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IMPOL - HST Imaging Polarimetry Reduction Package

Jeremy R. Walsh


Several HST instruments both current and decommisioned 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 (WF/PC2) is actively used for imaging polarimetry. 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 measure linear polarization and its position angle.

The STECF IRAF layered package impol provides five tasks specifically aimed at reduction of HST imaging polarimetry data. 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 impol.

Components of Package

  • hstpolima -- Reduce linear polarization data for HST instruments (currently FOC, NICMOS and WFPC2 are supported). A set of well aligned input images is required and the corresponding statistical error images. Images for the three Stokes parameters (I, Q and U), the linear polarization and position angle and their error images are output. IRAF/STSDAS help file available.
  • hstpolpoints -- Reduce linear polarization data for HST instruments for a set of circular apertures. From a set of input images and the error frames, the linear polarization results (and errors) are returned as an STSDAS table file. IRAF/STSDAS help file available.
  • hstpolsim -- Produce simulated polarization input data frames for HST instruments from images of the total signal, the linear polarization and the position angle of the polarization vectors. The output images can have noise added. Images of the statistical errors are also produced for full compatability with the input required by hstpolima. IRAF/STSDAS help file available.
  • polimodel -- Produce model linear polarization and position angle images given a total signal image. The polarization variation is specified by a power law and the position angles are orinted at right angles to the direction to the centre (centro-symmetric situation). Random noise variations in polarization and position angle can also be included. IRAF/STSDAS help file available.
  • polimplot -- Plot a polarization vector map with flexible control over vectors and labelling. A linear polarization image and a position angle image are the required inputs. By specifiying an image display as the plotting device, it is possible to overplot the vector map on the total signal (Stokes I) image. IRAF/STSDAS help file available.

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

Maintained by Richard Hook <>
Last Modified Thu, 08 Jun 2006 15:37:58 +0200