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The exploration of circumstellar environments by means of direct imaging to search for Earth-like exoplanets is one of the challenges of modern astronomy. One of the current limitations are evolving non-common path aberrations (NCPA) that originate from optics downstream of the main wavefront sensor. The self-coherent camera (SCC) is an integrated coronagraph and focal-plane wavefront sensor that generates wavefront information-encoding Fizeau fringes in the focal plane by adding a reference hole (RH) in the Lyot stop. Here, we aim to show that by featuring a polarizer in the RH and adding a polarizing beamsplitter downstream of the Lyot stop, the RH can be placed right next to the pupil. We refer to this new variant of the SCC as the polarization-encoded self-coherent camera (PESCC). We study the performance of the PESCC analytically and numerically, and compare it, where relevant, to the SCC. We show analytically that the PESCC relaxes the requirements on the focal-plane sampling and spectral resolution with respect to the SCC by a factor of 2 and 3.5, respectively. Furthermore, we find via our numerical simulations that the PESCC has effectively access to $\sim$16 times more photons, which improves the sensitivity of the wavefront sensing by a factor of $\sim4$. We also show that without additional measurements, the RH point-spread function (PSF) can be calibrated using PESCC images, enabling coherent differential imaging (CDI) as a contrast-enhancing post-processing technique for every observation. In idealized simulations (clear aperture, charge two vortex coronagraph, perfect DM, no noise sources other than phase and amplitude aberrations) and in circumstances similar to those of space-based systems, we show that WFSC combined with CDI can achieve a $1\sigma$ raw contrast of $\sim3\cdot10^{-11}- 8 \cdot 10^{-11}$ between 1 and 18 $\lambda / D$., Comment: Accepted for publication in A&A. Shortened abstract. 17 pages, 21 figures

Modern coronagraph design relies on advanced, large-scale optimization processes that require an ever increasing amount of computational resources. In this paper, we restrict ourselves to the design of Apodized Pupil Lyot Coronagraphs (APLCs). To produce APLC designs for future giant space telescopes, we require a fine sampling for the apodizer to resolve all small features, such as segment gaps, in the telescope pupil. Additionally, we require the coronagraph to operate in broadband light and be insensitive to small misalignments of the Lyot stop. For future designs we want to include passive suppression of low-order aberrations and finite stellar diameters. The memory requirements for such an optimization would exceed multiple terabytes for the problem matrix alone. We therefore want to reduce the number of variables and constraints to minimize the size of the problem matrix. We show how symmetries in the pupil and Lyot stop are expressed in the complete optimization problem, and allow removal of both variables and constraints. Each mirror symmetry reduces the problem size by a factor of four. Secondly, we introduce progressive refinement, which uses low-resolution optimizations as a prior for higher resolutions. This lets us remove the majority of variables from the high-resolution optimization. Together these two improvements require up to 256x less computer memory, with a corresponding speed increase. This allows for greater exploration of the phase space of the focal-plane mask and Lyot-stop geometry, and easier simulation of sensitivity to Lyot-stop misalignments. Moreover, apodizers can now be optimized at their native manufactured resolution., 17 pages, 8 figures, 2 tables, SPIE Proceedings 11443-165

In high-contrast imaging applications, such as the direct imaging of exoplanets, a coronagraph is used to suppress the light from an on-axis star so that a dimmer, off-axis object can be imaged. To maintain a high-contrast dark region in the image, optical aberrations in the instrument must be minimized. The use of phase-contrast-based Zernike Wavefront Sensors (ZWFS) to measure and correct for aberrations has been studied for large segmented aperture telescopes and ZWFS are planned for the coronagraph instrument on the Roman Space Telescope (RST). ZWFS enable subnanometer wavefront sensing precision, but their response is nonlinear. Lyot-based Low-OrderWavefront Sensors (LLOWFS) are an alternative technique, where light rejected from a coronagraph's Lyot stop is used for linear measurement of small wavefront displacements. Recently, the use of Deep Neural Networks (DNNs) to enable phase retrieval from intensity measurements has been demonstrated in several optical configurations. In a LLOWFS system, the use of DNNs rather than linear regression has been shown to greatly extend the sensor's usable dynamic range. In this work, we investigate the use of two different types of machine learning algorithms to extend the dynamic range of the ZWFS. We present static and dynamic deep learning architectures for single- and multi-wavelength measurements, respectively. Using simulated ZWFS intensity measurements, we validate the network training technique and present phase reconstruction results. We show an increase in the capture range of the ZWFS sensor by a factor of 3.4 with a single wavelength and 4.5 with four wavelengths.

Coronagraph is a powerful instrumentation that can be used to realize direct exoplanet imaging. Because the stars are far brighter than the exoplanets, a very high requirement for extinction is raised to obtain a high contrast imaging. By optimizing the transmittance of the entrance pupil and the Lyot stop in Lyot Coronagraph, a sequential optimization procedure is proposed to obtain even higher contrast imaging. In this manuscript, the usually adopted joint optimization in the existing literatures is divided into two steps, which we call the sequential optimization procedure. First, the entrance pupil is optimized for maximum transmission while the contrast constraint is imposed on the focal plane of the lens behind it. Second, the Lyot stop is optimized for maximum transmission with the contrast constraint imposed on the imaging plane. Compared with the joint optimization procedure, the sequential one can provide additional advantages. In terms of performance, under the same conditions, an IWA(Inner working angle) of 1.53λ/D can be obtained while the IWA for joint one is 2.01λ/D. Moreover, the sequential optimization is much faster. Referring to practical applications, the optimum transmittance of Lyot stop in sequential case becomes binary and therefore easy to fabricate. However sequential optimization reduces the throughput approximately 36.81%, which is a drawback that should be compensated.

We propose a new approach for high-contrast imaging at the diffraction limit using segmented telescopes in a modest observation bandwidth. This concept, named "spectroscopic fourth-order coronagraphy", is based on a fourth-order coronagraph with a focal-plane mask that modulates the complex amplitude of the Airy disk along one direction. While coronagraphs applying the complex amplitude mask can achieve the theoretical limit performance for any arbitrary pupils, the focal plane mask severely limits the bandwidth. Here, focusing on the fact that the focal-plane mask modulates the complex amplitude along one direction, we noticed that the mask can be optimized for each spectral element generated by a spectrograph. We combine the fourth-order coronagraph with two spectrographs to produce a stellar spectrum on the focal plane and reconstruct a white pupil on the Lyot stop. Based on the wavefront analysis of an optical design applying an Offner-type imaging spectrograph, we found that the achievable contrast of this concept is 10^{-10} at 1.2 - 1.5 times the diffraction limit over the wavelength range of 650 - 750 nm for the entrance pupil of the LUVOIR telescope. Thus, this coronagraph concept could bring new habitable planet candidates not only around G- and K-type stars beyond 20 - 30 pc but also around very nearby M-type stars. This approach potentially promotes the characterization of the atmospheres of nearby terrestrial planets with future on- and off-axis segmented large telescopes., 26 Pages, 12 Figures, Accepted for publication in The Astronomical Journal

We propose a coronagraphic system with fourth-order null for off-axis segmented telescopes, which is sufficiently insensitive to the telescope pointing errors and finite angular diameter of the host star to enable high-contrast imaging of potentially habitable planets. The inner working angle of the coronagraphic system is close to $1\lambda/D$, and there is no outer limit. The proposed coronagraphic system is made up of a new focal plane mask and an optimized Lyot stop with the second-order null. The new focal plane mask is an extension of the band-limited masks with a phase modulation. We construct a coronagraphic system with fourth-order null by placing two of the new coronagraph systems in succession to be orthogonal to each other. The proposed system is limited to narrow-band usage. The characteristics of the proposed coronagraph system are derived analytically, which includes: (1)the leak of stellar lights due to finite stellar diameter and pointing jitter of a telescope, and (2)the peak throughput. We achieve the performance simulations of this coronagraphic system based on these analytical expressions, considering a monochromatic light of 0.75$\mathrm{\mu}$m and off-axis primary mirror with a diameter of 8.5m. Thanks to the wide working area of the mask, the result shows that terrestrial planets orbiting K and G-dwarfs can be detected under the condition that the telescope pointing jitter is less than $0.01\lambda/D\approx240$as. The proposed coronagraphic system is promising for detection of potentially habitable planets with future space off-axis hexagonally segmented telescopes., Comment: Accepted for Publication in The Astronomical Journal

We are developing an innovative compact coronagraph for studying the physical conditions in the solar wind acceleration region. This paper presents the new development of the compact coronagraph for the investigation of temperature and speed of electrons in the solar corona. The proposed compact coronagraph is a one stage externally occulted coronagraph without internal occulter or Lyot stop mask. The key of the new idea is to set the inner field cutoff at the External Occulter (EO) much smaller than the specified inner field cutoff. A second occulter on the surface of the detector array removes the remaining diffraction. The occulter on the detector surface functions similar to an internal occulter with the Inner Field of View Cutoff (IFoVC) exactly the same as specified. For BITSE, the desired inner field cutoff is 3 R, but the cutoff at EO is only 1.5 R☉. The diffraction analysis shows that in the sensor plane, the diffraction intensity at the 3 R☉ is not sensitive to the EO cutoff, either at 1.5 R☉ or close to 3 R☉. The advantage of having a smaller EO cutoff is that the vignetting decreased for the Field of View (FoV) near 3 R☉, therefore, the signal increases. Meanwhile, the diffraction of Point Spread Function is much less in the radial direction, which not only increases the image quality around 3 R☉, but also increases the encircled energy and signal to noise ratio. In other words, the data is useful right at 3 R☉! The BITSE optical design and diffraction analysis will be presented in detail. The simulation shows the signal to noise ratio obtained from the diffraction and vignetting data enables corona temperature and speed measurement.

Context. Direct detection of exoplanets requires imaging in a highly dynamic range and exquisite image quality and stability. Wavefront error (atmospheric errors, manufacturing errors on optics, cophasing residuals, temperature variations, etc.) will limit the efficiency of this endeavor by creating various flavors of speckles that evolve with different timescales. Active wavefront-error correction using a deformable mirror requires measuring the wavefront aberrations in the science image with high accuracy and in a shorter time than the duration of the dominant speckle lifetime. Aims. The self-coherent camera (SCC) is a focal plane wavefront sensor exploiting the coherence of light to generate Fizeau fringes in the image plane to spatially encode speckles. The SCC combines a coronagraph and a modified Lyot stop to which a reference channel is added. The conventional SCC is restricted to long-exposure measurements because the light transmitted through the reference channel is limited. The SCC can correct quasi-static aberrations but precludes short-lived atmospheric aberrations from the measurement. Methods. I propose an alternative to the conventional SCC that I call the fast-modulated SCC. It uses a simplified Lyot stop design and an adequate Fourier filtering algorithm. The theory is established and confirmed by means of numerical simulations. Results. The SCC theory dictates that the separation between the classical Lyot stop and the reference channel must be larger than 1.5 times the Lyot stop diameter. The fast-modulated SCC allows for the reference channel to be placed anywhere, in particular in the vicinity of the pupil where the maximum of diffracted light is found. This alternative represents a complete game changer for the sensor: full compatibility with any type of coronagraph, easy installation in existing instruments, and versatility by accessing short- and long-time exposure measurements. Conclusions. While the conventional SCC can almost not be implemented in existing instruments because the optical beam footprint in the instrument must be wide enough to illuminate the reference channel, which is often seen as a significant shortcoming, the fast-modulated SCC does not require any constraint on this. The fast-modulated SCC also relaxes the high sampling requirement to resolve the fringes, which is usually incompatible with the observation of fainter targets because the fringes are larger. The fast-modulated SCC simultaneously counteracts the two original shortcomings of the SCC concept.

While an ideal optical vortex coronagraph operating behind a clear, circular, unaberrated telescope aperture can theoretically provide perfect rejection of the incident plane wave from an unresolved star, use of a telescope with an on-axis secondary mirror limits the rejection. In theory, a dual-stage vortex coronagraph can provide improved starlight rejection for an on-axis telescope, and here we provide experimental confirmation of the predicted distribution of the residual light in the output pupil plane of a dual-stage vortex coronagraph. In addition, a simple method of further improving the rejection of such a coronagraph is suggested: by slightly oversizing the first Lyot stop and phase-shifting the light within the exposed annulus by half a wave, the residual starlight within the pupil can be canceled to deeper levels.

Combining active pupil correction via deformable mirrors (DMs) with coronagraphs such as the Apodized Pupil Lyot Coronagraph (APLC) provides a powerful tool for creating high contrast dark holes with obstructed pupils featuring central obstructions, spiders, and gaps. We investigate optimal combinations of DM pupil remapping via Active Compensation of Aperture Discontinuities- Optimized Stroke Minimization (ACAD-OSM) and binary mask pupil apodization to obtain dark holes with contrasts of 1010 for the APLC. We examine the space of possible configurations for an APLC apodized with a circularly symmetric pupil mask and a pair of DMs using a modified MCMC algorithm that allows us to probe previously unexamined combinations of pupil apodization, focal plane mask size, and Lyot stop size. We find designs with ~ 20% encircled energy throughput for a focal plane mask radius of 4.5λ/D and a bandwidth of 20%, as well as for a focal plane mask radius of 3.18 λ/D and a bandwidth of 10%. We also find solutions for focal plane mask radii of 2 λ/D and 20% bandwidths that can obtain encircled energy throughputs of up to ~ 4%. Our strategy of combining circularly symmetric binary masks with DMs to create dark holes with obstructed pupils can be expanded to optimize the APLC for terrestrial exoplanet yield, and we conclude by exploring the possibility of optimizing coronagraphs using a simple parametric expression for yield.

Imaging Earth-like exoplanets with future space telescopes will require a coronagraph instrument that is capable of creating a dark zone in the starlight at the image plane that is ten orders of magnitude fainter than the off-axis image of the host star. What is more, the coronagraph must simultaneously provide a stable dark zone and high throughput over the angular separations that correspond to habitable zones around nearby Sun-like stars (~10-100 milliarcseconds). Since the pupils of most large-aperture space telescope architectures are likely to be obstructed by secondary mirrors, spider support structures, and gaps between mirror segments, the coronagraph optics must also be specially tailored to passively suppress starlight diffracted from the obstructions and discontinuities in the telescope pupil. Here, we demonstrate an apodized vortex coronagraph optimized for an off-axis segmented telescope on the new High Contrast Spectroscopy Testbed for Segmented Telescopes (HCST) at Caltech. The coronagraph consists of a microdot apodizer, a liquid crystal vortex phase mask in the focal plane, and a Lyot stop. The microdot apodizer is an AR-coated glass window with 10um gold microdots to be used in reflection around lambda=800nm. We describe the HCST optical system; the apodizer optimization, fabrication, and metrology procedures; and present end-to-end testbed results of the coronagraph coupled with a 32x32 Boston Micromachines deformable mirror for wavefront control. We aim to achieve a dark zone 10^-7 times fainter than the simulated host star over a wavelength range of 800±40nm in Spring 2018. Finally, we will outline future plans to demonstrate coronagraph concepts for centrally obscured telescopes.

By adding a dedicated coronagraph, ESO in collaboration with the Breakthrough Initiatives, modifies the Very Large Telescope mid-IR imager (VISIR) to further boost the high dynamic range imaging capability this instru- ment has. After the VISIR upgrade in 2012, where coronagraphic masks were first added to VISIR, it became evident that coronagraphy at a ground-based 8m-class telescope critically needs adaptive optics, even at wavelengths as long as 10μm. For VISIR, a work-horse observatory facility instrument in normal operations, this is ”easiest” achieved by bringing VISIR as a visiting instrument to the ESO-VLT-UT4 having an adaptive M2. This “visit” enables a meaningful search for Earth-like planets in the habitable zone around both α-Cen1,2. Meaningful here means, achieving a contrast of ≈ 10^(-6) within ≈ 0.8arcsec from the star while maintaining basically the normal sensitivity of VISIR. This should allow to detect a planet twice the diameter of Earth. Key components will be a diffractive coronagraphic mask, the annular groove phase mask (AGPM), optimized for the most sensitive spectral band-pass in the N-band, complemented by a sophisticated apodizer at the level of the Lyot stop. For VISIR noise filtering based on fast chopping is required. A novel internal chopper system will be integrated into the cryostat. This chopper is based on the standard technique from early radio astronomy, conceived by the microwave pioneer Robert Dicke in 1946, which was instrumental for the discovery of the 3K radio background.

The High Contrast Spectroscopy Testbed for Segmented Telescopes (HCST) at Caltech is aimed at filling gaps in technology for future exoplanet imagers and providing the U.S. community with an academic facility to test components and techniques for high contrast imaging with future segmented ground-based telescope (TMT, E-ELT) and space-based telescopes (HabEx, LUVOIR). The HCST will be able to simulate segmented telescope geometries up to 1021 hexagonal segments and time-varying external wavefront disturbances. It also contains a wavefront corrector module based on two deformable mirrors followed by a classical 3-plane single-stage corona- graph (entrance apodizer, focal-plane mask, Lyot stop) and a science instrument. The back-end instrument will consist of an imaging detector and a high-resolution spectrograph, which is a unique feature of the HCST. The spectrograph instrument will utilize spectral information to characterize simulated planets at the photon-noise limit, measure the chromaticity of new optimized coronagraph and wavefront control concepts, and test the overall scientific functions of high-resolution spectrographs on future segmented telescopes.

Detecting faint companions in close proximity to stars is one of the major goals of current/planned ground- and space-based high-contrast imaging instruments. High-performance coronagraphs can suppress the diffraction features and gain access to companions at small angular separation. However, the uncontrolled pointing errors degrade the coronagraphic performance by leaking starlight around the coronagraphic focal-plane mask, preventing the detection of companions at small separations. A Lyot-stop low-order wavefront sensor (LLOWFS) was therefore introduced to calibrate and measure these aberrations for focal-plane phase mask coronagraphs. This sensor quantifies the variations in wavefront error decomposed into a few Zernike modes by reimaging the diffracted starlight rejected by a reflective Lyot stop. The technique was tested with several coronagraphs on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system at the Subaru Telescope. The wavefront was decomposed into 15 and 35 Zernike modes with an occulting and focal-plane phase mask coronagraph, respectively, which were used to drive a closed-loop correction in the laboratory. Using a 2000-actuator deformable mirror, a closed-loop pointing stability between 10−3–10^(−4) λ/D was achieved in the laboratory in H-band, with sub nanometer residuals for the other Zernike modes (Noll index >4). On-sky, the low-order control of 10+ Zernike modes for the phase-induced amplitude apodization and the vector vortex coronagraphs was demonstrated, with a closed-loop pointing stability of 10^(-4) λ /D under good seeing and 10^(-3) λ/D under moderate seeing conditions readily achievable.

The Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph (PICTURE-C) mission will directly image debris disks and exozodiacal dust around nearby stars from a high-altitude balloon using a vector vortex coronagraph. The PICTURE-C low-order wavefront control (LOWC) system will be used to correct time-varying low-order aberrations due to pointing jitter, gravity sag, thermal deformation, and the gondola pendulum motion. We present the hardware and software implementation of the low-order ShackHartmann and reflective Lyot stop sensors. Development of the high-speed image acquisition and processing system is discussed with the emphasis on the reduction of hardware and computational latencies through the use of a real-time operating system and optimized data handling. By characterizing all of the LOWC latencies, we describe techniques to achieve a framerate of 200 Hz with a mean latency of ∼378 μs

With the recent commissioning of ground instruments such as SPHERE or GPI and future space observatories like WFIRST-AFTA, coronagraphy should probably become the most efficient tool for identifying and characterizing extra-solar planets in the forthcoming years. Coronagraphic instruments such as Phase mask coronagraphs (PMC) are usually based on a phase mask or plate located at the telescope focal plane, spreading the starlight outside the diameter of a Lyot stop that blocks it. In this communication is investigated the capability of a PMC to act as a phase-shifting wavefront sensor for better control of the achieved star extinction ratio in presence of the coronagraphic mask. We discuss the two main implementations of the phase-shifting process, either introducing phase-shifts in a pupil plane and sensing intensity variations in an image plane, or reciprocally. Conceptual optical designs are described in both cases. Numerical simulations allow for better understanding of the performance and limitations of both options, and optimizing their fundamental parameters. In particular, they demonstrate that the phase-shifting process is a bit more efficient when implemented into an image plane, and is compatible with the most popular phase masks currently employed, i.e. four-quadrants and vortex phase masks., 22 pages, 9 figures, 3 tables

A vortex coronagraph is now available for high contrast observations with the Keck/NIRC2 instrument at L band. Reaching the optimal performance of the coronagraph requires fine control of the wavefront incident on the phase mask. In particular, centering errors can lead to significant stellar light leakage that degrades the contrast performance and prevents the observation of faint planetary companions around the observed stars. It is thus critical to correct for the possible slow drift of the star image from the phase mask center, generally due to mechanical flexures induced by temperature and/or gravity field variation, or to misalignment between the optics that rotate in pupil tracking mode. A control loop based on the QACITS algorithm for the vortex coronagraph has thus been developed and deployed for the Keck/NIRC2 instrument. This algorithm executes the entire observing sequence, including the calibration steps, initial centering of the star on the vortex center and stabilisation during the acquisition of science frames. On-sky data show that the QACITS control loop stabilizes the position of the star image down to 2.4 mas rms at a frequency of about 0.02 Hz. However, the accuracy of the estimator is probably limited by a systematic error due to a misalignment of the Lyot stop with respect to the entrance pupil, estimated to be on the order of 4.5 mas. A method to reduce the amplitude of this bias down to 1 mas is proposed. The QACITS control loop has been successfully implemented and provides a robust method to center and stabilize the star image on the vortex mask. In addition, QACITS ensures a repeatable pointing quality and significantly improves the observing efficiency compared to manual operations. It is now routinely used for vortex coronagraph observations at Keck/NIRC2, providing contrast and angular resolution capabilities suited for exoplanet and disk imaging., 9 pages, 13 figures. Accepted for publication in A&A

Direct imaging of exoplanets requires the detection of very faint objects orbiting close to very bright stars. In this context, the SPICES mission was proposed to the European Space Agency for planet characterization at visible wavelength. SPICES is a 1.5m space telescope which uses a coronagraph to strongly attenuate the central source. However, small optical aberrations, which appear even in space telescopes, dramatically decrease coronagraph performance. To reduce these aberrations, we want to estimate, directly on the coronagraphic image, the electric field, and, with the help of a deformable mirror, correct the wavefront upstream of the coronagraph. We propose an instrument, the Self-Coherent Camera (SCC) for this purpose. By adding a small "reference hole" into the Lyot stop, located after the coronagraph, we can produce interferences in the focal plane, using the coherence of the stellar light. We developed algorithms to decode the information contained in these Fizeau fringes and retrieve an estimation of the field in the focal plane. After briefly recalling the SCC principle, we will present the results of a study, based on both experiment and numerical simulation, analyzing the impact of the size of the reference hole., Comment: Space Telescopes and Instrumentation 2012: Optical, Infrared, and Millimeter Wave. Proceedings of the SPIE, Volume 8442, article id. 844250, 10 pp. (2012)

We show that the use of apodizing functions at the coronagraph Lyot plane may be useful for improving the image contrast of ground-based coronagraphs. An experimental set-up consisting of a tip–tilt mirror, a coronagraph and a low-noiseEMCCDcamerawas implemented at theWilliam Herschel Telescope. Images were taken in the I band, which meant that the D/r0 value was around 10. Experimental results confirm that, for moderately aberrated wavefronts, our instrument works as theoretically expected, and that the contrast value attained is high enough to provide direct detection of faint companions. This research was supported by the Ministerio de Economía y Competitividad under project FIS2012-31079 and the Fundación Séneca of Murcia under projects 15419/PI/10 and 15345/PI/10.

Optical coronagraphy is a high contrast image technique used in astronomy to reduce light around a host star and make viable the detection of faint companions and the exploration of circumstellar disks. Digital coronagraphy consists of the digital processing of non-coronagraphic images acquired by space telescopes in order to reproduce the operation of a standard optical coronagraph. Digital coronagraphy presents significant advantages as no real coronagraph or extra device has to be manufactured and sent to space. In this paper, comparison of digital and optical coronagraph performances is accomplished both by numerical simulations that include detection noise and the use of archived images from the Hubble Space Telescope. Our analysis indicates that the attainable contrast with both techniques is comparable, though the required Lyot stop in digital coronagraphy differs from the standard one. Furthermore, the evolution of contrast as a function of the distance to the main star that we have encountered with the optical coronagraph is similar to that shown by different authors for the optical NIC2/COR coronagraph. Finally, although digital coronagraphy cannot substitute optical coronagraphs, it can be considered as an interesting tool for the analysis of actual system performance.

Context. The solar coronagraph ASPIICS will fly on the future ESA formation flying mission Proba-3. The instrument combines an external occulter of diameter 1.42 m and a Lyot solar coronagraph of 5 cm diameter, located downstream at a distance of 144 m. Aims. The theoretical performance of the externally occulted Lyot coronagraph has been computed by assuming perfect optics. In this paper, we improve related modelling by introducing roughness scattering effects from the telescope. We have computed the diffraction at the detector, that we compare to the ideal case without perturbation to estimate the performance degradation. We have also investigated the influence of sizing the internal occulter and the Lyot stop, and we performed a sensitivity analysis on the roughness. Methods. We have built on a recently published numerical model of diffraction propagation. The micro-structures of the telescope are built by filtering a white noise with a power spectral density following an isotropic ABC function, suggested by Harvey scatter theory. The parameters were tuned to fit experimental data measured on ASPIICS lenses. The computed wave front error was included in the Fresnel wave propagation of the coronagraph. A circular integration over the solar disk was performed to reconstruct the complete diffraction intensity. Results. The level of micro-roughness is 1.92 nm root-mean-square. Compared to the ideal case, in the plane of the internal occulter, the diffraction peak intensity is reduced by ≃0.001%. However, the intensity outside the peak increases by 12% on average, up to 20% at 3 R⊙, where the mask does not filter out the diffraction. At detector level, the diffraction peak remains ≃10−6 at 1.1 R⊙, similar to the ideal case, but the diffraction tail at large solar radius is much higher, up to one order of magnitude. Sizing the internal occulter and the Lyot stop does not improve the rejection, as opposed to the ideal case. Conclusions. Besides these results, this paper provides a methodology to implement roughness scattering in the wave propagation model for the solar coronagraph.

Recent efforts combining the optimization techniques of apodized pupil Lyot coronagraphs (APLC) and shaped pupils have demonstrated the viability of a binary-transmission mask architecture for extremely high contrast (10-10) exoplanet imaging. We are now building on those innovations to carry out a survey of Lyot coronagraph performance for large, segmented telescope apertures. These apertures are of the same kind under considera- tion for NASA's Large UV/Optical/IR (LUVOIR) observatory concept. To map the multi-dimensional design parameter space, we have developed a software toolkit to manage large sets of mask optimization programs and execute them on a computing cluster. Here we summarize a preliminary survey of 500 APLC solutions for 4 reference hexagonal telescope apertures. Several promising designs produce annular, 10-10 contrast dark zones down to inner working angle 4λ0=D over a 15% bandpass, while delivering a half-max PSF core throughput of 18%. We also report our progress on devising solutions to the challenges of Lyot stop alignment/fabrication tolerance that arise in this contrast regime.

Coronagraphy is a very efficient technique for identifying and characterizing extra-solar planets orbiting in the habitable zone of their parent star, especially when used in a space environment. An important family of coronagraphs is based on phase plates located at an intermediate image plane of the optical system, that spread the starlight outside the "Lyot" exit pupil plane of the instrument. In this communication we present a set of candidate phase functions generating a central null at the Lyot plane, and study how it propagates to the image plane of the coronagraph. These functions include linear azimuthal phase ramps (the well-known optical vortex), azimuthally cosine-modulated phase profiles, and circular phase gratings. Numerical simulations of the expected null depth, inner working angle, sensitivity to pointing errors, effect of central obscuration located at the pupil or image planes, and effective throughput including image mask and Lyot stop transmissions are presented and discussed. The preliminary conclusion is that azimuthal cosine functions appear as an interesting alternative to the classical optical vortex of integer topological charge., Comment: 12 pages

International audience; The VLT/SPHERE instrument includes a unique long-slit spectroscopy (LSS) mode coupled with Lyot coronagraphy dedicated to the spectral characterization of directly imaged giant exoplanets. The performance of this mode is limited by its non-optimal coronagraph, but in a previous work we demonstrated that it could be significantly improved at small inner-working angles using the stop-less Lyot coronagraph (SLLC). A prototype of the SLLC was installed in VLT/SPHERE in 2014 during the reintegration of the instrument in Paranal, and it was extensively tested in 2015 to characterize its performance. The performance is tested in both imaging and spectroscopy using data acquired on the internal source of SPHERE. In imaging, we obtain a raw contrast gain of a factor 10 at 0.3" with the SLLC. We also demonstrate that no Lyot stop is required to reach the full performance, which validates the SLLC concept. Comparison with a realistic simulation model shows that we are currently limited by the internal phase aberrations of SPHERE. In spectroscopy, we obtain a gain of 1 mag in a limited range of angular separations. Simulations show that although the main limitation comes from phase errors, the performance in the non-SLLC case is very close to the ultimate limit of the LSS mode. We present the very first on-sky data with the SLLC, which appear extremely promising for the future scientific exploitation of an apodized LSS mode in SPHERE. Finally, we explore a new possibility for the speckle subtraction in the LSS mode that could significantly improve the data analysis with respect to methods based on spectral differences.

International audience; Context. High-contrast hybrid coronagraphs, which combine an external occulter and a Lyot-style coronagraph became a reality in recent years, despite the lack of analytic and numerical end-to-end performance studies. The solar coronagraph ASPIICS which will fly on the future ESA Formation Flying mission Proba-3 is a good example of such a hybrid coronograph. Aims. We aim to provide a numerical model to compute theoretical performance of the hybrid externally occulted Lyot-style coronagraph, which we then aim to compare to the performance of the classical Lyot coronagraph and the externally occulted solar coronagraph. We will provide the level and intensity distribution of the stray light, when the Sun is considered as an extended source. We also investigate the effect of different sizes for the internal occulter and Lyot stop.Methods. First, we have built on a recently published approach, to express the diffracted wave front from Fresnel diffraction produced by an external occulter at the entrance aperture of the coronagraph. Second, we computed the coherent propagation of the wave front coming from a given point of the Sun through the instrument. This is performed in three steps: from the aperture to the image of the external occulter, where the internal occulter is set, from this plane to the image of the entrance aperture, where the Lyot stop is set, and from there to the final image plane. Making use of the axis-symmetry, we considered wave fronts originating from one radius of the Sun and we circularly average the intensities. Our numerical computation used the parameters of ASPIICS.Results. The hybrid externally occulted Lyot coronagraph rejects sunlight below 10E-8Mean Solar Brightness from 1,3 solar radius - in the particular configuration of ASPIICS. The Lyot coronagraph effectively complements the external occultation. We show that reducing the Lyot stop allows a clear gain in rejection, being even better than oversizing the internal occulter, that tends to exclude observations very close to the solar limb. As an illustration, we provide a graph that allows us to estimate performance as a function of the internal occulter and Lyot stop sizes.Conclusions. Our work consists of a methodological approach to compute the end-to-end performance for solar coronagraph.

SPHERE (which stands for Spectro-Polarimetric High-contrast Exoplanet REsearch) is a second-generation Very Large Telescope (VLT) instrument dedicated to high-contrast direct imaging of exoplanets whose first-light is scheduled for 2011. Within this complex instrument one of the central components is the apodized Lyot coronagraph (ALC). The principal aim of this paper is to report the first laboratory experiment of the ALC designed for the SPHERE instrument. The performance and sensitivity of the optical configuration was first numerically studied with an end-to-end approach (see the results in paper I subtitled “Detailed numerical study”). Made confident by the results, we then tested a prototype on an infrared coronagraphic bench. We measured the transmission profiles of the apodizer prototype and the coronagraphic performance of the apodized Lyot coronagraph in Y, J, and H bands. The coronagraph sensitivity to lateral and longitudinal misalignments of its three main components (apodizer, coronagraphic mask and Lyot stop) was finally studied in H band. We can conclude that the prototype meets the SPHERE technical requirements for coronagraphy.

We describe a coronagraph approach where the performance of a Phase-Induced Amplitude Apodization (PIAA) coronagraph is improved by using a partially transmissive phase-shifting focal plane mask and a Lyot stop. This approach combines the low inner working angle offered by phase mask coronagraphy, the full throughput and uncompromized angular resolution of the PIAA approach, and the design flexibility of Apodized Pupil Lyot Coronagraph. A PIAA complex mask coronagraph (PIAACMC) is fully described by the focal plane mask size, or, equivalently, its complex transmission which ranges from 0 (opaque) to -1 (phase shifting). For all values of the transmission, the PIAACMC theoretically offers full on-axis extinction and 100% throughput at large angular separations. With a pure phase focal plane mask (complex transmission = -1), the PIAACMC offers 50% throughput at 0.64 {lambda}/D while providing total extinction of an on-axis point source. This performance is very close to the 'fundamental performance limit' of coronagraphy derived from first principles. For very high contrast level, imaging performance with PIAACMC is in practice limited by the angular size of the on-axis target (usually a star). We show that this fundamental limitation must be taken into account when choosing the optimal value of the focal planemore » mask size in the PIAACMC design. We show that the PIAACMC enables visible imaging of Jupiter-like planets at {approx}1.2 {lambda}/D from the host star, and can therefore offer almost three times more targets than a PIAA coronagraph optimized for this type of observation. We find that for visible imaging of Earth-like planets, the PIAACMC gain over a PIAA is probably much smaller, as coronagraphic performance is then strongly constrained by stellar angular size. For observations at 'low' contrast (below {approx} 10{sup 8}), the PIAACMC offers significant performance enhancement over PIAA. This is especially relevant for ground-based high contrast imaging systems in the near-IR, where PIAACMC enables high contrast high efficiency imaging within 1 {lambda}/D. Manufacturing tolerances for the focal plane mask are quantified for a few representative PIAACMC designs.« less

This paper gives analytical expressions of the intensity in a Lyot coronagraph when the object is a resolved uniform disk. Intensities are given inside the Lyot stop and in the final plane. The derivation relies on the decomposition of the impinging waves on Zernike functions which allows the computation of the expansion of the intensities on infinite series. The expression in the final plane assumes that the sizes of the Lyot stop and the pupil are the same. An analysis of the truncation error is provided. These expressions are validated by computer simulations which makes it possible to recover known results such as the so-called diffraction ring observed by solar astronomers inside the Lyot stop. These analytical results and simulations bring deeper insight into the understanding of a Lyot coronagraph, for example, the fact that although the geometrical image of the source is behind the mask, a ghost image of the source can still be observed in the final plane.

We report numerical simulations and laboratory demonstrations of a four-quadrant phase mask (FQPM) coronagraph equipped with a Jacquinot pupil as a Lyot stop. We demonstrate that the Jacquinot-Lyot stop can effectively suppress the residual stellar intensity due to tip-tilt errors for unresolved stars. We also show that the achievable contrast with the Jacquinot-Lyot stop depends on the direction of the tip-tilt errors, which suggests that the contrast can be improved if the direction of these errors is monitored simultaneously with the data acquisition. Furthermore, we conduct laboratory experiments with a polarization differential imager in which the Jacquinot-Lyot stop is introduced into a four-quadrant polarization mask (FQPoM) coronagraph. In these experiments we also demonstrate two image processing techniques, a cross-correlation technique and a polarization-degree analysis, to extract planetary signal from residual stellar speckle noise.

The optical differentiation coronagraph relies on the optical differentiation technique implemented on a standard coronagraph. The use of a coronagraphic mask to estimate the first derivative of the incoming field shows high starlight suppression (theoretically infinite), but it is highly sensitive to pointing errors and suffers from monochromaticity drawbacks. In order to overcome these limitations, we generalize the optical differentiation concept to higher order derivatives. Here we describe a novel set of coronagraphic masks that estimate the second derivative of the incoming field. A mathematical description of the optical differentiation coronagraph is presented. These new masks also achieve a theoretical perfect suppression of the on-axis light, and furthermore, they are less sensitive to pointing errors (fourth-order sensitivity to tip/tilt error leakage). Moreover, they are pure amplitude masks, and hence they do not require a complementary phase mask, which represents an additional advantage. The use of a Gaussian roll-off helps to concentrate the diffracted starlight near the pupil borders, where it could be removed more efficiently by a Lyot stop, and transforms the coronagraph into a nearly band-limited coronagraph.

As adaptive optics technology continues to improve, the stellar coronagraph will play an ever increasing role in ground-based high-contrast imaging observations. Though several different image masks exist for the most common type of coronagraph, the Lyot coronagraph, it is not yet clear what level of wavefront correction must be reached in order to gain, either in starlight suppression or observing efficiency, by implementing a more sophisticated design. In this paper, we model image plane Lyot-style coronagraphs and test their response to a range of wavefront correction levels, in order to identify regimes of atmospheric compensation where the use of hard-edge, Gaussian, and band-limited image masks becomes observationally advantageous. To delineate performances, we calculate the speckle noise floor mean intensity. We find that apodized masks provide little improvement over hard-edge masks until on-sky Strehl ratios exceed $\sim0.88 \: S_{qs}$, where $S_{qs}$ is the intrinsic Strehl ratio provided by the optical system. Above this value, 4th-order band-limited masks out-perform Gaussian masks by generating comparable contrast with higher Lyot stop throughput. Below this level of correction, hard-edge masks may be preferentially chosen, since they are less susceptible to low-order aberration content. The use of higher-order band-limited masks is relegated to situations where quasi-static residual starlight cannot be sufficiently removed from the search area with speckle-nulling hardware.

We present methods for optimizing pupil and focal plane optical elements that improve the performance of vortex coronagraphs on telescopes with obstructed or segmented apertures. Phase-only and complex masks are designed for the entrance pupil, focal plane, and the plane of the Lyot stop. Optimal masks are obtained using both analytical and numerical methods. The latter makes use of an iterative error reduction algorithm to calculate "correcting" optics that mitigate unwanted diffraction from aperture obstructions. We analyze the achieved performance in terms of starlight suppression, contrast, off-axis image quality, and chromatic dependence. Manufacturing considerations and sensitivity to aberrations are also discussed. This work provides a path to joint optimization of multiple coronagraph planes to maximize sensitivity to exoplanets and other faint companions.

We present an overview of the design of a new testbed for studying coronagraphic imaging and wavefront control using a variety of pupil and coronagraph architectures. The testbed is designed to explore optimal use of starlight (including starlight rejected by the coronagraph) for wavefront control, system self-calibration, and point spread function (PSF) calibration. It is also compatible with coronagraph designs for centrally obscured and segmented apertures, and includes shaped or apodized pupils, a range of focal plane masks and Lyot stops of multiple sizes, and an optional PIAA apodizing stage. Starlight is reflected and imaged from the focal plane mask and Lyot stop for low-order wavefront sensing. Both a segmented and a continuous sheet MEMS DM are included to simulate segmented telescope pupils, apply known test phase patterns, and implement a controllable phase apodization coronagraph. The testbed is adaptable and is currently being used to investigate three different techniques: (1) the differential optical transfer function (dOTF), (2) low-order wavefront sensing (LOWFS) with a hybrid-Lyot coronagraph, and (3) linear dark field control (LDFC).

The existing high contrast imaging techniques cannot be applied directly to the telescope of the diameter 30–100 m without taking into account the specific characteristics of a segmented surface. While the increase in telescope diameter is an advantage for the high contrast range science, the segmentation sets a limit on the performance of the coronagraph. In particular, diffraction from intersegment gaps sets a floor to the achievable extinction of the starlight. Masking out the gaps in the Lyot plane although helps increasing the contrast, does not solve completely the problem: the high spatial frequency component of the diffractive light remains. In the paper I suggest using a Lyot stop which produces a phase-amplitude modulation in order to reduce the effect of segmentation.

Several recent designs for planet-finding telescopes use coronagraphs operating at visible wavelengths to suppress starlight along the telescope's optical axis while transmitting any off-axis light from circumstellar material. We describe a class of graded coronagraphic image masks that can, in principle, provide perfect elimination of on-axis light, while simultaneously maximizing the Lyot stop throughput and angular resolution. These ``band-limited'' masks operate on the intensity of light in the image plane, not the phase. They can work with almost any entrance pupil shape, provided that the entrance pupil transmissivity is uniform, and can be combined with an apodized Lyot stop to reduce the sensitivity of the coronagraph to imperfections in the image mask. We discuss some practical limitations on the dynamic range of coronagraphs in the context of a space-based terrestrial planet finder (TPF) telescope, and emphasize that fundamentally, the optical problem of imaging planets around nearby stars is a matter of precision fabrication and control, not Fraunhofer diffraction theory., Comment: 24 pages, including 6 figures. To appear in the Astrophysical Journal, May 2002

The electric field conjugation (EFC) algorithm has shown promise for removing scattered starlight from high-contrast imaging measurements, both in numerical simulations and laboratory experiments. To prepare for the deployment of EFC using ground-based telescopes, we investigate the response of EFC to unaccounted for deviations from an ideal optical model. We explore the linear nature of the algorithm by assessing its response to a range of inaccuracies in the optical model generally present in real systems. We find that the algorithm is particularly sensitive to unresponsive deformable mirror (DM) actuators, misalignment of the Lyot stop, and misalignment of the focal plane mask. Vibrations and DM registration appear to be less of a concern compared to values expected at the telescope. We quantify how accurately one must model these core coronagraph components to ensure successful EFC corrections. We conclude that while the condition of the DM can limit contrast, EFC may still be used to improve the sensitivity of high-contrast imaging observations. Our results have informed the development of a full EFC implementation using the Project 1640 coronagraph at Palomar observatory. While focused on a specific instrument, our results are applicable to the many coronagraphs that may be interested in employing EFC.

The Apodized Pupil Lyot Coronagraph (APLC) is a di_raction suppression system adopted in the baseline of several new and recent high-contrast imaging instruments (Palomar P1640, Gemini Planet Imager, VLT/SPHERE) to enable direct imaging of exoplanets at small angular separations (> 0.2 arcsec) from their host star. This coronagraph combines an entrance pupil apodizer, a hard-edge focal plane mask (FPM) and a Lyot stop in a relayed pupil plane to form the coronagraphic image of an observed star onto a camera located in the image plane. The APLC designs underlying these instruments take advantage of the eigen-properties between entrance pupil and Lyot stop, and rely on prolate apodization to reach a contrast performance of 10 7 over a 20% spectral bandwidth and with a moderate inner working angle (IWA, ~ 5 λ/=D) in the presence of central obstruction and support structures. In this communication we propose novel designs relying on the linearity between the coronagraphic electric field at the science camera and the apodization function. We use this relationship to devise a numerical optimization scheme that extends the APLC performance (contrast, IWA, apodizer through- put, chromaticity) beyond the intrinsic properties of prolate apodizations. We explore the parameter space by considering different aperture geometries, contrast levels, dark-zone size, spectral bandwidth and FPM size. We present the application of these new solutions to the case of the High-Contrast Imager for Complex Aperture Telescopes (HiCAT).

In this paper, we present the infrared coronagraphic test bench of the University of Li\`ege named VODCA (Vortex Optical Demonstrator for Coronagraphic Applications). The goal of the bench is to assess the performances of the Annular Groove Phase Masks (AGPMs) at near- to mid-infrared wavelengths. The AGPM is a subwavelength grating vortex coronagraph of charge two (SGVC2) made out of diamond. The bench is designed to be completely achromatic and will be composed of a super continuum laser source emitting in the near to mid-infrared, several parabolas, diaphragms and an infrared camera. This way, we will be able to test the different AGPMs in the M, L, K and H bands. Eventually, the bench will also allow the computation of the incident wavefront aberrations on the coronagraph. A reflective Lyot stop will send most of the stellar light to a second camera to perform low-order wavefront sensing. This second system coupled with a deformable mirror will allow the correction of the wavefront aberrations. We also aim to test other pre- and/or post-coronagraphic concepts such as optimal apodization.

In the first paper in this series, we described the principle of a coronagraph utilizing a four-quadrant phase mask and the results of numerical simulations obtained in the perfect case. In this second paper, we performed additional numerical simulations to assess in more detail the performances and limitations of this coronagraph under real conditions. The effect of geometrical parameters such as shape and size of both the phase mask and the Lyot stop is studied. We also analyze the effect of low- and high-order aberrations generated, for instance, by the atmospheric turbulence. An important issue is the wavelength dependence of the phase mask. We show that the performance decreases rapidly as the spectral bandwidth is increased, and as a consequence, we discuss the manufacturing of achromatized masks using multiple thin films. An optical concept is proposed.

We describe a new type of coronagraph, based on the principle of a phase mask as proposed by Roddier and Roddier a few years ago but using an original mask design found by one of us (D. R.), a four-quadrant binary phase mask (0, n) covering the full Ðeld of view at the focal plane. The mutually destructive interferences of the coherent light from the main source produce a very efficient nulling. The computed rejection rate of this coronagraph appears to be very high since, when perfectly aligned and phase-error free, it could in principle reduce the total amount of light from the bright source by a factor of 108, corresponding to a gain of 20 mag in brightness at the location of the Ðrst Airy ring, relative to the Airy peak. In the real world the gain is of course reduced by a strong factor, but nulling is still performing quite well, provided that the perturbation of the phase, for instance, due to the EarthIs atmosphere, is efficiently corrected by adaptive optics. We show from simulations that a detection at a contrast of 10 mag between a star and a faint companion is achievable in excellent conditions, while 8 mag appears routinely feasible. This coronagraph appears less sensitive to atmospheric turbulence and has a larger dynamic range than other recently proposed nulling techniques : the phase-mask coronagraph (by Roddier and Roddier) or the Achro- matic Interfero-Coronagraph (by Gay and Rabbia). We present the principle of the four-quadrant corona- graph and results of a Ðrst series of simulations. We compare those results with theoretical performances of other devices. We brieNy analyze the di†erent limitations in space or ground-based observations, as well as the issue of manufacturing the device. We also discuss several ways to improve the detection of a faint companion around a bright object. We conclude that, with respect to previous techniques, an instrument equipped with this coronagraph should have better performance and even enable the imaging of extrasolar giant planets at a young stage, when coupled with additional cleaning techniques.

METIS, the Multi Element Telescope for Imaging and Spectroscopy, is the solar coronagraph foreseen for the ESA Solar Orbiter mission. METIS is conceived to image the solar corona from a near-Sun orbit in two different spectral bands: in the HI UV narrow band at 121.6 nm, and in the polarized visible light band (580 - 640 nm). METIS is an externally occulted coronagraph which adopts an "inverted occulted" configuration. The inverted external occulter (IEO) is a small circular aperture after which a small spherical mirror M0 rejects back the disk-light through the IEO, then an annular mirror collects the signal coming from the corona and redirects it toward the telescope secondary mirror. This paper presents the error budget analysis for this new-concept coronagraph configuration, which incorporates two different sub-channels: the UV imaging and the polarimetric visible one. The two sub-channels are sharing the telescope optics, then an interference filter transmits the UV light towards the UV detector, while the visible-light is reflected towards the polarimetric unit. The tolerance analysis is rather complex, in fact not only the optical performance for the two sub-channels has to be maintained simultaneously, but also the positions of the M0 and the occulters (IEO, internal occulter and Lyot stop), which assure the optimal disk light suppression, have to be taken into account as tolerancing parameters. To guarantee the scientific requirements are optimally fulfilled for the two sub-channels, the preliminary results of manufacturing, alignment and stability tolerance analysis for the whole instrument will be described and discussed.

Spectral characterization of young, giant exoplanets detected by direct imaging is one of the tasks of the new generation of high-contrast imagers. For this purpose, the VLT/SPHERE instrument includes a unique long-slit spectroscopy (LSS) mode coupled with Lyot coronagraphy in its infrared dual-band imager and spectrograph (IRDIS). The performance of this mode is intrinsically limited by the use of a non-optimal coronagraph, but in a previous work we demonstrated that it could be significantly improved at small inner-working angles using the stop-less Lyot coronagraph (SLLC). We now present the development, testing, and validation of the first SLLC prototype for VLT/SPHERE. Based on the transmission profile previously proposed, the prototype was manufactured using microdots technology and was installed inside the instrument in 2014. The transmission measurements agree well with the specifications, except in the very low transmissions (, 13 pages, 13 figures. Accepted for publication in A&A

The Self-Coherent Camera is dedicated to the direct detection of exoplanets. This instrument can be used as a focal plane wavefront sensor to measure static aberrations that induce speckles on the detector, which prevents the detection of faint companions. The Self-Coherent Camera creates a reference beam in the Lyot stop pupil plane in order to spatially modulate the speckle pattern with Fizeau fringes. We can then estimate for wavefront aberrations upstream of the coronagraphic mask and correct for them using a deformable mirror. Currently, the Self-Coherent Camera is combined with a deformable mirror located in the pupil plane upstream of a Four-Quadrant Phase Mask Coronagraph. In this paper, we present the formalism that explains how the Self-Coherent Camera encodes speckles and how we estimate the wavefront aberrations directly from the science image. We present numerical simulation results on speckle suppression in the focal plane. Then, we give experimental results on wavefront correction on our optical bench using a 32x32 actuators deformable mirror. We show that we can improve the contrast in the focal plane by a factor of more than 100 in the PSF wings up to 12λ/D. © 2012 SPIE., Ground-Based and Airborne Instrumentation for Astronomy IV, July 1-6, 2012, Amsterdam, The Netherlands, Series: Proceedings of SPIE; no. 8446

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 “dar k hole” in the monochromatic point-spread function with an inner working angle of 2.05 fO /D , with a mean intensity 3.5×10 i 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 i 9 performance but that an annular postapodizer is not necessary. Keywords: Exoplanets, coronagraphy, deformable mirror, wavefront control, wavefront estimation 1. INTRODUCTION 1.1 PIAA coronagraph principle This paper presents laboratory results from testing of a Phase Induced Amplitude Apodization (PIAA) coronagraph on the High Contrast Imaging Testbed (HCIT) at NASA’s Jet Propulsion Laboratory. PIAA is a high-throughput, small inner-working angle, high spatial resolution coronagraphic technique proposed for space-based exoplanet imaging and spectroscopy missions

An eight-octant phase-mask (EOPM) coronagraph is one of the highest performance coronagraphic concepts, and attains simultaneously high throughput, small inner working angle, and large discovery space. However, its application to ground-based telescopes such as the Subaru Telescope is challenging due to pupil geometry (thick spider vanes and large central obstruction) and residual tip-tilt errors. We show that the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system, scheduled to be installed onto the Subaru Telescope, includes key technologies which can solve these problems. SCExAO uses a spider removal plate which translates four parts of the pupil with tilted plane parallel plates. The pupil central obstruction can be removed by a pupil remapping system similar to the PIAA optics already in the SCExAO system, which could be redesigned with no amplitude apodization. The EOPM is inserted in the focal plane to divide a stellar image into eight-octant regions, and introduces a π-phase difference between adjacent octants. This causes a self-destructive interference inside the pupil area on a following reimaged pupil plane. By using a reflective mask instead of a conventional opaque Lyot stop, the stellar light diffracted outside the pupil can be used for a coronagraphic low-order wave-front sensor to accurately measure and correct tip-tilt errors. A modified inverse-PIAA system, located behind the reimaged pupil plane, is used to remove off-axis aberrations and deliver a wide field of view. We show that this EOPM coronagraph architecture enables high contrast imaging at small working angle on the Subaru Telescope. Our approach could be generalized to other phase-mask type coronagraphs and other ground-based telescopes.

The Gemini Planet Imager (GPI) is a new facility instrument to be commissioned at the 8-m Gemini South telescope in early 2011. It combines of several subsystems including a 1500 subaperture Extreme Adaptive Optics system, an Apodized Pupil Lyot Coronagraph, a near-infrared high-accuracy interferometric wavefront sensor, and an Integral Field Unit Spectrograph, which serves as the science instrument. GPI's main scientific goal is to detect and characterize relatively young ( 5λ/D, without active wavefront control (no deformable mirror). We use Fresnel propagation modeling to analyze the testbed results.

A phase retrieval algorithm based on transverse translation diversity is investigated as a method for retrieving the phase of the field as seen through the JWST NIRCam coronagraph using only existing hardware.

HiCIAO (the High‐Contrast Instrument with Adaptive Optics) is a high‐contrast instrument for the 8.2‐meter Subaru Telescope. The instrument is a near‐infrared camera which benefits from a new adaptive optics (AO) system on the Subaru Telescope (AO188). The instrument realizes the high contrast with a help of AO188, a classical Lyot coronagraph, and three differential imaging techniques (polarimetric, spectral, and angular). Besides the differential imaging modes, HiCIAO also offers a normal imaging mode which covers 20″×20″ FOV with 0.″01 pixel−1 resolution, and a pupil viewing mode for a precise alignment of the Lyot stop on the pupil image. The expected contrasts are 105.5 at 1.″0 separation and 104 at 0.″1 separation from a central star in the spectral differential imaging mode. The instrument is currently in its commissioning phase after the first‐light observation in December 2008. This paper is an introductory review of the instrument.

Practical band-limited occulting masks used in visible-light Lyot coronagraphs, such as those proposed for the Terrestrial Planet Finder Coronagraph (TPF-C), will exhibit some non-band-limited transmission errors that may limit their observing contrast ratios to unacceptable levels. The complex (both phase and amplitude) transmission profiles of these masks exhibit large dynamic ranges over small spatial scales, which are difficult to probe using ordinary optical microscopes and interferometers. We describe here a technique for making these complex transmission measurements with the mask in-place in a Lyot coronagraph, using point-source images taken in the focal and pupil planes, with the Lyot stop removed. We also present measurements taken from the High-Contrast Imaging Testbed (HCIT) at JPL.