A new instrument for high-precision spectroscopy with extremely accurate wavelength calibration

The new prototype instrument LARS (Lars is an Absolute Reference Spectrograph) at the German Vacuum Tower Telescope, Tenerife, provides unique possibilities for precision spectroscopy of the Sun and of light sources in the lab. Spectral resolution, achievable signal to noise ratio (SNR) and straylight properties are similar to the Fourier-transform spectrometer (FTS) of the Kitt Peak McMath facility. A laser frequency comb enables an absolute wavelength calibration that is accurate at some parts in 10⁹ and provides the spectral point spread function for each measurement as a by-product.
We present the first results from observing runs carried out in the early science phase of the instrument. A set of high-fidelity centre to limb observations of selected photospheric FeI lines was obtained. Absolute convective shifts were determined with respect to highly accurate laboratory measurements carried out with the same instrument. The statistics of many repeated observations of the quiet Sun at disc centre allow us to quantify the variability one has to expect in line position as well as in line shapes. We demonstrate the potential of such new, high-fidelity spectroscopic observations of the average solar photosphere for the verification and refinement of numerical simulations.