  The measurement of Doppler velocities in spectroscopic solar observations
requires a reference for the local frame of rest. The rotational and radial
velocities of the Earth and the rotation of the Sun introduce velocity offsets
in 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 from
solar observations for any heliocentric angle, calibrating the convective line
shift of spatially-averaged profiles on quiet sun from a 3D hydrodynamical
simulation. This method works whenever there is quiet sun in the field-of-view,
and it has the advantage of being relatively insensitive to uncertainties in
the atomic data. We carry out radiative transfer computations in LTE for
selected C I and Fe I lines, whereas the Ca II infrared lines are synthesized
in non-LTE. The degree of asymmetry and the line shifts, however, show a clear
dependence on the heliocentric angle and the properties of the lines. The
profiles at \mu = 1 are compared with observed profiles to prove their
reliability, and they are tested against errors induced by the LTE
calculations, inaccuracies in the atomic data and the 3D simulation.
Theoretical quiet-sun profiles of lines commonly used by solar observers are
provided to the community. Those can be used as absolute references for
line-of-sight velocities. The limb effect is produced by the projection of the
3D atmosphere along the line of sight. We estimate the precision of the
disk-center line shifts to be approximately 50 m s^-1, but the off-center
profiles remain to be tested against observations.
