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.
Fourier transform spectroscopy (FTS) is a type of spectroscopy that can be used to analyze components in the sample. The basic setup that is commonly used in this technique is “Michelson interferometer”. The interference signal obtained from interferometer can be Fourier transformed into the spectral pattern of the illuminating light source. To experimentally study the concept of the Fourier transform spectroscopy, the project started by setup the Michelson interferometer in the laboratory. The implemented system used a broadband light source in near infrared region (0.81-0.89 μm) and controlled the movable mirror by using computer controlled motorized translation stage. In the early study, there is no sample the interference path. Therefore, the theoretical spectral results after the Fourier transformation of the captured interferogram must be the spectral shape of the light source. One main challenge of the FTS is to retrieve the correct phase information of the inferferogram that relates with the correct spectral shape of the light source. One main source of the phase distortion in FTS that we observed from our system is the non-linear movement of the movable reference mirror of the Michelson interferometer. Therefore, to improve the result, we coupled a monochromatic light source to the implemented interferometer. We simultaneously measured the interferograms of the monochromatic and broadband light sources. The interferogram of the monochromatic light source was used to correct the phase of the interferogram of the broadband light source. The result shows significant improvement in the computed spectral shape.
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.
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