# 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`.