Starshades provide 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 at science-enabling levels and validation of optical models at this high level of suppression. These technologies are addressed in current efforts underway at the Princeton Starshade Testbed. Recent experimental data suggest we are observing the effects of vector (non-scalar) diffraction, which are limiting the starshade's performance and preventing the scalar optical models from agreeing with experimental results at the deepest levels of suppression. This report outlines a model developed to simulate vector diffraction in the testbed using a full solution to Maxwell's equations propagating through narrow features of the starshade. We find that experimental results can be explained by vector diffraction as light traverses the thickness of the starshade mask and that our model is in rough agreement with observations. We provide simulation results of a number of starshade geometries as a first attempt to understand the relation of these effects to properties of the starshade masks. Finally, we outline a number of possible solutions aimed to minimize vector effects and to allow us to reach our milestone of 10<sup>-9</sup> suppression.
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<sup>−9</sup> 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.
We present a starshade error budget with engineering requirements that are well within the current manufacturing and
metrology capabilities. The error budget is based on an observational scenario in which the starshade spins about its axis
on timescales short relative to the zodi-limited integration time, typically several hours. The scatter from localized petal
errors is smoothed into annuli around the center of the image plane, resulting in a large reduction in the background flux
variation while reducing thermal gradients caused by structural shadowing. Having identified the performance
sensitivity to petal shape errors with spatial periods of 3-4 cycles/petal as the most challenging aspect of the design, we
have adopted and modeled a manufacturing approach that mitigates these perturbations with 1-m long precision edge
segments positioned using commercial metrology that readily meets assembly requirements. We have performed
detailed thermal modeling and show that the expected thermal deformations are well within the requirements as well.
We compare the requirements for four cases: a 32 m diameter starshade with a 1.5 m telescope, analyzed at 75 and 90
mas, and a 40 m diameter starshade with a 4 m telescope, analyzed at 60 and 75 mas.
A flower-like starshade positioned between a star and a space telescope is an attractive option for blocking the starlight
to reveal the faint reflected light of an orbiting Earth-like planet. Planet light passes around the petals and directly enters
the telescope where it is seen along with a background of scattered light due to starshade imperfections. We list the
major perturbations that are expected to impact the performance of a starshade system and show that independent models
at NGAS and JPL yield nearly identical optical sensitivities. We give the major sensitivities in the image plane for a
design consisting of a 34-m diameter starshade, and a 2-m diameter telescope separated by 39,000 km, operating
between 0.25 and 0.55 um. These sensitivities include individual petal and global shape terms evaluated at the inner
working angle. Following a discussion of the combination of individual perturbation terms, we then present an error
budget that is consistent with detection of an Earth-like planet 26 magnitudes fainter than its host star.
Fifty meter-class external occulters have been proposed to detect earth-like planets. The THEIA concept<sup>1</sup>, a forty-meter
diameter occulter with twenty ten-meter petals has the necessary nominal performance to achieve this goal. This paper
examines whether this design is robust against expected manufacturing and deployment errors. The development of a
numerical algorithm that represents the mask defects as a collection of rectangular apertures mitigates the problems
associated with modeling diffraction phenomena produced by an occulter with characteristic physical dimensions that
span five orders of magnitude. The field from each of these rectangles, which is proportional to a two-dimensional sinc
function at the telescope, is added to the diffracted field from the nominal occulter. Results for a set of representative
defects are presented. A single-petal, single-defect error budget, based on a minimum contrast of 10<sup>-12</sup> at 75 or 118
milli-arcseconds from the host star from 0.3 μ to 0.9 μ, is quoted. A Monte Carlo-type simulation that predicts the
performance of the occulter in the presence of random combinations of all of the error demonstrates that the system
contrast can maintained to better than 10<sup>-11</sup> from 0.3 μ to 0.9 μ if the values in the error budget can be achieved.
An occulter is an instrument designed to suppress starlight by diffraction from its edges; most are designed
to be circular, with a set of identical "petals" running around the outside. Proposed space-based occulters
are lightweight, deployed screens tens of meters in diameter with challenging accuracy requirements. In
this paper we describe the design of an occulter for the THEIA mission concept. THEIA consists of a
4-meter telescope diffraction limited to 300 nm, and a 40-meter external occulter to provide high-contrast
imaging. Operating from 250 to 1000 nm, it will provide a rich family of science projects, including exoplanet
characterization, ultraviolet spectroscopy, and very wide-field imaging. Originally conceived of as a hybrid
system employing both an occulter and internal coronagraph, THEIA now uses a single occulter to achieve
all of the starlight suppression but at two different distances from the telescope in order to minimize size and
distance. We describe the basic design principles of the THEIA occulter, its final configuration, performance,
High-contrast imaging, particularly direct detection of extrasolar planets, is a major science driver for the next
generation of extremely large telescopes such as the segmented Thirty Meter Telescope. This goal requires
more than merely diffraction-limited imaging, but also attention to residual scattered light from wavefront errors
and diffraction effects at the contrast level of 10<sup>-8</sup>-10<sup>-9</sup>. Using a wave-optics simulation of adaptive optics
and a diffraction suppression system we investigate diffraction from the segmentation geometry, intersegment
gaps, obscuration by the secondary mirror and its supports. We find that the large obscurations pose a greater
challenge than the much smaller segment gaps. In addition the impact of wavefront errors from the primary
mirror, including segment alignment and figure errors, are analyzed. Segment-to-segment reflectivity variations
and residual segment figure error will be the dominant error contributors from the primary mirror. Strategies to
mitigate these errors are discussed.
Direct detection of exo-planets from the ground may be feasible with the advent of extreme-adaptive optics
(ExAO) on large telescopes. A major hurdle to achieving high contrasts behind a star suppression system
(10<sup>-8</sup>/hr<sup>-1/2</sup>) at small angular separations, is the "speckle noise" due to residual atmospheric and telescope-based
quasistatic amplitude and phase errors at mid-spatial frequencies. We examine the potential of a post-coronagraphic,
interferometric wavefront sensor to sense and adaptively correct just such errors. Pupil and focal
plane sensors are considered and the merits and drawbacks of each scheme are outlined. It is not inconceivable to
implement both schemes or even a hybrid scheme within a single instrument to significantly improve its scientific
capabilities. This work was carried out in context of the proposed Planet Formation Imager instrument for
Thirty Meter Telescope (TMT) project.
Direct detection of extrasolar Jovian planets is a major scientific motivation for the construction of future extremely
large telescopes such as the Thirty Meter Telescope (TMT). Such detection will require dedicated high-contrast AO
systems. Since the properties of Jovian planets and their parent stars vary enormously between different populations, the
instrument must be designed to meet specific scientific needs rather than a simple metric such as maximum Strehl ratio.
We present a design for such an instrument, the Planet Formation Imager (PFI) for TMT. It has four key science
missions. The first is the study of newly-formed planets on 5-10 AU scales in regions such as Taurus and Ophiucus -
this requires very small inner working distances that are only possible with a 30m or larger telescope. The second is a
robust census of extrasolar giant planets orbiting mature nearby stars. The third is detailed spectral characterization of
the brightest extrasolar planets. The final targets are circumstellar dust disks, including Zodiacal light analogs in the
inner parts of other solar systems. To achieve these, PFI combines advanced wavefront sensors, high-order MEMS
deformable mirrors, a coronagraph optimized for a finely- segmented primary mirror, and an integral field spectrograph.
Integrated Modeling is currently being used to assess the feasibility of a baseline design concept (pre-phase A), developed for the Coronagraph version of the Terrestrial Planet Finder (TPF) mission. This design concept incorporates many challenging design elements for a space-born observatory: including a monolithic 8 by 3.5 meter elliptical primary mirror; a 12 meter long deployable secondary mirror support structure; as well as a 14 meter long deployable, tensioned-membrane, V-groove sunshield. Unprecedented thermal and dynamic stability is required by this flight system to allow observation of enough contrast between planets and their parent stars. This stringent performance requirement necessitates a balanced system, designed to optimize the various interacting disciplines: optical, thermal, structural & control. To support design feasibility studies, a MATLAB-environment-based integrated modeling tool (IMOS: Integrated Modeling of Optical Systems) was employed for analyzing the end-to-end system performance for typical in-orbit maneuvers. Our integrated modeling goal is to use a single model definition file to specify the thermal, structural, and optical modeling and analysis parameters, improving results accuracy, configuration control and data management. In working towards that goal, we have had parallel efforts in IMOS capability development, as well as design concept modeling and analysis. Typical system performance metrics studied include the relative motions of the optical elements, as well as the deformation of individual optics, decomposed into best-fitting Zernike polynomials.
We report on the progress in developing cryogenic delay lines and
integrated optics components. These are some of the critical components needed to enable far-IR direct-detection interferometers. To achieve background-limited performance in the 40 to 400 μm region, th einterferometer optics and delay lines must be cooeld to near liquid Helium temperatures. Our cryogenic delay line designs incorporate a number of novel features and has been operated at liquid nitrogen temperatures. Our integrated optics effort has focued on producing single-mode spatial filters and beam combiners.
Laser induced, micro-chemical etching is a promising new technology that can be used to fabricate three dimensional structures many millimeters across with micrometer accuracy. Laser micromachining possesses a significant edge over more conventional techniques. It does not require the use of masks and is not confined to crystal planes. A non-contact process, it eliminates tool wear and vibration problems associated with classical milling machines. At the University of Arizona we have constructed the first such laser micromaching system optimized for the fabrication of THz and far IR waveguide and quasi-optical components. Our system can machine many millimeters across down to a few microns accuracy in a short time, with a remarkable surface finish. This paper presents the design, operation and performance of our system, and its applications to waveguide devices for sub millimeter and far IR interferometry.
The Palomar Testbed Interferometer is a long-baseline interferometer that uses both phase and group-delay measurements for narrow-angle astrometry. The group-delay measurements are performed using 5 spectral channels across the band from 2.0 to 2.4 micrometers . Group delay is estimated from phasors (sine and cosine of fringe phase) calculated for each spectral channel using pathlength modulation of one wavelength. Normally the group delay is estimated to be the delay corresponding to the peak of the power spectrum of these complex phasors. The Fourier transform does not however yield a least-squares estimate of the delay. Nevertheless, the precision of phase estimation can be achieved in a group-delay estimate using a least-squares approach. We describe the least-squares group-delay estimator that has been implemented at PTI and illustrate its performance as applied to narrow-angle astrometry.
The Palomar Testbed Interferometer (PTI) is an infrared, phase-tracking interferometer in operation at Palomar Mountain since July 1995. It was funded by NASA for the purpose of developing techniques and methodologies for doing narrowangle astrometry for the purpose of detecting extrasolar planets. The instrument employs active fringe trackingin the infrared (2.0-2.4 μm) to monitor fringe phase. It is a dual-star interferometer; it is able to measure fringes on two separate stars simultaneously. An end-to-end heterodyne laser metrology system is used to monitor the optical path length of the starlight. Recently completed engineering upgrades have improved the initial instrument performance. These upgrades are:extended wavelength coverage, a single mode fiber for spatial filtering, vacuum pipes to relay the beams, accelerometers on the siderostat mirrors and a new baseline. Results of recent astrometry data indicate the instrument is approaching the astrometric limit as set by the atmosphere.
As part of a technology development program for realizing a space based submillimeter telescope, two different approaches to the absolute phasing of a segmented primary mirror using focal plane measurements have been implemented for feasibility. The method of optimization by simulated annealing evaluates the image quality of a point spread function after all the telescope segments have been randomly moved. It accepts each iteration which improves the image quality, as well as a random number of iterations which do not, thus keeping the Strehl in the initialization procedure from falling into local maxima. Methods for determining the annealing schedule, and the step size for random segment movements are presented and discussed. Using phase diversity and a model for the telescope imaging system, a nonlinear least squares algorithm has also been implemented which parameterizes each of the segment actuator movements. Using multiple out of focus images, the segment actuator positions are estimated using an iterative procedure. Nonlinear least squares, although computationally intensive, offers a large savings in the actuator movements over simulated annealing and pairwise phasing methods for large numbers of segments. These algorithms have been integrated into a general simulation program which models the behavior of the telescope under anticipated space conditions.