 header of the Mice routine <a href="cspice_getfov.html">cspice_getfov</a> for a
               description of the required parameters associated with
               an instrument.

      raydir   the ray pointing toward a target. The ray emanates from
               the location of the ephemeris object designated by the
               input argument 'obsrvr' and is expressed relative to the
               reference frame designated by 'rframe' (see descriptions
               below).

                [3,1]