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The Solar observatory at the Institute for Astrophysics and Geophysics Göttingen makes use of the ultra-high resolving power (R < 900, 000 at 600 nm) of a Fourier transform spectrograph (FTS) to obtain spectra of the resolved and integrated Sun. To improve the radial velocity (RV) stability of the FTS measurements we develop a new calibration unit based on a passively stabilized Fabry-Pérot Etalon (FP) (FSR= 3.6 GHz and F ≈ 7) for simultaneous calibration in the near-infrared. The FP is illuminated by two LEDs, covering the wavelength range from 800 to 1000 nm. To mitigate environmental effects, the FP is placed in a temperature and pressure controlled vessel. We explore the impact of the choice of input fiber as well as fiber coupler focal length on the calibration spectrum. In 150 laser frequency comb calibrated measurements over 8 hours we achieve an an RMS of the FP-RV of 0.58 m s−1 .
Characterization and calibration of a Fourier-transform spectrometer using a laser frequency comb
(2019)
We have used a laser frequency comb with a repetition frequency of 𝜈rep≈1 GHz to measure the drift and dispersion of a Fourier-transform spectrometer (FTS). We used the electronic measurements of 𝜈rep and 𝜈CEO to create a reference line list. We measured 28 interferograms and computed the phase and power spectra. The analysis of the interferograms and phase spectra allowed for compensation of several spectroscopic artifacts. In the computed power spectra, we detected ∼64.000 suitable lines in the near-infrared bandwidth Δ𝜈=308.79–374.74 THz. The residual dispersion of the FTS can be described by two factors, a linear dispersion and a constant offset. Both are highly correlated and need to be computed simultaneously. The factors were computed from the comparison of a reference with measured line lists. The linear dispersion factor is found to be varying on the order of 10−8 Hz/Hz, while the constant offset is of the order of 107 Hz. Using two factors for calibration, the difference between the reference and the measured line list can be removed completely with an uncertainty of ∼65 kHz corresponding to a precision of 0.5·10−9 Hz/Hz.
Fourier Transform Spectrographs (FTS) are versatile tools for measuring accurate, high resolution spectra. They are internally calibrated by a reference laser that runs in parallel to the science light. Therefore it is crucial to properly align these two beams with respect to each other. We show how this can be achieved by feeding a part of the reference light into the optical path of the science beam. For astronomical applications it’s often useful to use optical fibers. We present a coupling setup for our Bruker Optics IFS 125 FTS, consisting of (1) two hexagonal input fibers, (2) dichroic beam-combining for measuring two light sources simultaneously and (3) optimized optics to match the original Bruker design. The hexagonal shape of the fiber cores secures sufficient mode scrambling inside the fibers, resulting in constant beam parameters and a more homogeneous illumination of the entrance aperture of the FTS.