The Fourier Transform Spectrograph (FTS) stands as a powerful tool for astronomers in characterizing the composition of celestial bodies through their emitted light. In this study, we introduce the development and initial performance evaluation of a fiber-fed FTS, specifically tailored for solar observations within the 600-1000 nm wavelength range. To improve measurement precision, we integrated a stabilized He-Ne laser as a metrology wavelength source. This setup generates a monochromatic interferogram in parallel with the scientific interferogram, allowing for adaptive correction of the instrument's non-linear scan characteristics that affect the phase information of the scientific interferogram. For wavelength calibration, we employed well-defined oxygen (O2) lines as a reference. The comparison of the solar spectrum measured with our system against a simulated model showed good agreement affirming the system's efficacy. Additionally, we discuss the wavelength calibration using O2 lines in the telluric region, offering insights into the system's repeatability. The analysis of the Fe-I absorption line within these lines further enabled us to determine the Sun's rotational velocity.
The Fourier transform spectrograph (FTS) is one important tool that has been used to analyze and characterize the radiated energy distribution of the stellar objects through an atmosphere. Most of the current spectrographs were installed at the telescope focal plane of the telescope, which required the space and complexity of alignments. The implementation of a fiber to feed the light from the telescope has been implemented to overcome those limitations. However, a small flux due to the single point field of view becomes a main challenge of this system. In this work, we report the development of the laboratory prototype of a fiber-fed FTS by using off-the-shelf components specifically designed for the Thai National Telescope (TNT). The method used to process the data relies on the cubic spline interpolation for resampling of both scientific and metrology interferograms and producing the spectrum from the raw measurements. The current maximum optical path difference is about 30 mm with an achievable spectral resolving power higher than 19,000 based on the instrument line shape of the system. The results obtained by concentrating the Sun light with the signal-to-noise-ratio (SNR) greater than 20 are presented. We also present the preliminary results of the low flux detection from a dim halogentungsten source comparable to the magnitude of a bright star in the order of a few nano-watts. The implementation of a phase-lock amplifier has been investigated to detect the signal and improve the signal-to-noise-ratio of the spectrum.
We present the design and performance verification of a fiber-fed Fourier transform spectrograph (FTS) for spectroscopy in the optical band with the ability to reach a maximum optical path difference of 15 mm and allowing for an adjustable spectral resolution (λ / Δλ) between 1 and 15,000. The designed FTS system was successfully constructed using only off-the-shelf optical components. The technique for correction of the phase distortion in the FTS using a metrology interferogram and cubic spline interpolation was investigated and discussed. The contrast performance and the instrument line shape of the FTS were measured and analyzed. To further verify the performance of the developed system, the absorption spectrum of the sunlight was measured and compared with a synthetic model with identified telluric and absorption lines. The result shows that the developed FTS can detect the absorption lines with a spectral resolution close to 15,000.
The Center for Optics and Photonics of the National Astronomical Research Institute of Thailand, together with the Institut d’Optique Graduate School and the Centre de Recherche Astrohpysique de Lyon (CRAL), is currently developing the Evanescent Wave Coronagraph (EvWaCo). The coronagraph relies on the tunneling effect to produce a fully achromatic focal plane mask (FPM) with an adjustable size. The full instrument comprises a coronagraph and adaptive optics system that will be mounted on the Thai National Telescope and is specified to reach a raw contrast of 10−4 at an inner working angle of 3 Airy radii. The coronagraph will be used to perform high contrast observations of stellar systems during on-sky observations over the spectral domain [600 nm, 900 nm]. In this paper, we present the opto-mechanical design of the EvWaCo prototype and the performance measured in laboratory conditions. We also discuss the potential applications for space-based observations and the development plan under this project in the next five years.
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