THE NICMOS POLARIMETRIC CALIBRATION1

An accurate knowledge of the absolute calibration of the polarizing elements in the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) becomes especially important when conducting studies that require measuring degrees of polarization of close to 1% in the near-infrared. We present a comprehensive study of all previously observed polarimetric standards using the NIC2 camera on NICMOS. Considering both pre- and post-NICMOS Cooling System observations, we find variations in the polarimetry consistent with the effects of subpixel misalignments and the point-spread function. We also measure nonzero results from unpolarized standards, indicating an instrumental polarization of p approximate to 1.2%, theta approximate to 88 degrees. The lack of polarized and unpolarized standard stars with which to perform a comprehensive calibration study means that we cannot be confident that the current calibration will be effective for a number of recent large NICMOS GO programs. Further observations of polarimetric standards are needed in order to fully characterize the behavior of NICMOS at around p = 1%.