  Granular convective motions reach into the lower solar atmosphere, typically
causing photospheric spectral lines to exhibit a differential line shift. This
Doppler shift to shorter wavelength is commonly known as convective blueshift.
We performed systematic observations of the quiet Sun with the Laser Absolute
Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. The solar
disk was scanned along the meridian and the equator, from the disk center
toward the limb. The solar spectrum around 6173{\AA} was calibrated with a
laser frequency comb on an absolute wavelength scale with an accuracy of a few
m/s. We applied a bisector analysis on the spectral lines to reveal the changes
of convective blueshift and line asymmetry at different heliocentric positions.
Being a signature for convective motions, the bisector curve of Fe i
6173.3{\AA} describes a C-shape at disk center. When approaching the solar
limb, the bisector transforms into a \-shape. The analysis of the time- and
bisector-averaged line shifts yields three distinct results. Firstly, the
center-to-limb variation of Doppler velocities measured with LARS reveals a
significant discrepancy (up to 200m/s) to the full-disk Dopplergrams of the
Helioseismic and Magnetic Imager (HMI). Secondly, we obtained a significant
decrease of convective blueshift toward the solar limb. Thirdly, the
line-of-sight effect of solar activity leads to a scatter of up to 100m/s at
intermediate heliocentric positions. The accurate observation of the absolute
convective blueshift with LARS allows the identification of systematic
discrepancy with Doppler velocities measured by HMI. The center-to-limb
variation of HMI suffers from an additional blueshift toward the limb that is
incompatible with our results. LARS measurements can be taken as reference for
the correction of systematic errors in the synoptic HMI Dopplergrams.
