Thanks to the use of aspheric optics for lossless apodization, the Phase Induced Amplitude Apodization (PIAA)
coronagraph offers full throughput, high contrast and small inner working angle. It is therefore ideally suited for space-based
direct imaging of potentially habitable exoplanets. The concept has evolved since its original formulation to
mitigate manufacturing challenges and improve performance. Our group is currently aiming at demonstrating PIAA
coronagraphy in the laboratory to 1e-9 raw contrast at 2 λ/D separation. Recent results from the High Contrast Imaging
Testbed (HCIT) at JPL demonstrate contrasts about one order of magnitude from this goal at 2 λ/D. In parallel with our
high contrast demonstration at 2 λ/D, we are developing and testing new designs to reduce inner working angle and
improve performance in polychromatic light. The newly developed PIAA complex mask coronagraph (PIAACMC)
concept provides total starlight extinction and offers full throughput with a sub- λ/D inner working angles. We also
describe a recent laboratory demonstration of fine pointing control with PIAA.
In a setup similar to the self coherent camera, we have added a set of pinholes in the diffraction ring of the Lyot plane in a high-contrast stellar Lyot coronagraph. We describe a novel complex electric field reconstruction from image plane intensity measurements consisting of light in the coronagraph's dark hole interfering with light from the pinholes. The image plane field is modified by letting light through one pinhole at a time. In addition to estimation of the field at the science camera, this method allows for self-calibration of the probes by letting light through the pinholes in various permutations while blocking the main Lyot opening. We present results of estimation and calibration from the High Contrast Imaging Testbed along with a comparison to the pair-wise deformable mirror diversity based estimation technique. Tests are carried out in narrow-band light and over a composite 10% bandpass.
In this paper we describe the complex electric field reconstruction from image plane intensity measurements for
high contrast coronagraphic imaging. A deformable mirror (DM) surface is modified with pairs of complementary
shapes to create diversity in the image plane of the science camera where the intensity of the light is measured.
Along with the Electric Field Conjugation correction algorithm, this estimation method has been used in various
high contrast imaging testbeds to achieve the best contrasts to date both in narrow and in broad band light. We
present the basic methodology of estimation in easy to follow list of steps, present results from HCIT and raise
several open questions we are confronted with using this method.
The Phase Induced Amplitude Apodization (PIAA) concept uses aspheric optics to apodize a telescope beam
for high contrast imaging. The lossless apodization, achieved through geometrical redistribution of the light
(beam shaping) allows designs of high performance coronagraphs, ideally suited for direct imaging of exoplanets
similar to Earth around nearby stars. The PIAA coronagraph concept has evolved since its original formulation
to mitigate manufacturing challenges and improve performance. Our group is currently aiming at demonstrating
PIAA coronagraphy in the laboratory to 1e-9 raw contrast at 2 λ/D separation. Recent results from the High
Contrast Imaging Testbed (HCIT) at NASA JPL and the PIAA testbed at NASA Ames demonstrate contrasts
about one order of magnitude from this goal at 2 λ/D. In parallel with our high contrast demonstration at
2λ/D, we are developing and testing new designs at a complementary testbed at NASA Ames, and solving
associated technical challenges. Some of these new PIAA designs have been tested that can further mitigate PIAA
manufacturing challenges while providing theoretically total starlight extinction and offering 50% throughput at
less than 1 λ/D. Recent tests demonstrated on the order of 1e-6 contrast close to 1 λ/D (while maintaining 5e-8
contrast at 2 λ/D).
The High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory employs a broadband wavefront correction
algorithm called Electric Field Conjugation (EFC) to obtain the required 10-10 contrast. This algorithm works with one
deformable mirror (DM) to estimate the electric-field to be controlled, and with one or multiple DM's to create a "darkhole"
in a predefined region of the image plane where terrestrial planets would be found. We have investigated the
effects of DM actuator errors and the optic position errors on the efficiency of the EFC algorithm in a Lyot coronagraph
configuration. The structural design of the optical system as well as the parameters of various optical elements used in
the analysis are drawn from those of the HCIT system that have been implemented with one DM. The simulation takes
into account the surface errors of various optical elements. Results of some of these studies have been verified by actual
measurements.
We present high-contrast images from laboratory testing of a Phase Induced Amplitude Apodization (PIAA)
coronagraph at NASA's High Contrast Imaging Testbed (HCIT). Using a deformable mirror and wavefront estimation
and control algorithms, we create a "dark hole" in the monochromatic point-spread function with an inner working angle
of 2.05 fλ/D, with a mean intensity 3.5×10-8. We discuss the contributions to this floor, and the techniques being
developed to improve it. We also present simulations that investigate the effect of Lyot stops of various sizes, and
conclude that a Lyot stop is necessary for 10-9 performance but that an annular postapodizer is not necessary.
The NASA Exoplanet program and the Cosmic Origins program are exploring technical options to combine the visible
to NIR performance requirements of a space coronagraph with the general astrophysics requirements of a space
telescope covering the deep UV spectrum. Are there compatible options in terms of mirror coatings and telescope
architecture to satisfy both goals? In this paper, we address some of the main concerns, particularly relating to
polarization in the visible and throughput in the UV. Telescope architectures employing different coating options
compatible with current technology are considered in this trade study.
A 4-8m diameter telescope carrying a coronagraph instrument is a leading candidate for an anticipated flagship mission
to detect and characterize Earth-size exoplanets in the 2020s.1 Many candidate coronagraph instruments have been proposed,
and one is close to meeting some of the principal requirements for that mission. But the telescope and instrument
will need exquisite stability and precise control of the incoming wavefront to enable detection of faint companions (10-10of the star) at an angular separation of 2-4 Airy radii. In particular, wavefront errors cause speckles in the image, and
variations in those speckles can confound the exoplanet detection. This challenge is compounded by the background
light from zodiacal dust around our Sun and the target star, which limits the speed with which we can estimate and correct
the speckles. We are working on developing coherent speckle detection techniques that will allow rapid calibration
of speckles on the science detector, allowing subtraction in post-processing or correction with deformable mirrors. The
expected speed improvement allows a much quicker timeline for measurement & calibration, which reduces the required
telescope stability requirement and eases both the flight system design and the challenge of ground testing. We will describe
the experiments and summarize progress to date.
The High Contrast Imaging Testbed Phase Induced Amplitude Apodization (HCIT/PIAA) coronagraph system at JPL
relies on an Electric-Field Conjugation (EFC) wavefront correction algorithm to create a high contrast point-spread
function (PSF). This algorithm works with one deformable mirror (DM) to estimate the electric-field to be controlled,
and with one or multiple DM's to create a "dark-hole" in the image plane. We have investigated the effects of DM
actuator errors on the efficiency of the EFC algorithm. The structural design of the optical system as well as the
parameters of various optical elements used in the analysis are drawn from those of the HCIT/PIAA system that have
been and will be implemented with one or two DM's. The simulation takes into account the surface errors of various
optical elements. In this paper, we report our findings in the case of narrowband wavelength light.
High contrast imaging is an ongoing theme in the domain of astronomy, both for ground-based and space-based
telescopes. Achieving 106 - 107 contrasts expected with GPI and SPHERE or 1010 contrast for space projects,
requires extreme wavefront correction as well as good coronagraphic systems. With the testbed located at the
Laboratory of Adaptive Optics in Santa Cruz, we statically correct the wavefront to 0.5 nm in band and reach
contrast of a few 107 with an Apodized Lyot Coronagraph (APLC). The Electric Field Conjugation (EFC) allows
us to further improve on this performance. EFC is a formalism of the correction problem that computes the
actuator commands for the deformable mirror (DM) to correct for both amplitude and phase in a pre-defined
region in the final image plane. In order to take into account
non-common-path errors and potential amplitude
aberrations, the proper actuator commands are computed using an image plane-based DM diversity by means of
reconstructing the complex electric field. Already successfully tested for space-based telescopes, we here attempt
to adapt this method to ground-based observations, using the EFC high contrast solution to record new reference
centroids for a Shack-Hartmann wavefront sensor, which in turn can be used to recreate the far-field image. This
paper shows results of this first use of the EFC method with an APLC. We achieved 4.108 contrast on a [4-9]
λ/d square region. We also show that it can be applied to ground based adaptive optics, using Shack-Hartmann
wavefront sensors.
The detection and characterization of earth-like exo-planets with space coronagraph instruments could be adversely
affected by contamination of the many optical surfaces from the telescope primary mirror to the coronagraph mask.
Particulate contamination that may accumulate even in clean room conditions over the period of integration, testing, and
launch can cause performance degradation due to both coherent and incoherent scatter. While the coherent components
can be compensated in broad-band light using a sequential deformable mirror architecture, incoherent scatter would
remain. We show the challenges and effects of particulate contamination based on measurements and estimates, and
discuss the requirements throughout the coronagraph system while accounting for the wavefront control system.
In this communication we address two outstanding issues pertaining the modeling of PIAA coronagraphs, accurate
numerical propagation of edge effects and fast propagation of mid spatial frequencies for wavefront control.
In order to solve them, we first derive a quadratic approximation of the Huygens wavelets that allows us to
develop an angular spectrum propagator for pupil remapping. Using this result we introduce an independent
method to verify the ultimate contrast floor, due to edge propagation effects, of PIAA units currently being
tested in various testbeds. We then delve into the details of a novel fast algorithm, based on the recognition that
angular spectrum computations with a pre-apodised system are computationally light. When used for the propagation
of mid spatial frequencies, such a fast propagator will ultimately allow us to develop robust wavefront
control algorithms with DMs located before the pupil remapping mirrors.
We present the current status of our testing of a phase-induced amplitude apodization (PIAA) coronagraph at the Jet
Propulsion Lab's High Contrast Imaging Testbed (HCIT) vacuum facilities. These PIAA optics were designed to
produce a point-spread function containing a region whose intensity is below 10-9 over a 20-percent fractional bandpass,
comparable to the requirements for direct imaging of exoplanets from space. The results presented here show contrast
levels of 4×10-7 in monochromatic light, with an inner working angle of 2.4 λ/D. The instrumentation is described here,
as well as the testing procedures, wavefront control, and results.
This paper introduces a unified formulism to describe many of the high contrast correction methods, namely,
phase conjugation, classical speckle nulling and energy minimization. This unified formalism led to the Electric
Field Conjugation (EFC) algorithm where the solution found is such that minimizes the sum of the estimated
electric field at a desired plane and the electric field due to the corrective elements in the system. Applying this
formalism led to the conclusion that all the other methods are special cases of EFC.
The Pupil-mapping Exoplanet Coronagraphic Observer (PECO) mission concept uses a coronagraphic 1.4-m
space-based telescope to both image and characterize extra-solar planetary systems at optical wavelengths.
PECO delivers 10-10 contrast at 2 λ/D separation (0.15") using a high-performance Phase-Induced Amplitude
Apodization (PIAA) coronagraph which remaps the telescope pupil and uses nearly all of the light coming into
the aperture. For exoplanet characterization, PECO acquires narrow field images simultaneously in 16 spectral
bands over wavelengths from 0.4 to 0.9 μm, utilizing all available photons for maximum wavefront sensing and
sensitivity for imaging and spectroscopy. The optical design is optimized for simultaneous low-resolution spectral
characterization of both planets and dust disks using a moderate-sized telescope. PECO will image the habitable
zones of about 20 known F, G, K stars at a spectral resolution of R≈15 with sensitivity sufficient to detect
and characterize Earth-like planets and to map dust disks to within a fraction of our own zodiacal dust cloud
brightness. The PIAA coronagraph adopted for PECO reduces the required telescope diameter by a factor of two
compared with other coronagraph approaches that were considered for Terrestrial Planet Finder Coronagraph
Flight Baseline 1, and would therefore also be highly valuable for larger telescope diameters. We report on
ongoing laboratory activities to develop and mature key PECO technologies, as well as detailed analysis aimed
at verifying PECO's wavefront and pointing stability requirement can be met without requiring development of
new technologies.
The primary, secondary and tertiary mirrors of the Thirty Meter Telescope (TMT), taken together, have approximately
12,000 degrees of freedom in optical alignment. The Alignment and Phasing System (APS) will use
starlight and a variety of Shack-Hartmann based measurement techniques to position the segment pistons, tips,
and tilts, segment figures, secondary rigid body motion, secondary figure and the tertiary figure to correctly align
the TMT. We present a conceptual design of the APS including the requirements, alignment modes, predicted
performance, software architecture, and an optical design.
The Pupil mapping Exoplanet Coronagraphic Observer (PECO) mission concept is a 1.4-m telescope aimed at
imaging and characterizing extra-solar planetary systems at optical wavelengths. The coronagraphic method
employed, Phase-Induced Amplitude Apodization or PIAA (a.k.a. pupil mapping) can deliver 1e-10 contrast at
2 lambda/D and uses almost all the starlight that passes through the aperture to maintain higher throughput and
higher angular resolution than any other coronagraph or nuller, making PECO the theoretically most efficient
existing approach for imaging extra-solar planetary systems. PECO's instrument also incorporates deformable
mirrors for high accuracy wavefront control. Our studies show that a probe-scale PECO mission with 1.4 m
aperture is extremely powerful, with the capability of imaging at spectral resolution R≈∠15 the habitable zones
of already known F, G, K stars with sensitivity sufficient to detect planets down to Earth size, and to map
dust clouds down to a fraction of our zodiacal cloud dust brightness. PECO will acquire narrow field images
simultaneously in 10 to 20 spectral bands covering wavelengths from 0.4 to 1.0 μm and will utilize all available
photons for maximum wavefront sensing and imaging/spectroscopy sensitivity. This approach is well suited for
low-resolution spectral characterization of both planets and dust clouds with a moderately sized telescope.
We also report on recent results obtained with the laboratory prototype of a coronagraphic low order wavefront
sensor (CLOWFS) for PIAA coronagraph. The CLOWFS is a key part of PECO's design and will enable high
contrast at the very small PECO inner working angle.
The Shaped Pupil Coronagraph (SPC) is a high-contrast imaging system pioneered at Princeton for detection of extra-solar earthlike planets. It is designed to achieve 10-10 contrast at an inner working angle of 4λ/D in broadband light. A critical requirement in attaining this contrast level in practice is the ability to control wavefront phase and amplitude aberrations to at least λ/104 in rms phase and 1/1000 rms amplitude, respectively. Furthermore, this has to be maintained over a large spectral band. The High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Lab (JPL) is a state-of-the-art facility for studying such high contrast imaging systems and wavefront control methods. It consists of a vacuum chamber containing a configurable coronagraph setup with a Xinetics deformable mirror. Previously, we demonstrated 4x10-8 contrast with the SPC at HCIT in 10% broadband light. The limiting factors were subsequently identified as (1) manufacturing defects due to minimal feature size constraints on our shaped pupil masks and (2) the inefficiency of the wavefront correction algorithm we used (classical speckle nulling) to correct for these defects. In this paper, we demonstrate the solutions to both of these problems. In particular, we present a method to design masks with practical minimal feature sizes and show new manufactured masks with few defects. These masks were installed at HCIT and tested using more sophisticated wavefront control algorithms based on energy minimization of light in the dark zone. We present the results of these experiments, notably a record 2.4×10-9 contrast in 10% broadband light.
Space coronagraphy is a promising method for direct imaging of planetary systems orbiting the nearby stars. The High
Contrast Imaging Testbed is a laboratory facility at JPL that integrates the essential hardware and control algorithms
needed for suppression of diffracted and scattered light near a target star that would otherwise obscure an associated
exo-planetary system. Stable suppression of starlight by a factor of 5×10−10 has been demonstrated consistently in
narrowband light over fields of view as close as four Airy radii from the star. Recent progress includes the extension of
spectral bandwidths to 10% at contrast levels of 2×10−9, with work in progress to further improve contrast levels,
bandwidth, and instrument throughput. We summarize recent laboratory results and outline future directions. This
laboratory experience is used to refine computational models, leading to performance and tolerance predictions for
future space mission architectures.
Pupil mapping (a.k.a. Phase-Induced Amplitude Apodization, or PIAA) is a promising technique in high-dynamic range
stellar coronagraphy that obtains higher throughput and better inner working angle than any other known approach. As
with any coronagraph, the optical surface requirements and the diameter of the controllable region in the image plane
are tied to the wavefront control system and optical bandpass. For example, in a monochromatic bandpass, a single ideal
deformable mirror (DM) can create a dark hole with a diameter limited by its Nyquist frequency, even for highly
aberrated optics. In broadband light, the depth of the dark hole is linked to the wavelength dependence of aberrations,
their spatial frequency content, and their propagation through the system. We derive requirements on the surface height
and reflectivity power spectral densities for optics in the PIAA system and describe a sequential-DM architecture that
will achieve high-contrast over a large optical bandwidth.
Great strides have been made in recent years toward the goal of high-contrast imaging with a sensitivity adequate
to detect earth-like planets around nearby stars. It appears that the hardware − optics, coronagraph masks,
deformable mirrors, illumination systems, thermal control systems − are up to the task of obtaining the required
10-10 contrast. But in broadband light (e.g., 10% bandpass) the wavefront control algorithms have been a
limiting factor. In this paper we describe a general correction methodology that works in broadband light with
one or multiple deformable mirrors by conjugating the electric field in a predefined region in the image where
terrestrial planets would be found. We describe the linearized approach and demonstrate its effectiveness through
laboratory experiments. This paper presents results from the Jet Propulsion Laboratory High Contrast Imaging
Testbed (HCIT) for both narrow-band light (2%) and broadband light (10%) correction.
High contrast imaging from space must overcome photon noise of the diffracted star light and scattered light
from optical components defects. The very high contrast required (up to 10-10 for terrestrial planets) puts
severe requirements on the wavefront control system, as the achievable contrast is limited by the quality of the
wavefront. The "Peak-a-boo" correction algorithm, presented here, is a closed loop correction method for the
shaped pupil coronagraph to minimize the energy in a pre-defined region in the image where terrestrial planets
would be found. The reconstruction part uses three intensity measurements in the image plane with a pinhole
added to the shaped pupil for diversity. This method has been shown in simulations to converge to the nominal
contrast in 2-3 iterations. In addition, the "peak-a-boo" has shown to be effective in broadband conditions.
KEYWORDS: Wavefronts, Coronagraphy, Planets, Point spread functions, Stars, Light scattering, Imaging systems, Error analysis, High dynamic range imaging, Control systems
NASA is endeavoring to launch missions capable of detecting Earth-like planets around neighboring stars. In visible wavelengths, this requires better than one 10 to the minus ten suppression of scattered light as close as 50 milli-arcsec to the stellar image. This extraordinary requirement is within reach but it requires broad-band wave front control to sub-Angstrom levels. We describe several high dynamic range imaging solutions, describe the various factors that contribute to the scattered light level and introduce a novel closed-loop broad-band correction system, suitable for the Shaped Pupil Coronagraph and the Lyot Coronagraph.
The Shaped Pupil Coronagraph (SPC) is a high-contrast imaging system pioneered at Princeton for detection of extra-solar earthlike planets. It is designed to achieve 10-10 contrast at an inner working angle of 4λ/D. However, a critical requirement in attaining this contrast level in practice is the ability to control wavefront phase and amplitude aberrations to at least λ/104 in rms phase and 1/1000 rms amplitude, respectively. Furthermore, this has to be maintained over a large spectral band. The High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Lab (JPL) is a state-of-the-art facility for studying high contrast imaging systems and fine wavefront control methods. It consists of a vacuum chamber containing a configurable coronagraph setup with a Xinetics deformable mirror. In this paper, we present the results of testing Princeton's SPC in JPL's HCIT. In particular, we present the achievement of 4x10-8 contrast using a speckle nulling algorithm, and demonstrate that this contrast is maintained across wavelengths of 785, 836nm, and for broadband light having 10% bandwidth around 800nm.
The achievable contrast level for space-based detection of exo-planets will be limited by the stability of the optics. As a consequence, active amplitude and phase compensation will be needed. In order to mitigate these wavefront instabilities, we suggested, as an alternative to classical adaptive optics, the use of a Michelson interferometer equipped with two deformable mirrors. Simulations showed that this set up is able to create a symmetric "dark hole" in an appropriate area of the image plane. However, increasing the bandwidth of the incident light critically alters this nulling performance. A quantitative analysis of this effect will first be presented. An alternative to circumvent this problem is to introduce a dispersive element in one of the legs of the interferometer so that the path length difference does not exhibit the one over wavelength dependence. In the case of the insertion of a gaseous cell, the OPD could then be controlled by pressure variations. The last section of this paper will present a simulation-oriented proof of concept relying on the dispersive properties of nitrogen.
Conventional adaptive optics systems correct the wavefront by
adjusting a deformable mirror based on measurements of the phase
aberration taken in a pupil plane. The ability of this technique,
known as phase conjugation, to correct aberrations is normally
limited by the maximum spatial frequency of the DM. In this paper
we show that conventional phase conjugation is not able to achieve
the dark nulls needed for high-contrast imaging. Linear
combinations of high frequencies in the aberration at the pupil
plane "fold" and appear as low frequency aberrations at the
image plane. After describing the frequency folding phenomenon,
we present an alternative optimized solution for the shape of the
deformable mirror based on the Fourier decomposition of the
effective phase and amplitude aberrations.
Current plans call for the first Terrestrial Planet Finder mission, TPF-C, to be a monolithic space telescope with a coronagraph for achieving high contrast. Our group at Princeton pioneered the concept of shaped pupils for high contrast imaging and planet detection. In previous papers we introduced a number of families of optimal shaped pupils in square, circular, and elliptical apertures. Here, we show our most promising designs and present our laboratory results for the elliptical shaped pupil. We are currently achieving better than 10−7 contrast at 10 λ/D and 10−5 contrast at 4 λ/D, without wavefront control. We describe the deep ion etching manufacturing process to make free standing masks. We also discuss what is limiting contrast in the laboratory and our progress in wavefront correction.
The ultimate limitation of visible light high-dynamic-range imaging systems such as shaped pupil coronagraphs comes from scattering caused by imperfections in the optical surfaces of the collecting system, and from the non-uniform reflectivity of those surfaces. This paper focuses on the correction of these imperfections using two deformable mirrors in a zero path length difference Michelson interferometer. Simulations show the advantages and limitations of introducing such a device into a wavefront control loop. Laboratory work shows progress towards high resolution amplitude control.
Conventional adaptive optics methods use phase conjugation based
on measurements of the phase aberrations at the pupil plane. The
measurements are typically done using a Shack-Hartmann sensor
sampling at spatial frequencies determined by the spatial
frequency limitations of the deformable mirror. The work presented
here shows that the nulling needed for high contrast imaging
cannot be achieved using such a methodology. Linear combinations
of high frequencies in the aberration at the pupil plane "fold"
and appear as low frequency aberrations at the image plane. We
present an optimized solution for the shape of the deformable
mirror based on the Fourier decomposition of the effective phase
aberration.
The Princeton University Terrestrial Planet Finder (TPF) has been working on a novel method for direct imaging of extra solar planets using a shaped-pupil coronagraph. The entrance pupil of the coronagraph is optimized to have a point spread function (PSF) that provides the suppression level needed at the angular separation
required for detection of extra solar planets. When integration time is to be minimized, the photon count at the planet location in the image plane is a Poisson distributed random process. The ultimate limitation of these high-dynamic-range imaging systems comes from scattering due to imperfections in the optical surfaces of the collecting system. The first step in correcting the wavefront errors is the estimation of the phase aberrations. The phase aberration caused by these imperfections is assumed to be a sum of two-dimensional sinusoidal functions. Its parameters are estimated using a global search with a genetic algorithm and a local optimization with the BFGS quasi-Newton method with a mixed quadratic and cubic line search procedure.
The Princeton University Terrestrial Planet Finder (TPF) group has been working on a novel method for direct imaging of extra solar planets using a shaped-pupil coronagraph. The entrance pupil of the coronagraph is optimized to have a point spread function (PSF) that provides the suppression level needed at the angular separation required for detection of extra solar planets. When integration time is to be minimized, the photon count at the planet location in the image plane is a Poisson distributed random process. The ultimate limitation of these high-dynamic-range imaging systems comes from scattering due to imperfections in the optical surfaces of the collecting system. The first step in correcting the wavefront errors is the estimation of the phase aberrations. The phase aberration caused by these imperfections is assumed to be a sum of two-dimensional sinusoidal functions. Assuming one uses a deformable mirror to correct these aberrations, we propose an algorithm that
optimally decreases the scattering level in specified localized areas in the image plane independent of the choice of influence function of the deformable mirror.
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