The Evanescent Wave Coronagraph (EvWaCo) exploits the frustration of the total internal reflection (FTIR) between a prism and a lens put in contact. The starlight is transmitted through the contact area while the light from the companion is reflected. An EvWaCo prototype, equipped with an adaptive optics (AO) system, will be installed at the 2.4m Thai National Telescope as an on-sky demonstrator of the EvWaCo mask’s achromatic capabilities while testing new AO control techniques. To characterize the Extreme Adaptive Optics System (XAO) for this prototype, we developed a bench equipped with a DM192 ALPAO deformable mirror, a 15×15 sub-apertures Shack-Hartmann wavefront sensor (SH-WFS), and a two-track phase plate simulating an average seeing of 1.4" at the Thai National Telescope, and the best seeing at 1.00". Following our previous communications on the characterization of the DM and the phase plate, we present how we calibrate the sensor for the WFS and the interaction matrix. This paper presents preliminary results obtained from experiments after closing the loop using a leaky integrator.
The Evanescent Wave Coronagraph uses a focal plane mask comprising a lens and a prism placed in contact so that frustrated total internal reflection can occur - the principle governing starlight attenuation. This type of Lyot coronagraph has three main capabilities: i) the mask adapts itself to the wavelength, ii) the size of the mask is adjustable by pressure adjustment, and iii) both the light coming from the star and companion can be collected simultaneously. Previous experimental results, obtained without adaptive optics and in unpolarized light, showed a raw contrast of 10−4 at 3 λ/D in the I-band and at 4 λ/D in the R-band. Its performance has been limited so far by uncorrected residual aberrations of the optical bench that generate speckles close to the inner working angle. To study the mask performances close to the diffraction limit and compare them with theoretical models, a deformable mirror is installed in the optical path of the testbed to perform wavefront correction. In this work, we report the results obtained in the laboratory using this upgraded setup. We show the preliminary results of correcting the non-common path aberrations using the scientific camera as the wavefront sensor and compare them with expected theoretical performances. The corrections are applied after finding the optimal commands that maximize the variance at the detector plane.
The Evanescent Wave Coronagraph (EvWaCo) is a type of Lyot coronagraph that uses an achromatic focal plane mask comprising a lens and a prism in contact. The National Astronomical Research Institute of Thailand (NARIT) plans to install an EvWaCo prototype equipped with an adaptive optics system (AO) to correct the aberrated wavefront in real-time at the unused left Nasmyth port of the Thai National telescope. To prepare for this installation, a large adapter with a diameter of 1.3 m and twelve carbon fiber poles serve as the supporting beams to hold the prototype. This work focuses on the mechanical design and testing of the large adapter, considering the prototype requirements and installation limitations. In particular, mechanical deformations and stress distributions are analyzed under survival conditions. The maximum weight of the prototype is 200 kg, and a folding mirror installed in a translation stage is placed inside the large adapter. The structural optimization uses the finite element method to deal with the constraints and ensures a high performance. The carbon fiber poles comprise carbon fiber-reinforced polymer (CFRP) that reduce the weight by approximately 30% compared to an all-aluminum structure. Each carbon fiber pole weighs about 1.75 kg, and our testing results show that it can support up to eight times the prototype's weight. The epoxy adhesive, used to join different materials, can withstand a pull-out strength of up to three times the prototype's weight. The installation of this adapter is expected to start by the end of 2024.
NASA is embarking on an ambitious program to develop the Habitable Worlds Observatory (HWO) flagship to perform transformational astrophysics, as well as directly image ∼ 25 potentially Earth-like planets and spectroscopically characterize them for signs of life. This mission was recommended by Astro2020, which additionally recommended a new approach for flagship formulation based on increasing the scope and depth of early, pre-phase A trades and technology maturation. A critical capability of the HWO mission is the suppression of starlight. To inform future architecture trades, it is necessary to survey a wide range of candidate technologies, from the relatively mature ones such as the ones described in the LUVOIR and HabEx reports to the relatively new and emerging ones, which may lead to breakthrough performance. In this paper, we present a summary of an effort, funded by NASA’s Exoplanet Exoplaration Program (ExEP), to survey potential coronagraph options for HWO. In particular, our results consist of: (1) a database of different coronagraph designs sourced from the world-wide coronagraph community that are potentially compatible with HWO; (2) evaluation criteria, such as expected mission yields and feasibility of maturing to TRL 5 before phase A; (3) a unified modeling pipeline that processes the designs from (1) and outputs values for any machine-calculable criteria from (2); (4) assessments of maturity of designs, and other criteria that are not machine-calculable; (5) a table presenting an executive summary of designs and our results. While not charged to down-select or prioritize the different coronagraph designs, the products of this survey were designed to facilitate future HWO trade studies.
The National Astronomical Research Institute of Thailand, together with the Institut d’Optique Graduate School and Centre de Researche Astrophysique de Lyon, has been developing the Evanescent Wave Coronagraph (EvWaCo) a new kind of Lyot coronagraph that uses a lens and prism placed in contact as its focal plane mask. By the principle of frustrated total internal reflection, EvWaCo enables an achromatic rejection and ability to collect the light from the star and the companion. An EvWaCo prototype equipped with adaptive optics will be installed at the Thai National Telescope as an on-sky demonstrator. This demonstrator will work on a 1.2 × 0.8 m2 elliptical sub-aperture of the Thai National Telescope to reach a raw contrast of 10−4 at 3λ/D over the wavelength range [600 nm, 900 nm]. The completed optical design contains all the essential light path channels in high contrast imaging fitted inside a 960 mm×960 mm optical breadboard, namely the guiding camera channel, companion channel, star channel, and wavefront sensing channel. We also show the results of the tolerancing and straylight analysis.
We present the results obtained with an end-to-end simulator of an Extreme Adaptive Optics (XAO) system control loop. It is used to predict its on-sky performances and to optimise the AO loop algorithms. It was first used to validate a novel analytical model of the fitting error, a limit due to the Deformable Mirror (DM) shape. Standard analytical models assume a sharp correction under the DM cutoff frequency, disregarding the transition between the AO corrected and turbulence dominated domains. Our model account for the influence function shape in this smooth transition. Then, it is well-known that Shack-Hartmann wavefront sensors (SH-WFS) have a limited spatial bandwidth, the high frequencies of the wavefront being seen as low frequencies. We show that this aliasing error can be partially compensated (both in terms of Strehl ratio and contrast) by adding priors on the turbulence statistics in the framework of an inverse problem approach. This represents an alternative to the standard additional optical filter used in XAO systems. In parallel to this numerical work, a bench was aligned to experimentally test the AO system and these new algorithms comprising a DM192 ALPAO deformable mirror and a 15x15 SH-WFS. We present the predicted performances of the AO loop based on end-to-end simulations.
The Evanescent Wave Coronagraph (EvWaCo) is an achromatic coronagraph mask with adjustable size over the spectral domain [600nm, 900nm] that will be installed at the Thai National Observatory. We present in this work the development of a bench to characterise its Extreme Adaptive Optics system (XAO) comprising a DM192 ALPAO deformable mirror (DM) and a 15x15 Shack-Hartmann wavefront sensor (SH-WFS). In this bench, the turbulence is simulated using a rotating phase plate in a pupil plane. In general, such components are designed using a randomly generated phase screen. Such single realisation does not necessarily provide the wanted structure function. We present a solution to design the printed pattern to ensure that the beam sees a strict and controlled Kolmogorov statistics with the correct 2D structure function. This is essential to control the experimental conditions in order to compare the bench results with the numerical simulations and predictions. This bench is further used to deeply characterise the full 27 mm pupil of the ALPAO DM using a 54x54 ALPAO SH-WFS. We measure the average shape of its influence functions as well as the influence function of each single actuator to study their dispersion. We study the linearity of the actuator amplitude with the command as well as the linearity of the influence function profile. We also study the actuator offsets as well as the membrane shape at 0-command. This knowledge is critical to get a forward model of the DM for the XAO control loop.
The Evanescent Wave Coronagraph (EvWaCo) is a coronagraph that utilizes the principle of Frustrated Total Internal Reflection (FTIR) to simultaneously collect both the starlight and the companion light by using a focal plane mask composed of a convex diopter and a prism placed in contact. The mask exhibits an achromatic behavior, and its size can be varied by adjusting the pressure at the contact area. The National Astronomical Research Institute of Thailand (NARIT) is developing a prototype to demonstrate on-sky the performance of EvWaCo. This prototype will be installed at the Thailand National Telescope (TNT). In this paper, the mechanical design of the EvWaCo prototype is documented. The mechanical requirements of this prototype include a maximum weight equal to 180 kg, a maximum deformation of 120 μm, and an average deformation of 100 μm for every optical component. To achieve this, the structural parts are designed to achieve the high directional stiffness, and the passive thermal compensation is conceptualized for athermalization. Then, the lightweight, high-performance materials are selected. The Finite Element Analysis (FEA) method is used to simulate the performance of the prototype under the realistic conditions. The prototype performs with an average deformation of 43 ± 15 μm and a maximum deformation of 63 ± 18 μm at the average thermal condition of ΔT = 13.6 ⁰C. The instrument performs with an average deformation of 67 ± 16 μm and a maximum deformation of 92 ± 19 μm at the worst thermal condition of ΔT = 25 ⁰C. This instrument design weights 175.7 kg.
The Center for Optics and Photonics of the National Astronomical Research Institute of Thailand, together with the Institut d’Optique Graduate School and the Centre de Recherche Astrohpysique de Lyon (CRAL), is currently developing the Evanescent Wave Coronagraph (EvWaCo). The coronagraph relies on the tunneling effect to produce a fully achromatic focal plane mask (FPM) with an adjustable size. The full instrument comprises a coronagraph and adaptive optics system that will be mounted on the Thai National Telescope and is specified to reach a raw contrast of 10−4 at an inner working angle of 3 Airy radii. The coronagraph will be used to perform high contrast observations of stellar systems during on-sky observations over the spectral domain [600 nm, 900 nm]. In this paper, we present the opto-mechanical design of the EvWaCo prototype and the performance measured in laboratory conditions. We also discuss the potential applications for space-based observations and the development plan under this project in the next five years.
The EXOplanet high resolution SPECtrograph (EXOhSPEC) instrument is an echelle spectrograph dedicated to the detection of exoplanets by using the radial velocity method using 2m class telescopes. This spectrograph is specified to provide spectra with a spectral resolution R < 70, 000 over the spectral range from 400 to 700 nm and to reach a shortterm radial velocity precision of 3 m/s. To achieve this the separation between two adjacent spectral orders is specified to be greater than 30 pixels and to enable a wide range of targets the throughput of the instrument is specified to be higher than 4%. We present the results of the optimization of the spectrograph collimator performed and initial tests of its optical performance. First, we consider the spectrograph design and we estimate its theoretical performance. We show that the theoretical image quality is close to the diffraction limit. Second, we describe the method used to perform the tolerancing analyzes using ZEMAX software to estimate the optical performance of the instrument after manufacturing, assembly and alignment. We present the results of the performance budget and we show that the estimated image quality performance of EXOhSPEC are in line with the specifications. Third, we present the results of the stray light analysis and we show that the minimum ratio between the scientific signal and the stray light halo signal is higher than 1,000. Finally, we provide a status on the progress of the EXOhSPEC project and we show the first results obtained with a preliminary version of the prototype.
The Evanescent Wave Coronagraph (EvWaCo) is a coronagraph with an occulting mask based on the frustration of total internal reflection to i) produce an achromatic extinction of the central star and ii) reveal the faint companion surrounding the star. Results obtained in laboratory conditions show contrast performance of a few 10-6 between 10 λC/D and 20 λC/D over the full I-band centered at the wavelength λC = 800 nm with a spectral ratio of Δλ/ λC ≈ 20% in unpolarized light.
In this paper, we discuss the advantages of using EvWaCo to observe and characterize exoplanets with a space-based telescope. In the first section, we describe the system and present the current results obtained with the EvWaCo testbed. We also illustrate the capability of this coronagraph to detect the companion 30,000 times (respectively, 100,000 times) fainter than the central star at distances equal to 15 Airy radii (respectively, 30 Airy radii) from the PSF center in polychromatic and unpolarized light.
In the second section, we describe the design of the prototype dedicated to the on-sky tests of the instrument with the 2.4 m Thai National Telescope at horizon 2020. This prototype has been designed with the objective to reach a contrast equal to a few 10-4 at the inner working angle (IWA) equal to 3 λ/D from the star PSF center while observing through the atmosphere over the full photometric I-band. This prototype will include an adaptive optics specified to reach at λ ≈ 800 nm a Strehl ratio > 0.8 for magnitude m < 7.
In the third section, we show the theoretical performance of EvWaCo: a contrast comprised between a few 10-6 and 10-7 in the I-Band between 3 λ/D and 10 λ/D in the I-Band for an IWA equal to 3 λ/D with a Gaussian apodization in unpolarized light. We also show that similar contrasts performance are obtained in the V-, R-, bands, thus illustrating the EvWaCo quasi-achromaticity. Finally, we discuss the advantages and the limitation using the proposed concept for space-based observations and spectral characterization of exoplanets.
The objective of the Evanescent Wave Coronagraph (EvWaCo) project is to develop a new kind of simple and cost effective coronagraph, first for ground-based telescopes and then for space-based telescopes. The principle involves the tunneling effect to separate the star light from the companion light. The star light is directed transmitted toward a WaveFront Sensor (WFS) that measures the wavefront distortions in the immediate proximity of the occulting mask with minimum non-common path errors. The beam reflected by the mask propagates toward the Lyot stop and forms the images of the companion and of the star residuals on the camera.
The EvWaCo concept has been demonstrated and this instrument is achromatic over the I-band of the Johnson- Cousins photometric system in unpolarized light. We have measured over this photometric band an Inner Working Angle (IWA) equal to 6 λ/D and contrasts of a few 10-6 at distances greater than 10 Airy radii from the star Point Spread Function (PSF) center.
This paper describes the continuation of the project, from this setup of demonstration to the first prototype operating on the sky at horizon 2020. The objective is to show the capability of the full system to provide IWA and raw contrasts close to the state-of-art performance with the Thai National Telescope, by observing through an unobstructed elliptical pupil of major axis length equal to 1 m. The system will demonstrate over the full I-band an IWA close to 3 λ/D and raw contrasts equal to a few 10-4 at a distance equal to the IWA from the PSF.
The National Astronomical Research Institute of Thailand (NARIT) has developed since June 2014 an optical laboratory that comprises all the activities and facilities related to the research and development of new instruments in the following areas: telescope design, high dynamic and high resolution imaging systems and spectrographs. The facilities include ZEMAX and Solidwork software for design and simulation activities as well as an optical room with all the equipment required to develop optical setup with cutting-edge performance.
The current projects include: i) the development of a focal reducer for the 2.3 m Thai National Telescope (TNT), ii) the development of the Evanescent Wave Coronagraph dedicated to the high contrast observations of star close environment and iii) the development of low resolution spectrographs for the Thai National Telescope and for the 0.7 m telescopes of NARIT regional observatories. In each project, our activities start from the instrument optical and mechanical design to the simulation of the performance, the development of the prototype and finally to the final system integration, alignment and tests. Most of the mechanical parts are manufactured by using the facilities of NARIT precision mechanical workshop that includes a 3-axis Computer Numerical Control (CNC) to machine the mechanical structures and a Coordinate Measuring Machine (CMM) to verify the dimensions.
In this paper, we give an overview of the optical laboratory activities and of the associated facilities. We also describe the objective of the current projects, present the specifications and the design of the instruments and establish the status of development and we present our future plans.
The Evanescent Wave Coronagraph (EvWaCo) is a new kind of “band-limited coronagraph” that involves the tunneling effect to suppress the starlight, thus producing the coronagraphic effect. The first advantage is that this mask adapts itself to the wavelength due to the evanescent wave properties, yielding nearly an achromatization of the star extinction. The second advantage is that the starlight can be collected for astrometry and/or wavefront analysis and correction. NARIT has developed a specific optical setup operating over the spectral band [780 nm, 880nm] to demonstrate highlevel contrasts and inner working angles in line with the requirements for exoplanet detection. Our aims are: to test and characterize the EvWaCo performance in diffraction-limited regime, to install a simulator of turbulence and an adaptive optics setup to simulate ground-based observations, and to define the best scheme for the wavefront correction. The preliminary results obtained in diffraction-limited regime demonstrated contrasts equal to a few 10-6 at a distance between 10 and 20 λ/D from the Point Spread Function (PSF) center with an unpolarized source emitting at λ1 = 880 nm with a relative spectral bandwidth Δλ/λ1 ≈ 6%. In this paper, we first describe the upgraded setup and present the results of the performance characterization that investigates the variation of the contrast with the wavelength and with the polarization. Then, we show the results obtained on the star channel and demonstrate the capability to measure in real time the star PSF profile and position. Finally, we discuss the future improvements to optimize the performance.
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