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.
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.
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