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Hook, R. N., Walsh, J., Pirzkal, N., & Freudling, W. 2000, in ASP Conf. Ser., Vol. 216, Astronomical Data
Analysis Software and Systems IX, eds. N. Manset, C. Veillet, D. Crabtree (San Francisco: ASP), 671
The ``stecf'' IRAF Layered Package
R. N. Hook, J. Walsh, N. Pirzkal, W. Freudling
ST-ECF, Karl-Schwarzschild Str.2, D-85748 Garching bei München, Germany
Abstract:
We have collected together many of the astronomical applications
developed as IRAF tasks at the Space Telescope European Coordinating
Facility (ST-ECF) into a new IRAF layered package called ``stecf''.
This package contains several subpackages.
One contains a variety of image restoration algorithms including multiple
channel and photometric variants of the Richardson Lucy method. In addition
a wholly new package (``impol'') for the reduction of polarimetric imaging from
HST (and ground-based instruments) is made available.
Software for the post-pipeline processing of HST NICMOS data, with
particular emphasis on the association and pre-processing of images to be used
for grism spectral extraction and a ``quicklook'' grism
spectral extraction tool, originally for HST STIS data, are also included.
For many years the ST-ECF has developed IRAF tasks and packages for
HST and related data analysis. Up to now these have been distributed in
an ad hoc manner and supported at various levels. To allow wider and
more efficient distribution and to bring all components to a uniform level
of quality and documentation we have decided to combine these components into
a standard-format IRAF package to be called ``stecf''.
The ``stecf'' package will be made available as part of the next release
of STSDAS as well as on the IRAF archive site and its mirrors and from the
ST-ECF Web pages.
This paper gives an overview of the contents of
the packages and examples of their use along with pointers and links to
further information. The brief descriptions here cover an improved
photometric restoration method, grism reduction software and a
grism ``quicklook'' spectrum extraction tool as well as a more
complete description of a new package for the reduction of imaging
polarimetry data. Other tasks will be added in the future.
To address the problem of point-source photometry in restored images
Lucy (1994) suggested a two-channel photometric restoration method in
which the points are represented as 
functions in one channel and
the background, constrained to be smooth by an additional entropy
constraint, in the other. This method was implemented and made available
as PLUCY in the stsdas.contrib package.
PLUCY has the significant disadvantage that the stars were forced to be
at the centres of pixels. This led either to artifacts due to inexact alignment
or required the use of unreasonably large sub-sampled images. In the new
CPLUCY implementation the stars can be at arbitrary 
sub-pixel
positions but the entropy constraint on the background, used by
PLUCY, is retained. A superior new approach to this problem (GIRA,
Pirzkal et al. 2000) is being developed and CPLUCY may be viewed as an
intermediate code. Figure 1 shows an example of the application of CPLUCY
to a simulated HST/NICMOS image.
Figure 1:
An example of the application of CPLUCY. The image at the left is a
simulated HST/NICMOS image with bright point-sources and structured background.
The stars are at random sub-pixel positions. The centre image shows the true
background intensity distribution and on the right the CPLUCY reconstructed
background channel is shown.
 |
The stecf.nictools package
is a collection of IRAF cl scripts
to group, process and co-add NICMOS direct and grism images. It
was written to help the processing of NICMOS grism images after
they have been calibrated with the STScI pipeline calnica and
before spectra are extracted with the ST-ECF software NICMOSlook or
calnicc. It is also useful for simple co-adding (shift and add) of
direct images.
Slitless is an IRAF task originally written to allow the
``quicklook'' extraction of spectra from STIS direct/grism HST image
pairs. It allows a user to perform
unweighted non-flatfielded extraction and wavelength calibration with
a minimum of user interaction: specific objects or a
region of interest are selected in the direct
image, and the task automatically computes and selects, based on
the camera and grism modes that were used, the appropriate section to
extract from the associated grism image. The level of background to
subtract at each wavelength can be determined interactively or by
using some preset grism regions near the spectrum. Optionally, a Data
Quality image extension can be used to avoid extracting flagged
pixels. Slitless is configured to work primarily with the STIS G750L
grism mode, but more modes can be defined by the user using a simple
grism dispersion configuration file.
The impol package consists of five tasks to reduce, simulate and plot
imaging linear polarimetry data taken with the HST instruments with
polarizing facilities - currently FOC, NICMOS and WFPC2. The tasks are
written to require minimum user interaction so that specialist knowledge is
not required to produce scientifically useful polarization results.
Starting from well-aligned input images, along with statistical error maps,
images of the total signal (Stokes parameter I), the normalized Stokes
parameters Q/I and U/I, the linear polarization and position angle of the
E vector and their propagated errors are produced.
Figure 2:
An example of the output from stecf.impol showing
polarization vectors plotted on an HST WFPC2 F673N filter image of
the Homunculus nebula surrounding 
Carinæ.
 |
- All the instrumental set-up, such as filters, polarizers, rotation
angles, etc. are read from the image headers'
- Many input images (
) can be processed'
- HST instruments FOC, NICMOS and WFPC2 are handled; ACS will be
added in the near futuré
- Data from non-HST instruments with polarization analyzers can be
added by including a table file listing the instrument polarization
properties,
- Output images can be rebinned to increase the polarization
signal-to-noise per output pixel,
- Polarization vector maps can be flexibly plotted with user choice
of vector size, density of vectors and range of plotted magnitudes. Maps
can be over-plotted on images using standard IRAF facilities,
- Output error images have fully propagated statistical errors
from the input error images. The bias on the polarization values for
large fractional polarization errors is corrected.
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.
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.
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 compatibility with the input required by hstpolima.
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 oriented at right angles to the direction
to the centre. Random noise variations in polarization and position angle
can also be included.
polimplot: Plot a polarization vector map with flexible
control over
vectors and labeling. Linear polarization and position angle images are
required. A vector map can be over-plotted on the total signal image.
Figure 2 shows an example.
There is a web page
briefly describing the package
and giving example polarization maps for HST and model data.
A directory of demos is also available allowing simulated data with FOC,
NICMOS and WFPC2 and a non-HST instrument to be produced for a model bipolar
reflection nebula.
References
Lucy, L. B. 1994, in ``The Restoration of HST Images and
Spectra-II'',
R. Hanisch & R. White (eds), 79
Pirzkal, N., Hook, R. N., & Lucy, L. B. 2000,
this volume, 655
© Copyright 2000 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
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