The IAG solar flux atlas: Accurate wavelengths and absolute convective blueshift in standard solar spectra

We present a new solar flux atlas with the aim to understand wavelength precision and accuracy in solar benchmark data. The atlas covers the wavelength range 405--2300 nm and was observed at the Institut f\"ur Astrophysik, G\"ottingen (IAG) with a Fourier Transform Spectrograph. In contrast to other FTS atlases, the entire visible wavelength range was observed simultaneously using only one spectrograph setting. We compare the wavelength solution of the new atlas to the Kitt Peak solar flux atlases and to the HARPS frequency-comb calibrated solar atlas. Comparison reveals systematics in the two Kitt Peak FTS atlases resulting from their wavelength scale construction, and shows consistency between the IAG and the HARPS atlas. We conclude that the IAG atlas is precise and accurate on the order of $\pm 10$ m s$^{-1}$ in the wavelength range 405--1065 nm while the Kitt Peak atlases show deviations as large as several ten to 100 m s$^{-1}$. We determine absolute convective blueshift across the spectrum from the IAG atlas and report slight differences relative to results from the Kitt Peak atlas that we attribute to the differences between wavelength scales. We conclude that benchmark solar data with accurate wavelength solution are crucial to better understand the effect of convection on stellar RV measurements, which is one of the main limitations of Doppler spectroscopy at m s$^{-1}$ precision.