 the one-way light time between the observer
               and the central body associated 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          is the one-way light time between the obs