Joseph J. Green

Reflectivity and optical surface height requirements in a broadband coronagraph. 1.Contrast floor due to controllable spatial frequencies

We derive the broadband contrast floor in a coronagraphic telescope having nonideal optical surfaces. We consider only fundamental spatial frequencies within the control bandwidth of the coronagraphs deformable mirror. Cross terms arising from the beating of spatial frequencies beyond the deformable mirror control bandwidth will be considered in a second paper. Two wavefront control systems are analyzed:a zero-path difference Michelson interferometer with two deformable mirrors at a pupil image, and a sequential pair of deformable mirrors with one placed at a pupil image. We derive requirements on optical surface figure and reflectivity uniformity for both cases.

Coronagraph contrast demonstrations with the high-contrast imaging testbed

Predictions of contrast performance for the Eclipse coronagraphic telescope are based on computational models that are tested and validated with laboratory experience. We review recent laboratory work in the key technology areas for an actively-corrected space telescope designed for extremely high contrast imaging of nearby planetary systems. These include apodized coronagraphic masks, precision deformable mirrors, and coronagraphic algorithms for wavefront sensing and correction, as integrated in the high contrast imaging testbed at JPL. Future work will focus on requirements for the Terrestrial Planet Finder coronagraph mission.

Low-Order Aberration Sensitivity of Eighth-Order Coronagraph Masks

In a recent paper, Kuchner, Crepp, and Ge describe new image-plane coronagraph mask designs that reject to eighth order the leakage of starlight caused by image motion at the mask, resulting in a substantial relaxation of image centroiding requirements compared to previous fourth-order and second-order masks. They also suggest that the new masks are effective at rejecting leakage caused by low-order aberrations (e.g., focus, coma, and astigmatism). In this paper, we derive the sensitivity of eighth-order masks to aberrations of any order and provide simulations of coronagraph behavior in the presence of optical aberrations. We find that the masks leak light as the fourth power of focus, astigmatism, coma, and trefoil. This has tremendous performance advantages for the Terrestrial Planet Finder Coronagraph.

Hunting planets and observing disks with the JWST NIRCam coronagraph

The expected stable point spread function, wide field of view, and sensitivity of the NIRCam instrument on the James Webb Space Telescope (JWST) will allow a simple, classical Lyot coronagraph to detect warm Jovian-mass companions orbiting young stars within 150 pc as well as cool Jupiters around the nearest low-mass stars. The coronagraph can also be used to study protostellar and debris disks. At λ = 4.5 μm, where young planets are particularly bright relative to their stars, and at separations beyond ~0.5 arcseconds, the low space background gives JWST significant advantages over ground-based telescopes equipped with adaptive optics. We discuss the scientific capabilities of the NIRCam coronagraph, describe the technical features of the instrument, and present end-to-end simulations of coronagraphic observations of planets and circumstellar disks.

Adaptive cross-correlation algorithm for extended scene Shack-Hartmann wavefront sensing

We present an adaptive cross-correlation algorithm for a large dynamic range extended-scene Shack-Hartmann wavefront sensor. We show that it accurately measures very fine image shifts over many pixels under a variety of practical imaging conditions.

High-contrast imaging testbed for the Terrestrial Planet Finder coronagraph

One of the architectures under consideration for Terrestrial Planet Finder (TPF) is a visible coronagraph. To achieve TPF science goals, the coronagraph must have extreme levels of wavefront correction (less than 1 /spl Aring/ rms over controllable spatial frequencies) and stability to get the necessary suppression of diffracted starlight (10/sup -10/ contrast). The High Contrast Imaging Testbed is the TPF platform for laboratory validation of key coronagraph technologies, as well as demonstration of a flight-traceable approach to coronagraph implementation. Various wavefront sensing approaches are under investigation on the testbed, with wavefront control provided by a precision high actuator density deformable mirror. Diffracted light control is achieved through a combination of an occulting or apodizing mask and stop; many concepts exist for these components and are explored. Contrast measurements on the testbed establishes the technical feasibility of TPF requirements, while model and error budget validation are demonstrate implementation viability. This paper describes the current testbed design and preliminary experimental results.

High contrast imaging testbed for the Terrestrial Planet Finder coronagraph

One of the architectures under consideration for Terrestrial Planet Finder (TPF) is a visible coronagraph. To achieve TPF science goals, the coronagraph must have extreme levels of wavefront correction (less than 1 /spl Aring/ rms over controllable spatial frequencies) and stability to get the necessary suppression of diffracted starlight (10/sup -10/ contrast). The High Contrast Imaging Testbed is the TPF platform for laboratory validation of key coronagraph technologies, as well as demonstration of a flight-traceable approach to coronagraph implementation. Various wavefront sensing approaches are under investigation on the testbed, with wavefront control provided by a precision high actuator density deformable mirror. Diffracted light control is achieved through a combination of an occulting or apodizing mask and stop; many concepts exist for these components and are explored. Contrast measurements on the testbed establishes the technical feasibility of TPF requirements, while model and error budget validation are demonstrate implementation viability. This paper describes the current testbed design and preliminary experimental results.

Optimizing coronagraph designs to minimize their contrast sensitivity to low-order optical aberrations

The presence of optical aberrations in the entrance pupil of a coronagraph causes the stellar light to scatter about the occulting spot, reducing the effective contrast achievable. Even if these aberrations are sufficiently corrected with a deformable mirror to enable planet detection, small drifts in the optical alignment of the telescope introduce additional low-order aberrations. The design parameters of the coronagraph itself (e.g. occulting spot size, Lyot stop diameter, etc.) affect how these aberrations impact the contrast in the focal plane. In this study, we examine the sensitivity of contrast to low-order optical errors for several coronagraph concepts over their respective design parameters. By combining these sensitivities with the telescope throughput, we show that for each coronagraph concept there is an optimum selection of the design parameters that provides efficient, high-contrast imaging at the inner working distance in the presence of alignment errors.

The Sensitivity of Shaped Pupil Coronagraphs to Optical Aberrations

Unlike focal-plane coronagraphs that use occulting spots and Lyot stops to eliminate diffraction, pupil-plane coronagraphs operate by shaping the pupil to redirect the diffracted stellar light into a tight core. As with focal-plane coronagraphs, the optical aberrations in the telescope must be sufficiently corrected to enable high contrast imaging. However, in shaped-pupil coronagraphs, the low-order aberrations resulting from misalignment and optical figure drift have a much smaller influence upon the contrast at the inner working angle. These weaker sensitivities greatly relax the strict low-order wavefront stability required for high-contrast imaging at the cost of some throughput. In this paper, we present the simulated performance of the concentric ring shaped pupil concepts comparing them to focal-plane coronagraphs that are optimized for the same inner working angles.

Extreme wave front sensing accuracy for the Eclipse coronagraphic space telescope

The Eclipse coronagraphic telescope will allow for high contrast imaging near a target star to facilitate planet finding. One key element will be its high accuracy, high authority deformable mirror (DM) that controls the wave front error (WFE) down to an acceptable level. In fact, to achieve the desired contrast ratio of nine orders of magnitude (in intensity) to within 0.35 arcseconds of the target star, the WFE in the telescope must be controlled to level below 1Å rms within the controllable bandwidth of the DM. To achieve this extreme wave front sensing (WFS) accuracy, we employ a focus-diverse phase retrieval method extended from the Next Generation Space Telescope baseline approach. This method processes a collection defocused point-spread functions, measured at the occulting position in the Eclipse optical system, into a high accuracy estimate of the exit-pupil WFE. Through both simulation and hardware experiments, we examine and establish the key data requirements, such as the defocus levels and imaging signal-to noise level, that are necessary to obtain the desired WFS accuracy and bandwidth.