light time and stellar aberration corrections are
                         used, 'spoint' is selected such that, when 'spoint'
                         is corrected for light time and the vector from the
                         observer to the light-time corrected location of
                         'spoint' is corrected for stellar aberration, the
                         resulting vector is parallel to the ray defined by
                         the observer's location and 'dvec'.

                     