed models of the edge glint to explain laboratory measurements, to guide the development of edges with minimum glint, and to determine the fundamental glint floor which is set by diffraction. The models include finite difference time domain calculations, Sommerfeld's half-plane diffraction expressions, and a micro-facet scattering model. Models successfully reproduce the features and magnitude of the measured polarization-dependent scatter and show that measured edges are performing near the theoretical limit.
A starshade enables direct imaging of Earth-like exoplanets in the habitable zone of nearby stars by suppressing light from a target star so that orbiting planets are revealed. The perimeter of a starshade, known as the optical edge, has two critical functions. First, it must meet a precise in-plane profile specification to form a deep shadow in which the telescope is placed. Second, it must minimize reflected sunlight, as scattered sunlight significantly degrades the achievable contrast. Prior work on small scales and in a laboratory environment has shown that these requirements can be met using a chemically etched amorphous metal foil. This paper describes the next step of development, a first ever demonstration of assembled optical edge segments that meet both requirements simultaneously. The segments were constructed using space-compatible components and tested to relevant thermal and mechanical environments. A thorough assessment of edge performance, including in-plane profile, sunlight scatter and mechanical survivability was performed both before and after environmental testing. Furthermore, a custom scattered light testbed has been developed to quantify the magnitude of scattered sunlight over the entire length of the optical edge. The results of this study inform the future development of optical edge technology and pave the way towards eventual flight implementation.
Starshades, combined with future space telescopes, provide the ability to detect Earth-like exoplanets in the habitable zone by producing high contrast ratios at small inner working angles. The primary function of a starshade is to suppress light from a target star such that its orbiting planets are revealed. In order to do so, the optical edges of the starshade must maintain their precise in-plane profile to produce the necessary apodization function. However, an equally important consideration is the interaction of these edges with light emanating from our own Sun as scattered and/or diffracted sunlight can significantly degrade the achievable contrast. This paper describes the technical efforts performed to obtain precision, low-scatter optical edges for future starshades. Trades between edge radius (i.e. sharpness) and surface reflectivity have been made and small-scale coupons have been produced using scalable manufacturing processes. A custom scattered light testbed has been developed to quantify the magnitude of scattered light over all sun angles. Models have also been developed to make predictions on the level of reflected and/or diffracted light for various edge architectures. The results of these studies have established a current baseline approach which implements photochemical etching techniques on thin metal foils.