The NICMOS Polarimetric Calibration11Based on observations made with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS 5‐26555.

Abstract

ABSTRACTAn 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 ≈ 1.2%, θ ≈ 88°. 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%.