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.
Space telescopes are required to be lightweight and small without compromising high optical performance. A Metallic mirror is one of the technologies that can meet launch conditions, the harsh space environment and achieve the optical requirements of an imaging payload and have been widely used from JWST to new space payloads. Flexible mounting pads are one of the geometrical designs within a metallic mirror that is a very critical part which mounts the mirror to the supporting structure. Flexible pads improve optical stability by reducing screw pressure from mounting and increase vibration endurance by creating more flexibility in the design. This study will use Finite Element Analysis to optimize the shape of flexible pads, examining the effects on mechanical and optical performance by varying geometric dimensions in a parametric study under multiple scenarios from manufacturing to operating in orbit. The results highlight the parameters that have the biggest impact on mechanical and optical performance in each scenario and describe the relation between the parameters that affect mechanical and optical performance that improve the understanding of the opto-mechanical design of metallic mirrors. Finally, the design will be optimized with multiple objectives to get the most optimal design based on all scenario’s conditions.
The parametric study could be analyzed with the sensitivity study, response surface, and optimization. The results show the parameters that have the most impact on performance and show its effect on performance in various conditions such as manufacturing load, grounded based stability with screw pressure, natural frequency, thermal load, and gravity release. The optimization process can lead to the improvement of the optical design. This study improves understanding of opto-mechanical design of the flexible pads in metallic mirrors, which can be applied to other metallic mirror designs.
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 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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