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.