IRAF/STECF imaging linear polarimetry simulation and reduction package ST-ECF

IMPOL - HST Imaging Polarimetry Reduction Package

Jeremy R. Walsh


Several HST instruments, both current and decommissioned, provide or have provided imaging linear polarimetry capabilities. The original Wide Field and Planetary Camera (WF/PC), the Faint Object Camera (FOC) and the Near-Infrared Camera and Multi-Object Spectrograph (NICMOS) all had polarizing filters and the Wide Field and Planetary Camera 2 (WFPC2) is actively used for imaging polarimetry. In the near future the Advanced Camera for Surveys (ACS) is due for integration into HST (Servicing Mission 3B) and will provide near-UV and visible imaging polarimetry facilities.

Imaging polarimetry data requires special reduction facilities beyond that of image alignment and image arithmetic. Errors should be propagated through the reduction since their magnitude affects the derived polarization. In addition there can be substantial instrumental effects such as non-perfect polarizers and reflections which require to be considered in combining the input images to accurately measure and map linear polarization and its position angle.

The STECF IRAF layered package impol provides five tasks specifically aimed at reduction of HST imaging polarimetry data for ACS, FOC, NICMOS and WFPC2. All the routines are written in FORTRAN77 using the F77VOS library for full access to FITS and compatability with IRAF cl. Reduction of HST polarimetry data and simulation of raw polarimetry images, together with polarization vector plotting, are covered. The reduction is largely driven by the contents of the header keywords. In addition, by providing a table of instrument parameters, data from any non-HST polarizing instrument can be handled by the impol routines.

Components of Package

For reduction of data from imaging linear polarization instruments other than from HST, the instrument name is `SPECIAL'.

See the demo directory to try out the package.

Examples of simulated and real data

  • Figure 1. Model polarization vector plot of centro-symmetric reflection nebula produced with polimodel and plotted with polimplot.

  • Figure 2. Model polarization vector plot of bipolar reflection nebula overlaid on the total signal (Stokes I) image. This was formed from an intensity image produced by artdata/mkobjects together with polimodel. The intensity image was displayed and the polarization vector map overplotted using polimplot.

  • Figure 3. Polarization vector map of the Seyfert galaxy NGC 1068 made from archival pre-COSTAR HST data taken with the Faint Object Camera. The filter was U band (F320W) and the exposure time in each of the three polarizers was 1795s.

  • Figure 4. Polarization vector map of the bipolar reflection nebula CRL 2688 ("Cygnus Egg"), which surrounds an AGB star, overlaid on the Stokes I image. Data from three NICMOS images taken with the the NIC2 camera and the long pass polarizer filters (centre 1.91micron, width 0.20micron) were reduced.

    Directory of demos

    Last updated 02 February 2001

    [STECF HOME] Maintained by Jeremy Walsh