  The findings of a nine orbit calibration plan carried out during HST Cycle
15, to fully determine the NICMOS camera 2 (2.0 micron) polarization
calibration to high accuracy, are reported. Recently Ueta et al. and Batcheldor
et al. have suggested that NICMOS possesses a residual instrumental
polarization at a level of 1.2-1.5%. This would completely inhibit the data
reduction in a number of GO programs, and hamper the ability of the instrument
to perform high accuracy polarimetry. We obtained polarimetric calibration
observations of three polarimetric standards at three spacecraft roll angles
separated by ~60deg. Combined with archival data, these observations were used
to characterize the residual instrumental polarization in order for NICMOS to
reach its full potential of accurate imaging polarimetry at p~1%. Using these
data, we place an 0.6% upper limit on the instrumental polarization and
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, are seen to dominate the
accuracy of p and theta. However, the updated coefficients do allow imaging
polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15deg. This
work enables a new caliber of science with HST.
