IMPOL - HST Imaging Polarimetry Reduction Package
Jeremy R. Walsh
Introduction
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 <rhook@eso.org>
Last Modified Thu, 08 Jun 2006 15:37:58 +0200
