         frame's center is located elsewhere, then letting
               'ltcent' be 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 m