LARS: An Absolute Reference Spectrograph for solar observations Upgrade from a prototype to a turn-key system

Context. We designed a Laser-based Absolute Reference Spectrograph (LARS) for ultra-precise solar observations. The high-resolution echelle spectrograph of the Vacuum Tower Telescope is supported by a laser frequency comb to calibrate the solar spectrum on an absolute wavelength scale. In this article, we describe the scientific instrument and focus on the upgrades carried out in the last two years to turn the prototype into a turn-key system. Aims. The goal was to improve the short-term and long-term stability of the systems, and to enable a user-friendly and more versatile operation of the instrument. Methods. The first upgrade involved the modernization of the frequency comb. The laser system generating the comb spectrum was renewed. The Fabry-Perot cavities were adjusted to filter to a repetition frequency of 8 GHz. A technologically matured photonic crystal fiber was implemented for spectral broadening which simplified and stabilized the setup. The new control software facilitates an automated operation of the frequency comb. The second, quite recent upgrade was performed on the optics which feed the sunlight into a single-mode fiber connected to the spectrograph. A motorized translation stage was deployed to allow the automated selection of three different fields of view with diameters of 1 '', 3 '', and 10 '' for the analysis of the solar spectrum. Results. The successful upgrades allow for long-term observations of up to several hours per day with a stable spectral accuracy of 1ms(-1) limited by the spectrograph. The instrument covers a wavelength range between 480 nm and 700 nm in the visible. Stable, user-friendly operation of the instrument is supported. The selection of the pre-aligned fiber to change the field of view can now be done within seconds. Conclusions. LARS offers the possibility to observe absolute wavelength positions of spectral lines and Doppler velocities in the solar atmosphere. First results demonstrate the capabilities of the instrument for solar science. The accurate measurement of the solar convection, p-modes, and atmospheric waves will enhance our knowledge of the solar atmosphere and its physical conditions to improve current atmospheric models.