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shaped pupils
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pupil mapping
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occulters
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wavefront control
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subaru
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sim
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theia
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Shaped Pupils

Stellar coronagraphy aims to permit high-contrast imaging of exoplanets by blocking incoming light from the target star. Shaped pupils can be designed that aim for optimization of a wide variety of criteria including inner and outer angles, desired contrast levels between the parent star and the orbiting planet, rotational symmetry, manufacturability, removal of obscurations and support spiders, maximization of the detection region, ground or space observation, and many other considerations.

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Pupil Mapping

Phase Induced Amplitude Apodization (PIAA) is a coronagraph design which is unique in that rather than using opaque elements to produce the apodization function it remaps the on-axis light with optical elements to produce the apodization, allowing nearly 100% of the off-axis planet light through the coronagraph. This is easy to see qualitatively with ray optics where an even distribution of light at the entrance pupil is concentrated in the center. Quantitatively producing the correct profile for the target contrast in the image plane is a very challenging design problem which requires solving the Monge-Ampère equation across the aperture to find the surface profile required of the two optical elements. The current research topic involving PIAA here at Princeton is to create a hybrid coronagraph by using shaped pupils as pre- and post- apodizers for the PIAA coronagraph with two sequential deformable mirrors for wavefront control. The goal of this system is to create an optimized coronagraph with wavefront control that achieves the target 10-10 contrast while maximizing throughput in 10% Δλ/λ broadband light.

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Occulters

A promising alternative approach to coronagraphy is the usage of an external occulter to block stellar light in order to render the target planet visible. Because the occulter is external to the telescope, diffraction of light causes fewer optical problems thus easing the strict requirements on optical quality associated with a coronagraph. Key challenges associated with an external occulter are precise control of the occulter and telescope formation, optimization of the occulter shape for desired contrast levels  and robustness to deformations, and optical verification of theoretical results.

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Wavefront Control

Usage of shaped pupils for stellar coronagraphy unfortunately causes severe deformations in the incoming wavefront due to diffraction of light beyond the pupil within the telescope optics. Usage of deformable mirrors is required for correction of the wavefront to its original shape to permit extraction of target features from the aberrations. Key challenges include estimation of the incoming wavefront, design of control laws for the deformable mirror, deformable mirror surface modeling, and laboratory validation of these algorithms.

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Subaru

The Subaru Strategic Exploration of Exoplanets and Disks Survey (SEEDS) is the first strategic observing programs (SSOPs) awarded by the National Astronomical Observatory of Japan (NAOJ). This survey has been awarded 120 nights over 5 years time to use the new High Contrast Imaging System (HCIS) to study exoplanets through direct detection, explore the evolution of proto-planetary and debris disks, and investigate the link between exoplanets and circumstellar disks. As part of an MOU between Princeton and the NAOJ, Princeton scientists are able to participate fully as members of the SSOP teams.

This survey will produce high--contrast images able to detect 3 MJup (ΔH = 11 mag) planets at 5 AU (ρ = 0.5$”) around the best nearby, young stars, exceeding the sensitivity of previous surveys by more than a magnitude. SEEDS will observe ~500 targets including nearby stars, young stellar objects, open clusters, and white dwarfs. Princeton University is playing a lead role in the debris disks, nearby stars, and white dwarf categories. In addition, Princeton researchers are developing a data reduction software package, theoretical models of exoplanet atmospheres, and next generation instrumentation for the Subaru HCIS.

The SEEDS program will produce a uniform high-quality set of coronagraphic imaging data for a scientifically important set of target stars. The data will become publically available on the SMOKA archive in the form of user friendly sets of images. As many large-scale astronomy surveys have successfully demonstrated, publicly available data sets are a powerful way of engaging the broader astronomical community and producing novel scientific work.

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Astrometry

An indirect detection method for exoplanets is measurement of the parent star’s  position over time which is affected by the orbiting exoplanet. In conjunction with radial velocity measurements it is possible to use filters to extract planetary masses even for lower mass pbitable lanets.  Astrometric measurements as may be achieved using a pre-cursor mission such as SIM for a direct imaging mission can be used to pre-select candidate stars, with its utility towards the science yield a quantifiable measure.

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THEIA

The Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy (THEIA) is a direct exoplanetary mission concept study proposed to NASA ahead of the decadal survey. THEIA is a proposed flagship 4-meter on-axis optical/UV telescope operating over the 250-1000 nm science passband range with a single, 40-meter external occulter operating at two separation distances.

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