# New wavelength calibration of the HARPS spectrograph

## TL;DR

This paper presents an improved wavelength calibration method for the HARPS spectrograph, reducing systematic errors and enhancing its ability to detect low-amplitude exoplanet signals by correcting CCD stitching anomalies and using a new thorium line list.

## Contribution

It introduces a new calibration approach that accounts for CCD pixel anomalies and employs a higher-accuracy thorium line list, improving radial velocity precision for HARPS data.

## Key findings

- Reduced radial velocity dispersion in HARPS data.
- Attenuated one-year period signals in stellar RV measurements.
- Enhanced detection sensitivity for low-amplitude exoplanet signals.

## Abstract

(abridged) Even if the HARPS spectrograph has been operational for more than 15 years and it provides among the most precise Doppler measurements, improvements are still possible. One known problem, for instance, is the non-fully regular block-stitching of the CCDs, which introduces, in some cases, one-year period parasitic signals in the measured radial velocity. The aim is to improve the wavelength calibration of HARPS to push further its planet-detection capabilities. The properties of the CCD stitching-induced pixel-size anomalies are determined with LED flat-field frames, and then a physical, gap-corrected map of the CCDs is used for the fitting model of the spectral orders. We also use a new thorium line list, based on much higher-accuracy measurements than the one used up to now. We derive new wavelength solutions for the 15 years of HARPS data, both before and after the 2015 fibre upgrade. We demonstrate that we correct the gap anomalies by computing the wavelength solutions of laser frequency comb exposures, both with and without taking the gap correction into account. By comparing the rms of the most stable stars of the HARPS sample, we show that we globally decrease the radial velocity dispersion of the data, especially for the data acquired after the change of fibres. Finally, the comparative analysis of several individual systems shows that we manage to attenuate the periodogram power at one year in most cases. The analysis of the RVs derived from individual stellar lines also shows that we correct the stitching-induced RV variation. This improved calibration of the HARPS spectrograph allows to go deeper in the search for low-amplitude radial-velocity signals. It will be further improved by combining the thorium calibration spectra with laser frequency comb and Fabry-Perot calibration spectra, and not only for HARPS but notably also for HARPS-N and ESPRESSO.