Heliocentric radial velocity (cz) from optical measurements
The vopt parameter is computed from the optical
measurements in the
compilation of redshifts
by rejecting outliers and measurements marked for their
bad quality and computing a weighted average of the remaining
measurements.
The "radio" measurements are homogenized and combined separately to
produce another parameter, vrad, and
both vopt and vrad are combined to produce
v.
Details of the computation
The optical measurements are first selected and converted to a uniform
system as: vop = cz + zeropoint(iref), where cz is the compiled value
and zeropoint(iref) a zero-point correction for the reference iref.
The errors are also rescaled as: e_vop = e_cz * error_rescale(iref)
(if the precision is not given in the literature source, or if it was not entered in the database, an error of 60 km/s is assumed and rescaled according to the formula above).
Finally, the individual corrected measurements are combined. The error-weighted average is computed and the outliers are rejected.
The homogenization parameters zeropoint and error_rescale
were determined from an intercomparisons of the different sources of redshifts and
are read from the table vop_rsc.