Abstract
Over the past decades hollow-cathode lamps have been calibration standards for spectroscopic measurements. Advancing to cm/s radial velocity precisions with the next generation of instruments requires more suitable calibration sources with more lines and less dynamic range problems. Fabry-Perot interferometers provide a regular and dense grid of lines and homogeneous amplitudes making them good candidates for next generation calibrators. We investigate the usefulness of Fabry-Perot etalons in wavelength calibration, present an algorithm to incorporate the etalon spectrum in the wavelength solution and examine potential problems. The quasi periodic pattern of Fabry-Perot lines is used along with a hollow-cathode lamp to anchor the numerous spectral features on an absolute scale. We test our method with the HARPS spectrograph and compare our wavelength solution to the one derived from a laser frequency comb. The combined hollow-cathode lamp/etalon calibration overcomes large distortion (50 m/s) in the wavelength solution of the HARPS data reduction software. Direct comparison to the laser frequency comb bears differences of only maximum 10 m/s. Combining hollow-cathode lamps with Fabry-Perot Interferometers can lead to substantial improvements in the wavelength calibration of echelle spectrographs. Etalons can provide economical alternatives to the laser frequency comb, especially for smaller projects.
Highlights
Accurate wavelength calibration is a cornerstone for any measurement with high-resolution spectrographs
We investigate the usefulness of Fabry-Pérot etalons in wavelength calibration, present an algorithm to incorporate the etalon spectrum in the wavelength solution, and examine potential problems
As many astronomical research fields benefit from high-precision radial velocity (RV) measurements, the generation of spectrographs aims for cm/s precisions (D’Odorico & CODEX/ESPRESSO Team 2007)
Summary
Accurate wavelength calibration is a cornerstone for any measurement with high-resolution spectrographs. The advantage of the FPI against HCLs is its ability to produce a dense grid of lines with almost uniform intensity over the entire spectral range They can be manufactured to match the specifications of most spectrographs as the free spectral range is a function of the cavity width. This solves the problem of empty spectral regions and saturated lines, thereby increasing the robustness of the wavelength solution.
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