Starshades are a leading technology to enable the direct detection and spectroscopic characterization of Earth-like exoplanets. Two key aspects to advancing starshade technology are the demonstration of starlight suppression to the level required for flight and validation of optical models at this high level of suppression. These technologies are addressed in current efforts underway at the Princeton Starshade Testbed. We report on results from modeling the performance of the Princeton Starshade Testbed to help achieve the milestone 10−9 suppression. We use our optical model to examine the effects that errors in the occulting mask shape and external environmental factors have on the limiting suppression. We look at deviations from the ideal occulter shape such as over-etching during the lithography process, edge roughness of the mask, and random defects introduced during manufacturing. We also look at the effects of dust and wavefront errors in the open-to-atmosphere testbed. These results are used to set fabrication requirements on the starshade and constraints on the testbed environment. We use detailed measurements of the manufactured occulting mask to converge towards agreement between our modeled performance predictions and the suppression measured in the testbed, thereby building confidence in the validity of the optical models. We conclude with a discussion of the advantages and practicalities of scaling to a larger testbed to further advance the optical aspect of starshade technology.
A starshade is a specially designed opaque screen to suppress starlight and remove the effects of diffraction at the edge. The intensity at the pupil plane in the shadow is dark enough to detect Earth-like exoplanets by using direct imaging. At Princeton, we have designed and built a testbed that allows verification of scaled starshade designs whose suppressed shadow is mathematically identical to that of space starshade. The starshade testbed uses a 77.2 m optical propagation distance to realize the flight Fresnel number of 14.5. Here, we present lab result of a revised sample design operating at a flight Fresnel number. We compare the experimental results with simulations that predict the ultimate contrast performance.
Direct imaging using a starshade is a powerful technique for exoplanet detection and characterization. No current post-processing methods are specialized for starshade images and the ones for coronagraph images have not been applied to images produced by a starshade system ( starshade system means the light sources, starshade and telescope). Here, we report on the first step towards adapting these methods for starshade systems. We have built a starshade imaging model. We generate the image based on a simulation of the real astronomical scene and consider the effects of various starshade defects, misalignment, wavefront error, and detector noise. Future work will add the system dynamics of formation flying between the starshade and the telescope. The ultimate goal is to adapt coronagraphic image processing methods for starshade imaging.
The external starshade is a method for the direct detection and spectral characterization of terrestrial planets around other stars, a key goal identified in ASTRO2010. Tests of starshades have been and continue to be conducted in the lab and in the field using non-collimated light sources. We extend the current approach to performing night-time observations of astronomical objects using small-scale (10-30cm) starshades and the McMath-Pierce Solar Telescope at Kitt Peak National Observatory. This configuration allows us to make measurements of stars with a Fresnel number close to those expected in proposed full-scale space configurations. We present the results of our engineering runs conducted in 2015.
The direct detection and characterization of an Earth-like exoplanet is of the highest scientific priority and a leading technology that will enable such discovery is the starshade external occulter. We report on the latest results in ground-based efforts for demonstrating and advancing the technology of starshades. Using the McMath- Pierce Solar Telescope at the Kitt Peak National Observatory, we are able to track stars as they move across the night sky and stabilize a beam of starlight behind a starshade. This has allowed us to conduct the first astronomical observations achieving high-contrast with starshades. In our latest efforts, we have extended the separation between the starshade and telescope to reach an inner working angle of 10 arcseconds at a flight-like Fresnel number and resolution. In this report, we detail the development of a closed-loop feedback system to further stabilize the beam at the extended baseline and provide results on the contrast achieved. We conclude by laying out future work to design a dedicated siderostat-starshade facility for future testing of and observations with starshades. Our main result: we achieved a broadband contrast ratio of 3:2 x 10-5 at 15 arcseconds IWA, while at a flight-like Fresnel number and resolution.
The external starshade is a method for the direct detection and spectral characterization of terrestrial planets
around other stars, a key goal identified in ASTRO2010. In an effort to validate the starlight-suppression performance of
the starshade, we have measured contrast better than 1×10-9 using 60 cm starshades at points just beyond the starshade
tips. These measurements were made over a 50% spectral bandpass, using an incoherent light source (a white LED), and
in challenging outdoor test environments. Our experimental setup is designed to provide starshade to telescope
separation and telescope aperture size that are scaled as closely as possible to the flight system. The measurements
confirm not only the overall starlight-suppression capability of the starshade concept but also the robustness of the setup
to optical disturbances such as atmospheric effects at the test site. The spectral coverage is limited only by the optics and
detectors in our test setup, not by the starshade itself. Here we describe our latest results as well as detailed comparisons
of the measured results to model predictions. Plans and status of the next phase of ground testing are also discussed.
We report on laboratory demonstrations of a vision-based sensor to aid in the formation flying of suborbital vehicles. Precision formation flying of such vehicles will allow us to hold a starshade external occulter in the line of sight between a telescope and star at large separations. This will enable us to perform the first astronomical demonstrations of starshades as we attempt high-contrast imaging of the outer planetary systems of nearby stars. In this report, we identify two sensor architectures and detail the equations for a closed loop visual feedback system to be used for precision formation flying. We investigate the sensor's expected performance through a suite of Monte Carlo simulations and system-level demonstrations in the lab. We also report on the development and demonstration of a means for visual attitude and position determination.
We review the progress on the New Worlds Airship project, which has the eventual goal of suborbitally mapping the Alpha Centauri planetary system into the Habitable Zone. This project consists of a telescope viewing a star that is occulted by a starshade suspended from an airship. The starshade suppresses the starlight such that fainter planetary objects near the star are revealed. A visual sensor is used to determine the position of the starshade and keep the telescope within the starshade’s shadow. In the first attempt to demonstrate starshades through astronomical observations, we have built a precision line of sight position indicator and flew it on a Zeppelin in October (2012). Since the airship provider went out of business we have been redesigning the project to use Vertical Takeoff Vertical Landing rockets instead. These Suborbital Reusable Launch Vehicles will serve as a starshade platform and test bed for further development of the visual sensor. We have completed ground tests of starshades on dry lakebeds and have shown excellent contrast. We are now attempting to use starshades on hilltops to occult stars and perform high contrast imaging of outer planetary systems such as the debris disk around Fomalhaut.