Amir Give'on

Broadband wavefront correction algorithm for high-contrast imaging systems

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.

Closed Loop, DM Diversity-based, Wavefront Correction Algorithm for High Contrast Imaging Systems

High contrast imaging from space relies on coronagraphs to limit diffraction and a wavefront control systems to compensate for imperfections in both the telescope optics and the coronagraph. The extreme 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. This paper presents a general closed loop correction algorithm for high contrast imaging coronagraphs by minimizing the energy in a predefined region in the image where terrestrial planets could be found. The estimation part of the algorithm reconstructs the complex field in the image plane using phase diversity caused by the deformable mirror. This method has been shown to achieve faster and better correction than classical speckle nulling.

Demonstration of high contrast in 10% broadband light with the shaped pupil 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 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.

Pair-Wise, Deformable Mirror, Image Plane-Based Diversity Electric Field Estimation for High Contrast Coronagraphy

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.

A conceptual design for the Thirty Meter Telescope alignment and phasing system

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.

Phase induced amplitude apodization (PIAA) coronagraphy: recent results and future prospects

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).

Phase-Induced Amplitude Apodization (PIAA) coronagraph testing at the High Contrast Imaging Testbed

We present the current status of our testing of a phase-induced amplitude apodization (PIAA) coronagraph at the Jet Propulsion Labs 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.

Pupil mapping exoplanet coronagraphic observer (PECO)

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. PECOs 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 PECOs design and will enable high contrast at the very small PECO inner working angle.

Laboratory demonstrations of high-contrast imaging for space coronagraphy

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.

A unified formailism for high contrast imaging correction algorithms

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.