An internal coronagraph with an adaptive optical system for wavefront control is being considered for direct imaging of exoplanets with upcoming space missions and concepts, including WFIRST, HabEx, LUVOIR, EXCEDE and ACESat. The main technical challenge associated with direct imaging of exoplanets is to control of both diffracted and scattered light from the star so that even a dim planetary companion can be imaged. For a deformable mirror (DM) to create a dark hole with 10−10 contrast in the image plane, wavefront errors must be accurately measured on the science focal plane detector to ensure a common optical path. We present here a method that uses a set of random phase probes applied to the DM to obtain a high accuracy wavefront estimate even for a dynamically changing optical system. The presented numerical simulations and experimental results show low noise sensitivity, high reliability, and robustness of the proposed approach. The method does not use any additional optics or complex calibration procedures and can be used during the calibration stage of any direct imaging mission. It can also be used in any optical experiment that uses a DM as an active optical element in the layout.
We present the continued progress and laboratory results advancing the technology readiness of Multi-Star Wavefront Control (MSWC), a method to directly image planets and disks in multi-star systems such as Alpha Centauri. This method works with almost any coronagraph (or external occulter with a DM) and requires little or no change to existing and mature hardware. In particular, it works with single-star coronagraphs and does not require the off-axis star(s) to be coronagraphically suppressed. Because of the ubiquity of multistar systems, this method increases the science yield of many missions and concepts such as WFIRST, Exo-C/S, HabEx, LUVOIR, and potentially enables the detection of Earthlike planets (if they exist) around our nearest neighbor star, Alpha Centauri, with a small and low-cost space telescope such as ACESat. Our lab demonstrations were conducted at the Ames Coronagraph Experiment (ACE) laboratory and show both the feasibility as well as the trade-offs involved in using MSWC. We show several simulations and laboratory tests at roughly TRL-3 corresponding to representative targets and missions, including Alpha Centauri with WFIRST. In particular, we demonstrate MSWC in Super-Nyquist mode, where the distance between the desired dark zone and the off-axis star is larger than the conventional (sub-Nyquist) control range of the DM. Our laboratory tests did not yet include a coronagraph, but did demonstrate significant speckle suppression from two independent light sources at sub- as well as super-Nyquist separations.
We explore the capabilities of a starshade mission to directly image multi-star systems. In addition to the diffracted and scattered light for the on-axis star, a multi-star system features additional starlight leakage from the off-axis star that must also be controlled. A basic option is for additional starshades to block the off- axis stars. An interesting option takes the form of hybrid operation of a starshade in conjunction with an internal starlight suppression. Two hybrid scenarios are considered. One such scenario includes the coronagraph instrument blocking the on-axis star, with the starshade blocking off-axis starlight. Another scenario uses the wavefront control system in the coronagraph instrument and using a recent Super-Nyquist Wavefront Control (SNWC) technique can remove the off-axis stars leakage to enable a region of high-contrast around the on-axis star blocked by the starshade. We present simulation results relevant for the WFIRST telescope.
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.
An internal coronagraph with an adaptive optical system for wavefront correction for direct imaging of exoplanets is currently being considered for many mission concepts: a dedicated instrument undergoing development on the upcoming WFIRST mission, and prime instruments on the large-scale HabEx and LUVOIR mission studies, as well as smaller-scale missions such as ACESAT. To enable direct imaging of exoplanets with an internal coronagraph both diffraction and scattered light from the stellar point spread function must be directly suppressed using the coronagraph instrument or corrected in post-processing. Both of these tasks require estimation of the chromatically-dependent complex electric field in the focal plane either using the main science camera or the integral field spectrograph (IFS) camera. To date, the most common method to estimate the chromaticity of the complex electric field is using a heterodyne term generated by DM probes and requiring sequence of narrowband filters to increase coherence. We extend this concept to enable estimation using direct broadband images using a well-calibrated broadband response matrix of the DM probes. Our broadband focal plane estimation method can be used with a single broadband filter providing an alternative to more complicated methods that require several monochromatic channels or a dedicated integral field spectrograph. This capability can also enable low- cost, low-complexity coronagraph missions. We demonstrate the broadband estimation method using fully 30% bandwidth broadband input light with an optical simulator featuring a PIAA coronagraph.
We show preliminary laboratory results advancing the technology readiness of a method to directly image planets and disks in multi-star systems such as Alpha Centauri. This method works with almost any coronagraph (or external occulter with a DM) and requires little or no change to existing and mature hardware. Because of the ubiquity of multistar systems, this method potentially multiplies the science yield of many missions and concepts such as WFIRST, Exo-C/S, HabEx, LUVOIR, and potentially enables the detection of Earth-like planets (if they exist) around our nearest neighbor star, Alpha Centauri, with a small and low-cost space telescope such as ACESat. We identified two main challenges associated with double-star (or multi-star) systems and methods to solve them. “Multi-Star Wavefront Control” (MSWC) enables the independent suppression of starlight from more than one star, and Super-Nyquist Wavefront Control (SNWC) enables extending MSWC to the case where star separation is beyond the Nyquist limit of the deformable mirror (DM). Our lab demonstrations were conducted at the Ames Coronagraph Experiment (ACE) laboratory and proved the basic principles of both MSWC and SNWC. They involved a 32x32 deformable mirror but no coronagraph for simplicity. We used MSWC to suppress starlight independently from two stars by at least an order of magnitude, in monochromatic as well as broadband light as broad as 50%. We also used SNWC to suppress starlight at 32 l/D, surpassing the Nyquist limit of the DM.
A starshade or external occulter is a spacecraft flown along the line-of-sight of a space telescope to suppress starlight and
enable high-contrast direct imaging of exoplanets. Because of its large size and scale it is impossible to fully test a starshade
system on the ground before launch. Therefore, laboratory verification of starshade designs is necessary to validate the
optical models used to design and predict starshade performance. 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 a
comparable space starshade. The starshade testbed uses 77.2 m optical propagation distance to realize the flight-appropriate
Fresnel numbers of 14.5. Here we present the integration status of the testbed and simulations predicting the ultimate
contrast performance. We will also present our results of wavefront error measurement and its implementation of
suppression and contrast.
Model-based wavefront control methods such as electric field conjugation require accurate optical propagation models to create high-contrast regions in the focal plane using deformable mirrors (DMs). Recently, it has been shown that it is possible to exceed the controllable outer-working angle imposed by the Nyquist limit based on the number of actuators by utilizing a diffraction grating. The print-through pattern on MEMS-based DMs formed during the fabrication process creates both an amplitude and a phase diffraction grating that can be used to enable Super-Nyquist wavefront control. Using interferometric measurements of a DM-actuator, we develop a DM-diffraction grating model. We compare the total energy enclosed in the first diffraction order due to the phase, amplitude, and combined phase-amplitude gratings with laboratory measurements.
For performance verification of an external occulter mask (also called a starshade), scaled testbeds have been developed to measure the suppression of the occulter shadow in the pupil plane and contrast in the image plane. For occulter experiments the scaling is typically performed by maintaining an equivalent Fresnel number. The original Princeton occulter testbed was oversized with respect to both input beam and shadow propagation to limit any diffraction effects due to finite testbed enclosure edges; however, to operate at realistic space-mission equivalent Fresnel numbers an extended testbed is currently under construction. With the longer propagation distances involved, diffraction effects due to the edge of the tunnel must now be considered in the experiment design. Here, we present a diffraction-based model of two separate tunnel effects. First, we consider the effect of tunnel-edge induced diffraction ringing upstream from the occulter mask. Second, we consider the diffraction effect due to clipping of the output shadow by the tunnel downstream from the occulter mask. These calculations are performed for a representative point design relevant to the new Princeton occulter experiment, but we also present an analytical relation that can be used for other propagation distances.
Princeton University is upgrading our space occulter testbed. In particular, we are lengthening it to ~76m to achieve flightlike Fresnel numbers. This much longer testbed required an all-new enclosure design. In this design, we prioritized modularity and the use of commercial off-the-shelf (COTS) and semi-COTS components. Several of the technical challenges encountered included an unexpected slow beam drift and black paint selection. Herein we describe the design and construction of this long-travel laser enclosure.
The Phase Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC) is an architecture for directly observing extra-solar planets, and can achieve performance near the theoretical limits for any direct-detection instrument. The PIAACMC architecture includes aspheric PIAA optics, and a complex phase-shifting focal plane mask that provides a pi phase shift to a portion of the on-axis starlight. The phase-shifted starlight is forced to interfere destructively with the un-shifted starlight, causing the starlight to be eliminated, and allowing a region for high-contrast imaging near the star.
The PIAACMC architecture can be designed for segmented and obscured apertures, so it is particularly well suited for ground-based observing with the next generation of large telescopes. There will be unique scientific opportunities for directly observing Earth-like planets around nearby low-mass stars. We will discuss design strategies for adapting PIAACMC for the next generation of large ground-based telescopes, and present progress on the development of the focal plane mask technology. We also present simulations of wave-front control with PIAACMC, and suggest directions to apply the coronagraph architecture to future telescopes.
The EXoplanetary Circumstellar Environments and Disk Explorer (EXCEDE) science mission concept uses a visible-wavelength phase-induced amplitude apodization (PIAA) coronagraph to enable high-contrast imaging of circumstellar debris systems and some giant planets at angular separations reaching into the habitable zones of some of the nearest stars. We report on the experimental results obtained in the vacuum chamber at the Lockheed Martin Advanced Technology Center in 10% broadband light centered about 650 nm, with a median contrast of 1×10−5 between 1.2 and 2.0λ/D simultaneously with 3×10−7 contrast between 2 and 11λ/D for a single-sided dark hole using a deformable mirror (DM) upstream of the PIAA coronagraph. These results are stable and repeatable as demonstrated by three measurement runs with DM settings set from scratch and maintained on the best 90% out of the 1000 collected frames. We compare the reduced experimental data with simulation results from modeling observed experimental limits. The observed performance is consistent with uncorrected low-order modes not estimated by the low-order wavefront sensor. Modeled sensitivity to bandwidth and residual tip/tilt modes is well matched to the experiment.
Direct imaging of earth-like exoplanets in the Habitable Zone of sun-like stars requires image contrast of ~10^10 at angular separations of around a hundred milliarcseconds. One approach for achieving this performance is to fly a starshade at a long distance in front of the telescope, shading the telescope from the direct starlight, but allowing planets around the star to be seen. The starshade is positioned so that sunlight falls on the surface away from the telescope, so the sun does not directly illuminate it. However, sunlight scattered from the starshade edge can enter the telescope, raising the background light level and potentially preventing the starshade from delivering the required contrast. As a result, starshade edge design has been identified as one of the highest priority technology gaps for external occulter missions in the NASAs Exoplanet Exploration Program Technology Plan 2013. To reduce the sunlight edge scatter to an acceptable level, the edge Radius Of Curvature (ROC) should be 1μm or less (commercial razor blades have ROC of a few hundred nanometer). This poses a challenging manufacturing requirement and may make the occulter difficult to handle. In this paper we propose an alternative approach to controlling the edge scattering by applying a flexible metamaterial to the occulter edge. Metamaterials are artificially structured materials, which have been designed to display properties not found in natural materials. Metamaterials can be designed to direct the scatter at planned incident angles away from the space telescope, thereby directly decreasing the contaminating background light. Reduction of the background light translates into shorter integration time to characterize a target planet and therefore improves the efficiency of the observations. As an additional benefit, metamaterials also have potential to produce increased tolerance to edge defects.
One of the main candidates for creating high-contrast for future Exo-Earth detection is an external occulter or sharshade. A starshade blocks the light from the parent star by flying in formation along the line-of-sight from a space telescope. Because of its large size and scale it is impossible to fully test a starshade system on the ground before launch. Instead, we rely on modeling supported by subscale laboratory tests to verify the models. At Princeton, we are designing and building a subscale testbed to verify the suppression and contrast of a starshade at the same Fresnel number as a flight system, and thus mathematically identical to a realistic space mission. Here we present the mechanical design of the testbed and simulations predicting the ultimate contrast performance. We will also present progress in implementation and preliminary results.
Star light suppression technologies to find and characterize faint exoplanets include internal coronagraph instruments as well as external star shade occulters. Currently, the NASA WFIRST-AFTA mission study includes an internal coronagraph instrument to find and characterize exoplanets. Various types of masks could be employed to suppress the host star light to about 10-9 level contrast over a broad spectrum to enable the coronagraph mission objectives. Such masks for high contrast internal coronagraphic imaging require various fabrication technologies to meet a wide range of specifications, including precise shapes, micron scale island features, ultra-low reflectivity regions, uniformity, wave front quality, achromaticity, etc. We present the approaches employed at JPL to produce pupil plane and image plane coronagraph masks by combining electron beam, deep reactive ion etching, and black silicon technologies with illustrative examples of each, highlighting milestone accomplishments from the High Contrast Imaging Testbed (HCIT) at JPL and from the High Contrast Imaging Lab (HCIL) at Princeton University. We also present briefly the technologies applied to fabricate laboratory scale star shade masks.
Usage of an internal coronagraph with an adaptive optical system for wavefront correction for direct imaging of exoplanets is currently being considered for many mission concepts, including as an instrument addition to the WFIRST-AFTA mission to follow the James Web Space Telescope. The main technical challenge associated with direct imaging of exoplanets with an internal coronagraph is to effectively control both the diffraction and scattered light from the star so that the dim planetary companion can be seen. For the deformable mirror (DM) to recover a dark hole region with sufficiently high contrast in the image plane, wavefront errors are usually estimated using probes on the DM. To date, most broadband lab demonstrations use narrowband filters to estimate the chromaticity of the wavefront error, but this reduces the photon flux per filter and requires a filter system. Here, we propose a method to estimate the chromaticity of wavefront errors using only a broadband image. This is achieved by using special DM probes that have sufficient chromatic diversity. As a case example, we simulate the retrieval of the spectrum of the central wavelength from broadband images for a simple shaped- pupil coronagraph with a conjugate DM and compute the resulting estimation error.
This paper is the fourth in the series on the technology development for the EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer) mission concept, which in 2011 was selected by NASA's Explorer program for technology development (Category III). EXCEDE is a 0.7m space telescope concept operating with a Phase Induced Amplitude Apodization (PIAA) Coronagraph and a Deformable Mirror (DM) to create a "dark-hole" or a region of high-contrast starlight suppression at the focal plane to allow direct imaging of exoplanets. This will allow fundamental science in the form of direct detection and spatial resolution of low surface brightness circumstellar debris disks, and the direct imaging of giant planets with angular separations as close in as the habitable zone of the host star. Thus, EXCEDE can function as both a scientific and technological precursor for any mission capable of imaging exo-Earths.
Previously, we have reported experimental results on the first milestone, the demonstration of EXCEDE contrast in monochromatic light in air and more recently in vacuum. In this paper, we report on the procedure and the experimental results obtained for our second milestone demonstration of the EXCEDE starlight suppression system carried in a vacuum chamber at the Lockheed Martin Advanced Technology Center. This includes high contrast performance demonstrations at 1.2 λD, which includes a lab demonstration of 1x10-5 median contrast between 1.2 and 2.0 λD simultaneously with 3x10-7 median contrast between 2 and 11 λD in 10% bandwidth polychromatic light centered at 650 nm for a single-sided dark zone. The results are stable and repeatable as demonstrated by three measurement runs with DM settings set from scratch and maintained on the best 90% out of the 1000 collected frames per run. We compare reduced experimental data with simulation results from modeling experimental limits.
An external occulter is a spacecraft own along the line-of-sight of a space telescope to suppress starlight and enable high-contrast direct imaging of exoplanets. The shape of an external occulter must be specially designed to optimally suppress starlight; however, deviations from the ideal shape such as manufacturing errors can result in loss of suppression in the shadow. Due to the long separation distances and large dimensions involved for a space occulter, laboratory testing is conducted with scaled versions of occulters etched on silicon wafers. Using numerical simulations for a ight Fresnel occulter design, we show how the suppression performance of an occulter mask scales with the available propagation distance for expected random manufacturing defects along the edge of the occulter petal. We derive an analytical model for predicting performance due to such manufacturing defects across the petal edges of an occulter mask and compare this with the numerical simulations. We discuss the scaling of an extended occulter testbed.
An external occulter is a specially-shaped spacecraft own along the line-of-sight of a space telescope to block starlight before reaching its entrance pupil. Using optimization methods, occulter shapes can be designed to most effectively block starlight. A full-scale occulter cannot be tested on the ground and its performance must be predicted; therefore the fidelity of the optical propagation models used for design and performance prediction must be verified under scaled conditions. In this paper we present both contrast and suppression laboratory measurements for a scaled occulter, and perform a diffractive analysis to determine the factors limiting performance of the laboratory occulter.
An external occulter is a specially-shaped spacecraft own in formation with a telescope. It enables high-contrast imaging of the dim planetary companions of the neighboring solar system by blocking starlight before it reaches the entrance pupil. Occulters have to be designed via optimization methods that account for diffraction to most effectively block starlight. To predict occulter performance, we must verify the fidelity of the optical propagation models under scaled conditions. In this paper, we measure the contrast of a scaled occulter. The validity of the contrast calibration is determined using a baseline circular occulter. We verify contrast better than 10-10, however the measurements do not perform as well as the prediction from theoretical modelling. We attribute this difference to glint scattering off mask edges.
We present and compare experimental results in high contrast imaging representing the state of the art in coronagraph and starshade technology. These experiments have been undertaken with the goal of demonstrating the capability of detecting Earth-like planets around nearby Sun-like stars. The contrast of an Earth seen in reflected light around a Sun-like star would be about 1.2 × 10−10. Several of the current candidate technologies now yield raw contrasts of 1.0 × 10−9 or better, and so should enable the detection of Earths, assuming a gain in sensitivity in post-processing of a factor of 10. We present results of coronagraph and starshade experiments conducted at visible and infrared wavelengths. Cross-sections of dark fields are directly compared as a function of field angle and bandwidth. The strength and differences of the techniques are compared.
An external occulter precisely flown in formation with a space telescope has been recently studied as a mission
scenario for direct imaging of exoplanets. 10-10 contrast must be attained to permit imaging of the faint reflected
light of an Earth-like exoplanet. Here we report on experimental verification of a scaled occulter design that
maintains a constant Fresnel number to an equivalent 400 mas space mission in monochromatic light at 632 nm
using a diverging beam and an outer mask to suspend the occulter mask. We report contrast in regions of the
discovery zone at 4.71 × 10-10 and suppression in the pupil plane of 3.7 × 10-7.
An external occulter flown in precise formation with a telescope is being considered for high-contrast direct
imaging of exoplanets as a viable mission scenario. In this paper, the dynamics about the Sun-Earth L2 region for
an occulter-telescope constellation are considered in conjunction with fourth-body and solar radiation pressure
acting as disturbing forces. An optimal observation window is defined in terms of both thrust required and
the Sun-constellation geometry. By simulation, the effects of the stellar latitude and distance, the spacecraft
separation, the magnitude of the disturbing forces, and on-off versus continuous thrusting are quantified on the
thrusting profile needed to maintain precise alignment.
The Princeton occulter testbed uses long-distance propagation with a diverging beam and an optimized
occulter mask to simulate the performance of external occulters for finding extrasolar planets. We present
new results from the testbed in both monochromatic and broadband light. In addition, we examine sensing
and control of occulter position using out-of-band spectral leak around the occulter and occulter position
tolerancing. These results are validated by numerical simulations of propagation through the system.
An occulter is used in conjunction with a separate telescope to suppress the light of a distant star. To
demonstrate the performance of this system, we are building an occulter experiment in the laboratory at
Princeton. This experiment will use an etched silicon mask as the occulter, with some modifications to try
to improve the performance. The occulter is illuminated by a diverging laser beam to reduce the aberrations
from the optics before the occulter. We present the progress of this experiment and expectations for future
There is an emerging interest in vertically-integrated CMOS (VI-CMOS) image sensors. This trend arises from
the difficulty in achieving high SNR, high dynamic range, and high frame rate with planar technologies while
maintaining small pixel sizes, since the photodetector and electronics have to share the same pixel area and
use the same technology. Fabrication methods for VI-CMOS image sensors add new degrees of freedom to
the photodetector design. Having a model that gives a good approximation to the behavior of a device under
different operating conditions is important for device optimization. This work presents a new approach in
photodetector modeling, and uses it to optimize the thickness of the photosensitive layer in VI-CMOS image
sensors. We consider a simplified structure of an a-Si:H photodetector, and develop an analytical solution and a
numerical solution to state equations taken from semiconductor physics, which are shown to be comparable. If
the photosensitive layer is too thin, our model shows that the contact resistances dominate the device and, if it
is too thick, most charge carriers recombine on their way to the contacts. Therefore, an optimum thickness can