  The ability of NICMOS to perform high accuracy polarimetry is currently
hampered by an uncalibrated residual instrumental polarization at a level of
1.2-1.5%. To better quantify and characterize this residual we obtained
observations of three polarimetric standard stars at three separate space-craft
roll angles. Combined with archival data, these observations were used to
characterize the residual instrumental polarization to enable NICMOS to reach
its full polarimetric potential. Using these data, we calculate values of the
parallel transmission coefficients that reproduce the ground-based results for
the polarimetric standards. The uncertainties associated with the parallel
transmission coefficients, a result of the photometric repeatability of the
observations, dominate the accuracy of p and theta. However, the new
coefficients now enable imaging polarimetry of targets with p~1.0% at an
accuracy of +/-0.6% and +/-15 degrees.

Astrophysics