High-accuracy near-infrared imaging polarimetry with NICMOS

The findings of a nine-orbit calibration plan carried out during HST Cycle 15, to fully determine the NICMOS camera 2 (2.0 μm) 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 Polarimetrie calibration observations of three polarimetric standards at three spacecraft roll angles separated by ∼60°. 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 θ. However, the updated coefficients do allow imaging polarimetry of targets with p ≈ 1.0% at an accuracy of ±0.6% and ±15°. This work enables a new caliber of science with HST.