  NICMOS cameras 1 and 2 each carry a set of three polarizing elements to
provide high sensitivity observations of linearly polarized light. The
polarizers are bandpass limited and provide diffraction-limited imaging in
camera 1 at 0.8 - 1.3um, and in camera 2 at 1.9-2.1um. The NICMOS design
specified the intra-camera primary axis angles of the polarizers to be
differentially offset by 120 degree, and with identical polarizing efficiency
and transmittance. While this ideal concept was not strictly achieved, accurate
polarimetry in both cameras, over their full (11" and ~19.2" square) fields of
view was enabled through ground and on-orbit calibration of the as-built and
HST-integrated systems. The Cycle 7 & 7N calibration program enabled and
demonstrated excellent imaging polarimetric performance with uncertainties in
measured polarization fractions <=1%. After the installation of the NICMOS
Cooling System (NCS), the polarimetric calibration was re-established in Cycle
11, resulting in systemic performance comparable to (or better than) Cycle 7 &
7N. The NCS era NICMOS performance inspired the development of an earlier
conceived, but non-implemented, observing mode combining high contrast
coronagraphic imaging and polarimetry in camera 2. We successfully executed a
program to calibrate and commission the "Coronagraphic Polarimetry'' mode in
NICMOS in Cycle 13, and the mode was made available for GO use in Cycle 14. We
discuss the data reduction and calibration of direct and coronagraphic NICMOS
polarimetry. Importantly, NICMOS coronagraphic polarimetry provides unique
access to polarized light near bright targets over a range of spatial scales
intermediate between direct polarimetry and ground-based (coronagraphic)
polarimetry using adaptive optics.
