High-resolution spectrometers have proven to be an important tool for astronomical observations and continue to have an ever expanding set of applications, such as high resolution IFS (Integral Field Spectroscopy). With this in mind, we present an alternative approach to the design and construction of Echelle type spectrometers. The usual approach is to drive high resolving power through the use of large gratings and long focal length collimators, leading to great production costs in the order of $1,000,000. Our compact, proof-of-concept prototype, via the coupling of adaptive optics, achieves comparable performance and resolution; with a theoretical resolving power R<80,000 in the Vis-NIR regime (500nm- 1μm) at a cost <$10,000. This is attained through the use of COTS (Commercially-Off-The-Shelf) and economically designed components. The overall device footprint is compact, measuring the size of a ‘shoe-box’, approximately 30cm×15cm. The spectrometer prototype is fibre-fed with a single 10μm fibre and follows a double-pass design - applying a custom designed, 108.24mm focal length, collimating and re-focussing lens. The system follows an Echelle type design with high resolution achieved through the use of a compact R4 diffraction grating and a prism as the cross-disperser.
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 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.
We present an inexpensive (<US$500) and easily replicable integral field unit for use on small aperture telescopes.
Based on a commercial small spectrograph (SBIG Self-Guiding Spectrograph) and a 37 optical fibre bundle integral field
unit with each fibre having 50μm cores and a pitch of 125μm. It has an overall field-of-view of 40 arc seconds
(2.6arcsec/core), a resolution of 9Å from 3995Å to 7170Å and an average system efficiency of 9%, yielding a signal-tonoise
ratio of 10 for a 20min exposure of a 13mag/arcsec2 source. Still in commissioning, we present first light
observations of Vega and M57.
We present a conceptual design for a Precision Radial Velocity Spectrograph (PRVS) for the Gemini telescope. PRVS is
a fibre fed high resolving power (R~70,000 at 2.5 pixel sampling) cryogenic echelle spectrograph operating in the near
infrared (0.95 - 1.8 microns) and is designed to provide 1 m/s radial velocity measurements. We identify the various
error sources to overcome in order to the required stability. We have constructed models simulating likely candidates
and demonstrated the ability to recover exoplanetary RV signals in the infrared. PRVS should achieve a total RV error of
around 1 m/s on a typical M6V star. We use these results as an input to a simulated 5-year survey of nearby M stars.
Based on a scaling of optical results, such a survey has the sensitivity to detect several terrestrial mass planets in the
habitable zone around nearby stars. PRVS will thus test theoretical planet formation models, which predict an abundance
of terrestrial-mass planets around low-mass stars.We have conducted limited experiments with a brass-board instrument
on the Sun in the infrared to explore real-world issues achieving better than 10 m/s precision in single 10 s exposures and
better than 5 m/s when integrated across a minute of observing.