Journal of Astronomical Telescopes, Instruments, and Systems
VOL. 7 · NO. 2 | April 2021
ISSUES IN PROGRESS
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Starshade concepts must be stowed within rocket fairings for launch and then deployed in space. The in-plane deployment accuracy must be on the order of hundreds of micrometers for sufficient starlight suppression to enable the detection and study of Earth-like exoplanets around nearby Sun-like stars. We describe tests conducted to demonstrate deployment repeatability of two key structural subsystems of the “furled” starshade architecture—the petal and the inner disk. Together, the petals and the inner disk create the in-plane shape of a starshade. Test articles to represent the petal and inner disk subsystems were constructed at relevant scales for a 26-m-diameter starshade. These test articles were subjected to stowage-and-deployment cycles and their shapes were measured. The measured performance—tens of parts per million of petal strain after deployment, and hundreds of micrometers of inner disk deployment accuracy—was found to be within required allocations.
NASA is developing starshade technology to Technology Readiness Level 5 within a directed activity called S5. The objective of S5 is to mature starshade technology to the level that exoplanet imaging missions, such as Starshade Rendezvous and HabEx, can begin the formulation phase. This paper outlines the S5 activity as a whole, to show how it closes all starshade technology gaps in a mutually consistent way. It serves as a companion paper to several other papers in this special section that report progress in specific starshade technologies.
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.
High-contrast imaging enabled by a starshade in formation flight with a space telescope can provide a near-term pathway to search for and characterize temperate and small planets of nearby stars. NASA’s Starshade Technology Development Activity to TRL5 (S5) is rapidly maturing the required technologies to the point at which starshades could be integrated into potential future missions. We reappraise the noise budget of starshade-enabled exoplanet imaging to incorporate the experimentally demonstrated optical performance of the starshade and its optical edge. Our analyses of stray light sources—including the leakage through micrometeoroid damage and the reflection of bright celestial bodies—indicate that sunlight scattered by the optical edge (i.e., the solar glint) is by far the dominant stray light. With telescope and observation parameters that approximately correspond to Starshade Rendezvous with Roman and Habitable Exoplanet Observatory (HabEx), we find that the dominating noise source is exozodiacal light for characterizing a temperate and Earth-sized planet around Sun-like and earlier stars and the solar glint for later-type stars. Further, reducing the brightness of solar glint by a factor of 10 with a coating would prevent it from becoming the dominant noise for both Roman and HabEx. With an instrument contrast of 10 − 10, the residual starlight is not a dominant noise, and increasing the contrast level by a factor 10 does not lead to any appreciable change in the expected science performance. If unbiased calibration of the background to the photon-noise limit can be achieved, Starshade Rendezvous with Roman could provide nearly photon-limited spectroscopy of temperate and Earth-sized planets of F, G, and K stars <4 parsecs away, and HabEx could extend this capability to many more stars <8 parsecs. Larger rocky planets around stars <8 parsecs would be within the reach of Roman. To achieve these capabilities, the exozodiacal light may need to be calibrated to a precision better than 2% and the solar glint to better than 5%. Our analysis shows that the expected temporal variability of the solar glint is unlikely to hinder the calibration, and the main challenge for background calibration likely comes from the unsmooth spatial distribution of exozodiacal dust in some stars. Taken together, these results validate the optical noise budget and technology milestones adopted by S5 against key science objectives and inform the priorities of future technology developments and science and industry community partnerships.
NASA is studying a possible starshade flying in formation with the Nancy Grace Roman Space Telescope (Roman). The starshade would perform weeks-long translational retargeting maneuvers between target stars. A retargeting architecture that is based on chemical propulsion and does not require ground tracking or interactions with the telescope during the retargeting cruise is introduced. Feasibility is demonstrated through a covariance analysis of the starshade-telescope relative position over several weeks using realistic sensor and actuator assumptions. Performance is sufficient for Roman to reacquire the starshade after retargeting, and the architecture is shown to be applicable to other mission concepts such as the Habitable Exoplanet Observatory (HabEx). Results are verified through high-fidelity simulations, and driving sources of uncertainty are identified to confirm the robustness of the approach.
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Starshades are a leading technology to enable the detection and spectroscopic characterization of Earth-like exoplanets. We report on optical experiments of sub-scale starshades that advance critical starlight suppression technologies in preparation for the next generation of space telescopes. These experiments were conducted at the Princeton starshade testbed, an 80-m long enclosure testing 1/1000’th scale starshades at a flight-like Fresnel number. We demonstrate 10 − 10 contrast at the starshade’s geometric inner working angle (IWA) across 10% of the visible spectrum, with an average contrast at the IWA of 2 × 10 − 10 and contrast floor of 2 × 10 − 11. In addition to these high-contrast demonstrations, we validate diffraction models to better than 35% accuracy through tests of intentionally flawed starshades. Overall, this suite of experiments reveals a deviation from scalar diffraction theory due to light propagating through narrow gaps between the starshade petals. We provide a model that accurately captures this effect at contrast levels below 10 − 10. The results of these experiments demonstrate that there are no optical impediments to building a starshade that provides sufficient contrast to detect Earth-like exoplanets. This work also sets an upper limit on the effect of unknowns in the diffraction model used to predict starshade performance and set tolerances on the starshade manufacture.
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.
We present an analytical model for the desired kinematics of the starshade-telescope relative motion during exoplanet direct imaging observations. We combine this model with an existing deadbanding strategy published by the NASA JPL S5 Team to define a dynamics framework for deadbanding simulations. Global results of these simulations show that the fuel usage and the number of observation interruptions vary as a function of the target star ecliptic coordinates and time, meaning there exist optimal times to observe particular targets. We combine these results with the telescope pointing constraints due to the relative position of the Sun and other bright solar system objects. We show that optimally scheduling an observation could result in up to 30 more min of integration time and 26 fewer interruptions per observation, improvements of almost 300% in some cases. We also show how phasing the start time of the telescope on its halo orbit is paramount for ensuring optimal observations, providing up to 68 additional min and 31 fewer interruptions per observation. Choosing an optimal halo phasing can also increase, for some near-ecliptic target stars, the fraction of a year that the target is observable from a few percent to more than 30%.
Launching a starshade to rendezvous with the Nancy Grace Roman Space Telescope (Roman) would provide the first opportunity to directly image the habitable zones (HZs) of nearby sunlike stars in the coming decade. A report on the science and feasibility of such a mission was recently submitted to NASA as a probe study concept. The driving objective of the concept is to determine whether Earth-like exoplanets exist in the HZs of the nearest sunlike stars and have biosignature gases in their atmospheres. With the sensitivity provided by this telescope, it is possible to measure the brightness of zodiacal dust disks around the nearest sunlike stars and establish how their population compares with our own. In addition, known gas-giant exoplanets can be targeted to measure their atmospheric metallicity and thereby determine if the correlation with planet mass follows the trend observed in the Solar System and hinted at by exoplanet transit spectroscopy data. We provide the details of the calculations used to estimate the sensitivity of Roman with a starshade and describe the publicly available Python-based source code used to make these calculations. Given the fixed capability of Roman and the constrained observing windows inherent for the starshade, we calculate the sensitivity of the combined observatory to detect these three types of targets, and we present an overall observing strategy that enables us to achieve these objectives.
We present a fast algorithm for computing the diffracted field from arbitrary binary (sharp-edged) planar apertures and occulters in the scalar Fresnel approximation for up to moderately high Fresnel numbers (≲103). It uses a high-order areal quadrature over the aperture and then exploits a single 2D nonuniform fast Fourier transform to evaluate rapidly at target points (on the order of 107 such points per second, independent of aperture complexity). It thus combines the high accuracy of edge integral methods with the high speed of Fourier methods. Its cost is O ( n2 log n ) , where n is the linear resolution required in the source and target planes, to be compared with O ( n3 ) for edge integral methods. In tests with several aperture shapes, this translates to between two and five orders of magnitude acceleration. In starshade modeling for exoplanet astronomy, we find that it is roughly 104 × faster than the state-of-the art in accurately computing the set of telescope pupil wavefronts. We provide a documented, tested MATLAB/Octave implementation. An appendix shows the mathematical equivalence of the boundary diffraction wave, angular integration, and line integral formulas and then the analysis of a new non-singular reformulation that eliminates their common difficulties near the geometric shadow edge. This supplies a robust edge integral reference against which to validate the main proposal.
We present the optical requirement-driven observational constraints of the Remote Occulter, an orbiting starshade designed to work with ground-based telescopes to produce visible-band images and spectra of temperate planets around Sun-like stars. We then utilize these constraints to develop and present numerical simulations of time-dependent observable sky regions along with each region’s nightly available exposure duration and show that nearly the entire sky could be observed for up to 8 h a night. We further examine how changes introduced to our established constraints will impact such observational windows and discuss their implications, setting the ground for upcoming studies aiming to further investigate the Remote Occulter mission capabilities and architecture.
We present optimized observation schedules for a distributed configuration of the Remote Occulter Mission. Accounting for refueling rounds, we show that an Earth-orbiting Remote Occulter could enable up to 158 ground-based observations of 80 exoplanetary targets in a mission lifetime. We develop two target lists, provide exposure time estimates for each potential target star, present an analytic approach for determining target observability, and estimate the cost of station-keeping and retargeting maneuvers required to maintain such a mission. We optimize the mission observation schedule over these cost and science delivery estimates using deterministic and metaheuristic optimization methods with varying degrees of operator intervention and conclude by assessing mission profile sensitivity to both isolated and accumulated cost and design perturbations.