s the
                         target surface at 'spoint'.  'trgepc' is defined as
                         follows: letting 'lt' be the one-way light time
                         between the observer and the intercept point,
                         'trgepc' is the epoch et-lt, et+lt, or 'et' depending
                         on whether the requested aberration correction is,
                         respectively, for received radiation, transmitted
                         radiation, or omitted. 'lt' is