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LARS is an Absolute Reference Spectrograph designed for ultra-precise solarobservations. The high-resolution echelle spectrograph of the Vacuum TowerTelescope is supported by a state-of-the-art laser frequency comb to calibratethe solar spectrum on an absolute wavelength scale. In this article, wedescribe the scientific instrument and focus on the upgrades in the last twoyears to turn the prototype into a turn-key system. The pursued goal was toimprove the short-term and long-term stability of the systems, and enable auser-friendly and more versatile operation of the instrument. The first upgradeinvolved the modernization of the frequency comb. The Fabry-Perot cavities wereadjusted to filter to a repetition frequency of 8GHz. A technologically maturedphotonic crystal fiber was implemented for spectral broadening. The second,quite recent upgrade was performed on the optics feeding the sunlight into asingle-mode fiber connected to the spectrograph. A motorized translation stagewas deployed to allow the automated selection of three different fields-of-viewwith diameters of 1", 3", and 10" for the analysis of the solar spectrum. Thesuccessful upgrades allow for long-term observations of up to several hours perday with a stable spectral accuracy of 1 m/s limited by the spectrograph.Stable, user-friendly operation of the instrument is supported. The selectionof the pre-aligned fiber to change the field of view can now be done withinseconds. LARS offers the possibility to observe absolute wavelength positionsof spectral lines and Doppler velocities in the solar atmosphere. First resultsdemonstrate the capabilities of the instrument for solar science. The accuratemeasurement of the solar convection, p-modes, and atmospheric waves willenhance our knowledge of the solar atmosphere and its physical conditions toimprove current atmospheric models.

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