New wavelength calibration for echelle spectrographs using Fabry-Pérot etalons

Abstract

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 construct a combined wavelength solution which fits simultaneously the thorium emission lines and the Fabry-Pérot lines. The combined fit is anchored to the absolute thorium wavelengths, which provide the "zero-point" of the spectrograph, while the Fabry-Pérot lines are used to improve the (spectrally) local precision. The obtained wavelength solution is 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 thoriumonly 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 RV scatter could be reduced compared to the measurements using the previous calibration.Conclusions. The richness of the Fabry-Pérot spectrum helps improving 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.

Testing the Spectroscopic Extraction of Suppression of Convective Blueshift

“…When the linear equations defining RV 0 and RV 1 in terms of RV sppl and RV conv are inverted in the presence of noise introduced by this external variability, the RMS of noise in RV sppl is magnified to twice as large as the original RMS of noise in RV 0 (M17). Potential contributions from instrumental systematics due to wavelength calibration (Cosentino et al 2014;Dumusque 2018;Cersullo et al 2019;Coffinet et al 2019), or daily calibration sequences (Collier Cameron et al 2019), may also contribute significantly to this non-RV conv RV variability. This sensitivity to RV contributions other than RV conv motivates future consideration of different solar activity processes, especially those operating on different timescales such as magnetoconvection (Palle et al 1995;Del Moro 2004;Meunier 2018).…”

Measuring precise radial velocities on individual spectral lines

“…To mitigate this observed night-to-night RV offsets induced by the non-stability of the wavelength solutions, a possible solution is to use in combinaison to the Th-Ar spectrum, the much denser spectrum of the FP étalon to stabilise the wavelength solution on the edges of the HARPS CCD (Bauer et al 2015). This work will be soon implemented in the HARPS DRS (Cersullo et al 2018) and will be available to the community. Performing a wavelength solution every night simplify a lot the data analysis as this fix the zero velocity point of the instrument every night, and it is therefore not necessary to estimate the drift of the instrument from night-tonight.…”

“…be soon implemented in the HARPS DRS (Cersullo et al 2018) and will be available to the community. Performing a wavelength solution every night enables to fix the zero velocity point of the instrument, which simplify greatly the data reduction as it is not necessary to estimate the drift of the instrument from night-tonight.…”

The ACCELERATION programme: I. Cosmology with the redshift drift

“…The current gold standard in optical wavelength calibration is at the level of ∼ 0.5 m s −1 (e.g. Dumusque, et al 2012;Cersullo, et al 2019; using laser frequency comb technology (Murphy, et al 2007;Wilken, et al 2012). In the immediate future, the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) hopes to achieve a wavelength precision and stability of ∼ 10 cm s −1 , also using laser frequency combs.…”