ner_tags: O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Instrument O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Instrument O O B-Instrument I-Instrument I-Instrument I-Instrument I-Instrument I-Instrument B-Instrument B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O B-Instrument O O O O O O B-Formula I-Formula I-Formula I-Formula I-Formula I-Formula B-Instrument I-Instrument I-Instrument I-Instrument I-Instrument O O O B-Telescope I-Telescope O B-Observatory B-Observatory I-Observatory I-Observatory O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O
tokens: instrument (Pfeiffer, Frank Baumuller 1992 ). The experiment had two goals. The first one was to understand whether the precision of a single astrocomb is confirmed by an independent system, and the second one was to assess zero-point offsets in the wavelength calibration introduced by switching between the two astrocombs. Probst et al. ( 2020 ) analyses the same data set, but with a focus on describing the experimental set-up, astrocomb hardware and its optimization during the campaign before the astrocomb’s deployment in 2015 May. This paper, on the other hand, focuses on data analysis techniques and advanced algorithms that will provide wavelength calibration precision required by the ELT projects discussed above. The paper is divided as follows. In Section 2 , we briefly describe the experimental set-up – the spectrograph and the two astrocombs. Section 3 describes the data set and general properties of the spectra. Section 4 gives details on our automatic algorithm to detect astrocomb lines and obtaining their centres and wavelengths. Wavelength calibration is discussed in Section 5 , where we describe how we deal with instrumental effects that impact significantly on wavelength calibration precision and accuracy, e.g. defects associated with the CCD manufacturing process. In the same section, we describe our findings on the optimal wavelength calibration model for HARPS spectra. We present our findings on precision and consistency in Section 6 , where we also consider and model the contribution of flux dependent velocity shifts. Finally, our results are presented in Section 7 and discussed in Section 8 . 2 EXPERIMENTAL SET-UP 2.1 The HARPS instrument The High Accuracy Radial velocity Planet Searcher (HARPS; Mayor et al. 2003 ) was built for extreme stability and precision and is one of the most stable astronomical spectrographs in existence. HARPS is a fibre fed, high-resolution ( R = λ/Δλ = 115 000), R4 grism cross-dispersed echelle spectrograph installed on the 3.6m telescope at ESO’s La Silla Observatory and the first instrument to be equipped with an astrocomb for regular operations (Probst et al. 2016 ). The light of the two input fibres is dispersed into 72 echelle orders on the detector, simultaneously covering the wavelength range between 378 and 691 nm. The spectrograph is enclosed in a thermal and pressure controlled vacuum vessel, with long-term temperature variations at the 0.01 K level and operating pressure below . Several thermal and mechanical effects can slightly shift the positions of the spectrum on the detector with time, an unavoidable effect that is eliminated through simultaneous referencing: drifts in the science fibre are tracked by simultaneously observing a spectrum rich in velocity information content (e.g. ThAr, an astrocomb, or a Fabry–Pérot etalon) in the secondary fibre (Baranne et al. 1996 ). Each fibre has a static double scrambler. A servo controller (‘secondary guiding’) ensures that the object image is