  Using NICMOS on HST, we have performed imaging polarimetry of proto-planetary
nebulae. Our objective is to study the structure of optically thin
circumstellar shells of post-asymptotic giant branch stars by separating
dust-scattered, linearly polarized star light from unpolarized direct star
light. This unique technique allows us to probe faint reflection nebulae around
the bright central star, which can be buried under the point-spread-function of
the central star in conventional imaging. Our observations and archival search
have yielded polarimetric images for five sources: IRAS 07134+1005 (HD 56126),
IRAS 06530-0213, IRAS 04296+3429, IRAS (Z)02229+6208, and IRAS 16594-4656.
These images have revealed the circumstellar dust distribution in an
unprecedented detail via polarized intensity maps, providing a basis to
understand the 3-D structure of these dust shells. We have observationally
confirmed the presence of the inner cavity caused by the cessation of AGB mass
loss and the internal shell structures which is strongly tied to the progenitor
star's mass loss history on the AGB. We have also found that equatorial
enhancement in these circumstellar shells comes with various degrees of
contrast, suggesting a range of optical depths in these optically thin shells.
Our data support the interpretation that the dichotomy of PPN morphologies is
due primarily to differences in optical depth and secondary to the inclination
effect. The polarization maps reveal a range of inclination angles for these
optically thin reflection nebulae, dispelling the notion that elliptical
nebulae are pole-on bipolar nebulae.
