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تسجيل مستخدم جديدOptimization of Apodized Pupil Lyot Coronagraph for ELTs
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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.
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In the context of high contrast imaging, we propose to evaluate the performance of the Apodized Pupil Lyot Coronagraph (APLC) working without Lyot Stop, namely Stop-less Lyot Coronagraph (SLLC). This coronagraph is a combination of an entrance pupil
apodizer and an opaque mask in the following focal plane. However, contrary to APLC, SLLC is amputated by the traditional pupil stop. Our goal is to stress the interest of using this coronagraphic solution, in particular for instruments for which the introduction of a stellar coronagraph with Lyot stop is made impossible. We estimate the intensity attenuation achieved with SLLC and carry out our study with a focus on the case of Gran Telescopio Canarias (GTC). In a first step, numerical simulations are made assuming the absence of any aberration, thereafter SLLC performance is evaluated considering AO corrected wavefronts in our approach for ground-based instruments. SLLC performance proves to be equivalent to that obtained with APLC in presence of AO compensated atmospheric turbulence images, which Strehl ratio is S=0.552 at the wavelength lambda=1.57 mu m. This coronagraph allows to remove the peak intensity of a star image and therefore, avoid detector saturation. Moreover, it helps increasing the image dynamic range. A mean contrast gain in stellar magnitudes Delta m=0.23 is obtained with SLLC whereas APLC reaches a value Delta m=0.38.
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 pro
duce 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.
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.
A coronagraphic starlight suppression system situated on a future flagship space observatory offers a promising avenue to image Earth-like exoplanets and search for biomarkers in their atmospheric spectra. One NASA mission concept that could serve as
the platform to realize this scientific breakthrough is the Large UV/Optical/IR Surveyor (LUVOIR). Such a mission would also address a broad range of topics in astrophysics with a multiwavelength suite of instruments. The apodized pupil Lyot coronagraph (APLC) is one of several coronagraph design families that the community is assessing as part of NASAs Exoplanet Exploration Program Segmented aperture coronagraph design and analysis (SCDA) team. The APLC is a Lyot-style coronagraph that suppresses starlight through a series of amplitude operations on the on-axis field. Given a suite of seven plausible segmented telescope apertures, we have developed an object-oriented software toolkit to automate the exploration of thousands of APLC design parameter combinations. This has enabled us to empirically establish relationships between planet throughput and telescope aperture geometry, inner working angle, bandwidth, and contrast level. In parallel with the parameter space exploration, we have investigated several strategies to improve the robustness of APLC designs to fabrication and alignment errors. We also investigate the combination of APLC with wavefront control or complex focal plane masks to improve inner working angle and throughput. Preliminary scientific yield evaluations based on design reference mission simulations indicate the APLC is a very competitive concept for surveying the local exoEarth population with a mission like LUVOIR.
The apodized-pupil Lyot coronagraph is one of the most advanced starlight cancellation concepts studied intensively in the past few years. Extreme adaptive optics instruments built for present-day 8m class telescopes will operate with such coronagrap
h for imagery and spectroscopy of faint stellar companions. Following the development of an early demonstrator in the context of the VLT-SPHERE project (~2012), we manufactured and tested a second APLC prototype in microdots designed for extremely large telescopes. This study has been conducted in the context of the EPICS instrument project for the European-ELT (~2018), where a proof of concept is required at this stage. Our prototype was specifically designed for the European-ELT pupil, taking its large central obscuration ratio (30%) into account. Near-IR laboratory results are compared with simulations. We demonstrate good agreement with theory. A peak attenuation of 295 was achieved, and contrasts of 10^-5 and 10^-6 were reached at 7 and 12 lambda/D, respectively. We show that the APLC is able to maintain these contrasts with a central obscuration ratio of the telescope in the range 15% to 30%, and we report that these performances can be achieved in a wide wavelength bandpass (BW = 24%). In addition, we report improvement to the accuracy of the control of the local transmission of the manufactured microdot apodizer to that of the previous prototype. The local profile error is found to be less than 2%. The maturity and reproducibility of the APLC made with microdots is demonstrated. The apodized pupil Lyot coronagraph is confirmed to be a pertinent candidate for high-contrast imaging with ELTs.
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