  Past imaging observations of a post-AGB star, HD 161796, have probed
different parts of its circumstellar shell which correspond to different epochs
of the star's AGB mass loss history. While the overall structure of the shell
can be described by an axisymmetric model consisting of three layers of
characteristic structure, the mass distribution in the shell appears to rotate
its axis of symmetry in a continuous manner. It is thus interesting to observe
the halo region of the shell using from far-IR to sub-mm wavelengths and
increase the ``time resolution'' during this critical epoch of the star's mass
loss history.
