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.