The Exoplanet High-Resolution Spectrograph (EXOhSPEC) is a high-resolution spectrograph for the characterisation of exoplanets with the Thai National Telescope. The folded version of this instrument comprises one triplet lens to collimate the beam incident on the grating and to focus the beam reflected by the grating onto the camera. This collimator comprises three lenses L1, L2 and L3 of diameter varying between 50 mm and 60 mm. We specified the barrel to guarantee a maximum decenter of the lenses equal to 25 μm. The maximum error in the orientation of each single lens is specified to be lower than 0.03º. The proposed concept is based on a semi-kinematic mounting which is used to restrain these lenses with 6 and 30 N of preloads on the axial and lateral directions to ensure their stability. These preloads are applied to the lenses using the elastic pushing force of silicone elastomers and spring force from ball-plungers. We present the design of the collimator and the assembly method. Our Finite Element Analyses show that the maximum surface error induced by the preloads is lower than 60 nm Peak-To-Valley on each optical surface of L1, L2, and L3. We describe our manufacturing process using NARIT’s CNC machine and its validation using our Coordinate-Measuring Machine.
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.
We will review the development in the last decade of discrete beam combiners (DBC), phase sensors based on the propagation of light in photonic lattices. The latest results on the development of DBC for astronomical applications will be presented, along with a new application for the complete tomography of modes at the tip of a multi-mode fiber. The possible use of the DBC in monitoring and controlling modal instabilities in high power lasers will be discussed.
We conduct an extensive numerical study to single out the best performing rectangular array of evanescently coupled waveguides (discrete beam combiner) that can be used as an integrated optic beam combiner for 6-telescopes at once. We found that the performance of a discrete beam combiner only depends on the conditioning of the Visibility to Pixel Matrix (V2PM) describing it. However, we found that the condition number of V2PM pertaining to different beam combiner architectures cannot be compared. We further report on the possible input waveguide configuration of an 8-telescope discrete beam combiner featuring 8x8 or 9x9 waveguides.
Nulling interferometry has been identified as a competitive technique for the detection of extrasolar planets. The technique consists in combining out-of-phase pairs of telescopes to null effectively the light of a bright star an reveal the dim glow of the companion. We have manufactured and tested with monochromatic light an integrated optics component which combines a linear array of 4 telescopes in the nulling mode envisaged by Angel&Wolf.<sup>1</sup> Our testbench simulates the motion of a star in the sky. The tests have demonstrated a nulling scaling as the fourth power of the baseline delay.
In recent years, new coronagraphic schemes have been proposed, the most promising being the optical vortex
phase mask coronagraphs. In our work, a new scheme of broadband optical scalar vortex coronagraph is proposed
and characterized experimentally in the laboratory. Our setup employs a pair of computer generated phase
gratings (one of them containing a singularity) to control the chromatic dispersion of phase plates and achieves a
constant peak-to-peak attenuation below 1:1000 over a bandwidth of 120 nm centered at 700 nm. An inner working
angle of λ/D is demonstrated along with a raw contrast of 11.5magnitudes at 2λ/D. A more compact setup
achieves a peak-to-peak attenuation below 1:1000 over a bandwidth of 60 nm with the other results remaining