with the frame, the orientation of the frame is
            evaluated at et-LTCENT, et+LTCENT, or et depending
            on whether the requested aberration correction is,
            respectively, for received radiation, transmitted
            radiation, or is omitted. LTCENT is computed using
            the method indicated by 'abcorr'.

      lt    the value(s) of the one-way light time between the
            observer and target in seconds. If the target position
            is corrected for aberrat