High Accuracy Imaging Polarimetry with NICMOS

TL;DR

This paper improves NICMOS imaging polarimetry by calibrating residual instrumental polarization, enabling more accurate measurements of faint polarized targets.

Contribution

It provides a new calibration method for NICMOS that reduces instrumental polarization effects, enhancing polarimetric accuracy.

Findings

Residual instrumental polarization characterized at 1.2-1.5%

New calibration coefficients enable 1% polarization measurements with +/-0.6% accuracy

Achieved polarimetric angle accuracy of +/-15 degrees

Abstract

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.