Context. The study of Earth-mass extrasolar planets via the radial-velocity technique and the measurement of the potential cosmological variability of fundamental constants call for very-high-precision spectroscopy at the level of δλ/λ < 10-9. Only an accurate wavelength calibration of the spectrograph can guarantee that the aimed precision is achieved over a multi-exposure and multi-epoch data set. Wavelength accuracy is obtained by providing two fundamental ingredients: 1) an absolute and information-rich wavelength source and 2) the ability of the spectrograph and its data reduction of transferring the reference scale (wavelengths) to a measurement scale (detector pixels) in a repeatable manner.
Aims: The goal of this work is to improve the wavelength calibration accuracy of the HARPS spectrograph by combining the absolute spectral reference provided by the emission lines of a thorium-argon hollow-cathode lamp (HCL) with the spectrally rich and precise spectral information of a Fabry-Pérot-based calibration source.
Methods: On the basis of calibration frames acquired each night since the Fabry-Pérot etalon was installed on HARPS in 2011, we constructed a combined wavelength solution that fits simultaneously the thorium emission lines and the Fabry-Pérot lines. The combined fit was anchored to the absolute thorium wavelengths, which provide the "zero-point" of the spectrograph, while the Fabry-Pérot lines were used to improve the (spectrally) local precision. The obtained wavelength solution was verified for auto-consistency and tested against a solution obtained using the HARPS laser-frequency comb (LFC).
Results: The combined thorium+Fabry-Pérot wavelength solution shows significantly better performances compared to the thorium-only calibration. In both cases, the residuals of the LFC line positions to the fitted wavelength solution follow a Gaussian distribution with an rms value of about 14 m s-1 for the combined solution, and twice as large for the thorium-only solution (29 m s-1). Given these positive results, we have applied the new calibrations to scientific frames and tested the radial-velocity residual on three well-known stars: HD 10700, HD 20794, and HD 69830. In all three cases the radial-velocity (RV) scatter could be reduced compared to the measurements using the previous calibration.
Conclusions: The richness of the Fabry-Pérot spectrum helps to improve the wavelength calibration using thorium-argon lamps or extending the wavelength domain of LFCs with limited operational range. The presented techniques will therefore be used in the new HARPS and HARPS-N pipeline, and will be exported to the ESPRESSO spectrograph.
Astronomy and Astrophysics
- Pub Date:
- April 2019
- instrumentation: spectrographs;
- techniques: radial velocities;
- planets and satellites: detection;
- planets and satellites: general;
- methods: data analysis;
- Astrophysics - Instrumentation and Methods for Astrophysics
- 15 pages, 8 figures