... The technique is based on the iterative subtraction of continuum estimates and eigenvector sky models derived from singular value decompositions of sky spectra and sky spectra residuals. ... The spectra may also be redshifted to a common radial velocity, though this feature isn't used for sky subtraction. svdvec is a Fortran program, called by IRAF as a foreign task, which decomposes an array of spectra, such as that created by svdprep , into eigenvectors using Singular Value Decomposition. ...