  LARS is an Absolute Reference Spectrograph designed for ultra-precise solar
observations. The high-resolution echelle spectrograph of the Vacuum Tower
Telescope is supported by a state-of-the-art 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 in the last two
years to turn the prototype into a turn-key system. The pursued goal was to
improve the short-term and long-term stability of the systems, and enable a
user-friendly and more versatile operation of the instrument. The first upgrade
involved the modernization of the frequency comb. The Fabry-Perot cavities were
adjusted to filter to a repetition frequency of 8GHz. A technologically matured
photonic crystal fiber was implemented for spectral broadening. The second,
quite recent upgrade was performed on the optics feeding 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. The
successful upgrades allow for long-term observations of up to several hours per
day with a stable spectral accuracy of 1 m/s limited by the spectrograph.
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. 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.
