# On the optimum operating conditions of ThNe calibration lamps for measurements of radial velocity variations

## TL;DR

This study investigates optimal operating conditions for ThNe calibration lamps to enhance the precision of radial velocity measurements, balancing lamp lifetime and spectral line quality in high-resolution spectrographs.

## Contribution

It provides an analysis of how operating current and exposure time affect spectral line quality and calibration precision in ThNe lamps, offering guidelines for optimal usage.

## Key findings

- Higher current improves line strength and measurement precision.
- Optimal exposure time minimizes noise and saturation effects.
- ThNe lamps outperform ThAr lamps for precise radial velocity studies.

## Abstract

Previous studies have shown that ThNe lamps are very suitable for the precise measurement of wavelength variations. A critical issue for all hollow cathode lamps (HCL) is the current at which the lamp is operated - a low value has the advantage that the life time of the lamp is longer whereas the number of useful lines is lower. We investigate the number of suitable spectral lines depending on the current used and obtained spectra of a ThNe HCL coupled by a fibre to the Tautenburg Echelle spectrograph in a setting that is typical for many high-resolution spectrographs. Operating currents were chosen in the range specified by the manufacturer. We varied exposure time to identify the saturation level and effects of noise. A few thousands of Th lines were identified automatically in the wavelength range considered. We noticed a scatter of several hundred ms$^{-1}$ of Th lines when varying the settings but did not detect any systematic trends. The scatter in wavelength residuals of Th lines however indicates that a precise control of operating current can be necessary. An estimation of the calibration precision of individual lines indicates that a combination of strong Th lines allows one to reach a level of 1$\mathrm{ms}^{-1}$. Although a high operating current reduces the life-time of the lamp and accelerates its ageing it guarantees highest precision because of the numerous Th lines which become strong compared to the Ne lines. Then, noise, saturation, wavelength residuals of lines identified, and the pollution due to saturated Ne lines can be minimised by adjusting exposure time. Our results indicate that ThNe lamps can be better suited than ThAr lamps for all kinds of studies that involve the precise measurement of radial velocity variations, e.g. studies on late-type stars, brown dwarfs, objects that suffer from strong extinction, or high-redshift galaxies.