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The measurement of Doppler velocities in spectroscopic solar observationsrequires a reference for the local frame of rest. The rotational and radialvelocities of the Earth and the rotation of the Sun introduce velocity offsetsin the observations. Normally, good references for velocities are missing (e.g.telluric lines), especially in filter-based spectropolarimetric observations.We determine an absolute reference for line-of-sight velocities measured fromsolar observations for any heliocentric angle, calibrating the convective lineshift of spatially-averaged profiles on quiet sun from a 3D hydrodynamicalsimulation. This method works whenever there is quiet sun in the field-of-view,and it has the advantage of being relatively insensitive to uncertainties inthe atomic data. We carry out radiative transfer computations in LTE forselected C I and Fe I lines, whereas the Ca II infrared lines are synthesizedin non-LTE. The degree of asymmetry and the line shifts, however, show a cleardependence on the heliocentric angle and the properties of the lines. Theprofiles at \mu = 1 are compared with observed profiles to prove theirreliability, and they are tested against errors induced by the LTEcalculations, inaccuracies in the atomic data and the 3D simulation.Theoretical quiet-sun profiles of lines commonly used by solar observers areprovided to the community. Those can be used as absolute references forline-of-sight velocities. The limb effect is produced by the projection of the3D atmosphere along the line of sight. We estimate the precision of thedisk-center line shifts to be approximately 50 m s^-1, but the off-centerprofiles remain to be tested against observations.

Solar velocity references from 3D HD photospheric models