Abstract
Improved wavelength calibrators for high-resolution astrophysical spectrographs will be essential for precision radial velocity (RV) detection of Earth-like exoplanets and direct observation of cosmological deceleration. The astro-comb is a combination of an octave-spanning femtosecond laser frequency comb and a Fabry-Pérot cavity used to achieve calibrator line spacings that can be resolved by an astrophysical spectrograph. Systematic spectral shifts associated with the cavity can be 0.1-1 MHz, corresponding to RV errors of 10-100 cm/s, due to the dispersive properties of the cavity mirrors over broad spectral widths. Although these systematic shifts are very stable, their correction is crucial to high accuracy astrophysical spectroscopy. Here, we demonstrate an in-situ technique to determine the systematic shifts of astro-comb lines due to finite Fabry-Pérot cavity dispersion. The technique is practical for implementation at a telescope-based spectrograph to enable wavelength calibration accuracy better than 10 cm/s.
Highlights
High precision wavelength calibrators for astrophysical spectrographs will be key components of new precision radial velocity (RV) observations, including the search for Earth-like extrasolar planets [1] and direct observation of cosmological acceleration [2, 3]
In the results presented here, source comb modes neighboring the astro-comb line, with intensities after passing through the Fabry-Perot cavity (FPC) that differ by 0.1% of the main astro-comb peak, shift the measured line centroid by 1 MHz, which corresponds to an RV systematic error of 1 m/s
We demonstrate an in-situ technique to determine the systematic shifts of astrocomb lines due to FPC dispersion, which can be applied at a telescope-based spectrograph to enable wavelength calibration accuracy better than 10 cm/s
Summary
High precision wavelength calibrators for astrophysical spectrographs will be key components of new precision radial velocity (RV) observations, including the search for Earth-like extrasolar planets (exoplanets) [1] and direct observation of cosmological acceleration [2, 3]. In the results presented here, source comb modes neighboring the astro-comb line, with intensities after passing through the FPC that differ by 0.1% of the main astro-comb peak, shift the measured line centroid by 1 MHz, which corresponds to an RV systematic error of 1 m/s Such systematic RV shifts are inevitable over spectral bandwidths of 1000 Adue to the dispersive properties of the mirrors of the FPC. This imperfect suppression leads to systematic inaccuracies in astro-comb line centers at the 50 cm/s level, as observed on the TRES spectrograph across a 1000 Abandwidth The effects of these systematic shifts can be characterized by determining the FPC finesse (Fm) and the frequency difference between astro-comb lines and the FPC resonance over the full spectral width. The intensity of the astro-comb line varies with the change of the frequency of one FPC mode according to Eq (1), from which we can derive the finesse and the phase errors at all astro-comb line frequencies
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