NICMOS Polarimetry Update

Note that the coefficients presented below supersede all previous values!

Dean C. Hines
NICMOS Project
Steward Observatory
The University of Arizona
Tucson, AZ 85721
dhines@as.arizona.edu

NICMOS contains optics which enable imaging polarimetry at 1 & 2 microns with unprecedented detail. The preflight Thermal Vacuum tests revealed that each polarizer has a unique polarizing efficiency, and that the position angle offsets differ from the nominal positions of 0, 120 & 240. Therefore, to properly reduce polarimetry data obtained with NICMOS, a new algorithm different from that needed for an ideal polarizer was developed. Combined with calibration measurements of polarized and unpolarized stars, this algorithm allows accurate imaging polarimetry to about +/- 0.5% (in percentage polarization) over the entire field of view in both cameras.

The HSL Algorithm for Reducing NICMOS Polarimetry

The ``standard theory'' polarimetry reduction algorithm outlined in the original NICMOS Manual assumes that the polarizers have uniform and perfect (100%) polarizing efficiencies, and that the position angles of the primary axis of the polarizers are offset by exactly 120. The thermal vacuum tests showed that the NICMOS polarizers are not ideal, so a more complex technique is required. The new algorithm developed by Hines, Schmidt & Lytle (1997; hereafter HSL) is presented below.

The characteristics of the polarizers and the coefficients for the HSL algorithm are presented below. These were determined during Preflight Thermal Vacuum tests and from on-orbit measurements of polarized and unpolarized stars.

Note that the coefficients presented herein supersede all previous values!

Note: Ghost images are definitely present in POL0S and POL240S in exposures of very bright targets. No such ghosting is apparent in the NIC2 polarizers.

These ghosts will typically be seen as regions of 100% polarization (seen as white blobs in the lower right panel), since they appear in different positions in the POL0S and POL240S. There are no obvious ghosts in POL120S. Note that the object was very bright with J=7.32

References:

Hines, D.C., 1998: Imaging Polarimetry with NICMOS (Entire Report in PostScript)
[Contains updated coefficients as listed above].

Hines, D.C., Schmidt, G.D., & Lytle, D. 1997, The Polarimetric Capabilities of NICMOS, in The 1997 HST Calibration Workshop with a New Generation of Instruments, ed. Casertano et al. (Contains old coefficients)

NICMOS Instrument Manual (Contains standard theory algorithm)