Apodized-pupil Lyot coronagraphs: multistage designs for extremely large telescopes

Earlier apodized-pupil Lyot coronagraphs (APLC) have been studied and developed to enable high-contrast imaging for exoplanet detection and characterization with present-day ground-based telescopes. With the current interest in the development of the next generation of telescopes, the future extremely large telescopes (ELTs), alternative APLC designs involving multistage configuration appear attractive. The interest of these designs for application to ELTs is studied. Performance and sensitivity of multistage APLC to ELT specificities are analyzed and discussed, taking into account several ineluctable coronagraphic telescope error sources by means of numerical simulations. Additionally, a first laboratory experiment with a two-stages-APLC in the near-infrared (H-band) is presented to further support the numerical treatment. Multistage configurations are found to be inappropriate to ELTs. The theoretical gain offered by a multistage design over the classical single-stage APLC is largely compromised by the presence of inherent error sources occurring in a coronagraphic telescope, and in particular in ELTs. The APLC remains an attractive solution for ELTs, but rather in its conventional single-stage configuration.Comment: A&A accepte

Detection and Characterization of Exoplanets and Disks using Projections on Karhunen-Loeve Eigenimages

We describe a new method to achieve point spread function (PSF) subtractions for high- contrast imaging using Principal Component Analysis (PCA) that is applicable to both point sources or extended objects (disks). Assuming a library of reference PSFs, a Karhunen-Lo`eve transform of theses references is used to create an orthogonal basis of eigenimages, on which the science target is projected. For detection this approach provides comparable suppression to the Locally Optimized Combination of Images (LOCI) algorithm, albeit with increased robustness to the algorithm parameters and speed enhancement. For characterization of detected sources the method enables forward modeling of astrophysical sources. This alleviates the biases in the astrometry and photometry of discovered faint sources, which are usually associated with LOCI- based PSF subtractions schemes. We illustrate the algorithm performance using archival Hubble Space Telescope (HST) images, but the approach may also be considered for ground-based data acquired with Angular Differential Imaging (ADI) or integral-field spectrographs (IFS).Comment: 12 pages, 4 figure

New Completeness Methods for Estimating Exoplanet Discoveries by Direct Detection

We report new methods for evaluating realistic observing programs that search stars for planets by direct imaging, where observations are selected from an optimized star list, and where stars can be observed multiple times. We show how these methods bring critical insight into the design of the mission & its instruments. These methods provide an estimate of the outcome of the observing program: the probability distribution of discoveries (detection and/or characterization), & an estimate of the occurrence rate of planets (eta). We show that these parameters can be accurately estimated from a single mission simulation, without the need for a complete Monte Carlo mission simulation, & we prove the accuracy of this new approach. Our methods provide the tools to define a mission for a particular science goal, for example defined by the expected number of discoveries and its confidence level. We detail how an optimized star list can be built & how successive observations can be selected. Our approach also provides other critical mission attributes, such as the number of stars expected to be searched, & the probability of zero discoveries. Because these attributes depend strongly on the mission scale, our methods are directly applicable to the design of such future missions & provide guidance to the mission & instrument design based on scientific performance. We illustrate our new methods with practical calculations & exploratory design reference missions for JWST operating with a distant starshade to reduce scattered and diffracted starlight on the focal plane. We estimate that 5 habitable Earth-mass planets would be discovered & characterized with spectroscopy, with a probability of 0 discoveries of 0.004, assuming a small fraction of JWST observing time (7%), eta=0.3, and 70 observing visits, limited by starshade fuel.Comment: 27 pages, 4 figures, 6 tables, accepted for publication by Ap

Apodized Pupil Lyot Coronagraphs for Arbitrary Apertures. IV. Reduced Inner Working Angle and Increased Robustness to Low-Order Aberrations

The Apodized Pupil Lyot Coronagraph (APLC) is a diffraction suppression system installed in the recently deployed instruments Palomar/P1640, Gemini/GPI, and VLT/SPHERE to allow direct imaging and spectroscopy of circumstellar environments. Using a prolate apodization, the current implementations offer raw contrasts down to $10^{-7}$ at 0.2 arcsec from a star over a wide bandpass (20\%), in the presence of central obstruction and struts, enabling the study of young or massive gaseous planets. Observations of older or lighter companions at smaller separations would require improvements in terms of inner working angle (IWA) and contrast, but the methods originally used for these designs were not able to fully explore the parameter space. We here propose a novel approach to improve the APLC performance. Our method relies on the linear properties of the coronagraphic electric field with the apodization at any wavelength to develop numerical solutions producing coronagraphic star images with high-contrast region in broadband light. We explore the parameter space by considering different aperture geometries, contrast levels, dark-zone sizes, bandpasses, and focal plane mask sizes. We present an application of these solutions to the case of Gemini/GPI with a design delivering a $10^{-8}$ raw contrast at 0.19 arcsec and offering a significantly reduced sensitivity to low-order aberrations compared to the current implementation. Optimal solutions have also been found to reach $10^{-10}$ contrast in broadband light regardless of the telescope aperture shape (in particular the central obstruction size), with effective IWA in the $2-3.5\lambda/D$ range, therefore making the APLC a suitable option for the future exoplanet direct imagers on the ground or in space.Comment: 14 pages, 10 figures, accepted in Ap

Shaped pupil design for the Gemini Planet Imager

The Gemini Planet Imager (GPI) is an instrument designed for the Gemini South telescope to image young Jupiter-mass planets in the infrared. To achieve the high contrast needed for this, it employs an apodized pupil Lyot coronagraph (APLC) to remove most of the starlight. Current designs use a partially-transmitting apodizer in the pupil; we examine the use of binary apodizations in the form of starshaped shaped pupils, and present a design that could achieve comparable performance, along with a series of design guidelines for creating shaped pupil versions of APLCs in other systems.Comment: 20 pages, 7 figures, accepted for publication in Ap

Reference-less detection, astrometry, and photometry of faint companions with adaptive optics

We propose a complete framework for the detection, astrometry, and photometry of faint companions from a sequence of adaptive optics corrected short exposures. The algorithms exploit the difference in statistics between the on-axis and off-axis intensity. Using moderate-Strehl ratio data obtained with the natural guide star adaptive optics system on the Lick Observatory's 3-m Shane Telescope, we compare these methods to the standard approach of PSF fitting. We give detection limits for the Lick system, as well as a first guide to expected accuracy of differential photometry and astrometry with the new techniques. The proposed approach to detection offers a new way of determining dynamic range, while the new algorithms for differential photometry and astrometry yield accurate results for very faint and close-in companions where PSF fitting fails. All three proposed algorithms are self-calibrating, i.e. they do not require observation of a calibration star thus improving the observing efficiency.Comment: Astrophysical Journal 698 (2009) 28-4

The Strehl Ratio in Adaptive Optics Images: Statistics and Estimation

Statistical properties of the intensity in adaptive optics images are usually modeled with a Rician distribution. We study the central point of the image, where this model is inappropriate for high to very high correction levels. The central point is an important problem because it gives the Strehl ratio distribution. We show that the central point distribution can be modeled using a non-central Gamma distribution.Comment: 8 pages, 5 figure

Optimization of Apodized Pupil Lyot Coronagraph for ELTs

We study the optimization of the Apodized Pupil Lyot Coronagraph (APLC) in the context of exoplanet imaging with ground-based telescopes. The APLC combines an apodization in the pupil plane with a small Lyot mask in the focal plane of the instrument. It has been intensively studied in the literature from a theoretical point of view, and prototypes are currently being manufactured for several projects. This analysis is focused on the case of Extremely Large Telescopes, but is also relevant for other telescope designs. We define a criterion to optimize the APLC with respect to telescope characteristics like central obscuration, pupil shape, low order segment aberrations and reflectivity as function of the APLC apodizer function and mask diameter. Specifically, the method was applied to two possible designs of the future European-Extremely Large Telescope (E-ELT). Optimum configurations of the APLC were derived for different telescope characteristics. We show that the optimum configuration is a stronger function of central obscuration size than of other telescope parameters. We also show that APLC performance is quite insensitive to the central obscuration ratio when the APLC is operated in its optimum configuration, and demonstrate that APLC optimization based on throughput alone is not appropriate.Comment: 9 pages, 17 figures, accepted for publication in Astronomy & Astrophysic

High resolution imaging with Fresnel interferometric arrays: suitability for exoplanet detection

We propose a new kind of interferometric array that yields images of high dynamic range and large field. The numerous individual apertures in this array form a pattern related to a Fresnel zone plate. This array can be used for astrophysical imaging over a broad spectral bandwidth spanning from the U.V. (50 nanometers) to the I.R. (20 microns). Due to the long focal lengths involved, this instrument requires formation-flying of two space borne vessels. We present the concept and study the S/N ratio in different situations, then apply these results to probe the suitability of this concept to detect exoplanets.Comment: 12 pages, 19 figures, to be published in A&