Analysis of Polarized Light with NICMOS

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard the Hubble Space Telescope (HST ) contains optics in Cameras 1 and 2 (NIC1 and NIC2) that enable imaging polarimetry at D1 and 2 km, respectively, with unprecedented spatial detail. PreÑight thermal vacuum tests revealed that the three polarizers in each camera have unique polarizing efficiencies and that the rotational orientations within each set depart from the nominal 120¡ intervals. To properly interpret polarimetric data obtained with these optical systems, a reduction algorithm was developed which di†ers from the standard approach used for ideal polarizers. We discuss this technique, its successful application to selected NICMOS observations, and the uncertainty distributions associated with threeand four-polarizer schemes. We also present information on the NICMOS instrumental polarization, the nature and origin of image artifacts in the polarized images, and suggest observational techniques for obtaining high-quality polarimetry with the instrument. With the large number of observations already taken and the exciting prospects for an extended mission with a retroÐt closed-cycle turbine cooler, NICMOS will continue to produce high-resolution imaging polarimetry in the near-infrared for many years to come.