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Granular convective motions reach into the lower solar atmosphere, typicallycausing photospheric spectral lines to exhibit a differential line shift. ThisDoppler shift to shorter wavelength is commonly known as convective blueshift.We performed systematic observations of the quiet Sun with the Laser AbsoluteReference Spectrograph (LARS) at the German Vacuum Tower Telescope. The solardisk was scanned along the meridian and the equator, from the disk centertoward the limb. The solar spectrum around 6173{\AA} was calibrated with alaser frequency comb on an absolute wavelength scale with an accuracy of a fewm/s. We applied a bisector analysis on the spectral lines to reveal the changesof convective blueshift and line asymmetry at different heliocentric positions.Being a signature for convective motions, the bisector curve of Fe i6173.3{\AA} describes a C-shape at disk center. When approaching the solarlimb, the bisector transforms into a \-shape. The analysis of the time- andbisector-averaged line shifts yields three distinct results. Firstly, thecenter-to-limb variation of Doppler velocities measured with LARS reveals asignificant discrepancy (up to 200m/s) to the full-disk Dopplergrams of theHelioseismic and Magnetic Imager (HMI). Secondly, we obtained a significantdecrease of convective blueshift toward the solar limb. Thirdly, theline-of-sight effect of solar activity leads to a scatter of up to 100m/s atintermediate heliocentric positions. The accurate observation of the absoluteconvective blueshift with LARS allows the identification of systematicdiscrepancy with Doppler velocities measured by HMI. The center-to-limbvariation of HMI suffers from an additional blueshift toward the limb that isincompatible with our results. LARS measurements can be taken as reference forthe correction of systematic errors in the synoptic HMI Dopplergrams.

Convective blueshifts in the solar atmosphere II. High-accuracy observations of the Fe i 6173.3{\AA} line and deviations of full-disk Dopplergrams