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Space Telescope Imaging Spectrograph
Release history.

BASIC RELEASE NOTES

This directory contains a new version of the software used to extract
STIS spectra. The software is bundled as a temporary IRAF package
(sc2d), containing two tasks (sx1d and sc2dref) and one pset (algpars).
The "reffiles" subdirectory contains 100 Mb of new reference files used
by the algorithm. The new reference files are not yet available from
the calibration database maintained by the HST archive.

In the next full release of STSDAS, the new sc2d functionality will be
integrated as an option into the existing "stsdas$hst_calib/stis/x1d"
task, but for now the software is available only as a temporary package
available via anonymous ftp. Use of the two-dimensional scattered light
correction algorithm will likely become part of the standard pipeline
processing, as well.

BRIEF DESCRIPTION OF THE ALGORITHM

The new "sc2d" algorithm implemented in "sx1d" uses an iterative
deconvolution scheme to model and remove scattered light from the
two-dimensional image, prior to spectral extraction. The algorithm was
developed by Don Lindler (Advanced Computer Concepts) and Chuck Bowers
(Goddard), who helped design and test STIS on the ground. Ivo Busko
(STScI) implemented the algorithm as part of the STIS spectral
extraction software.

The new algorithm obtains a two-dimensional model of the scattered
light based on the extracted spectrum and known scattering properties
of the instrument. Echelle scatter is modelled by redistributing
extracted counts along lines of constant wavelength, using echelle line
spread functions. Scattering by the cross disperser is treated by
independently convolving each column with a scattering kernel.
Scattering due to the aperture-truncated telescope PSF and isotropic
detector halo are treated by 2-dimensional convolution. In the FUV, the
PSF is a strong function of wavelength, so scattered light models are
constucted at 2 or 3 different wavelengths, and then the results are
combined with weights that vary as a function of wavelength. After
three iterations, the final scattered light model is subtracted from
the raw image, and extraction proceeds using the standard
one-dimensional interpolation of the background remaining between
orders.

We are in the process of writing a detailed instrument science report
that will quantify the magnitude of the error due to residual scattered
light, using both the old and new algorithms. Here we provide only
preliminary information to help potential users assess whether the
new algorithm is required for particular science goals. Using the
one-dimensional algorithm, errors in the cores of saturated
interstellar lines are about 9% of the continuum at 1200 Angstroms
(E140H and E140M), 3% at 1670 Angstroms (E230H), and 0.6% at 2800
Angstroms (E230H). Errors in emission line spectra are typically 1% of
the peak line flux and can affect adjacent orders.

RUNNING THE NEW PACKAGE

Install the software as described in the INSTALLATION section below.

Before the new algorithm will run successfully, the headers of input
"_flt" images must be edited to include pointers to the new reference
files. This editing can be done by hand or with the task "sc2dref".
This convenient script inserts reference file pointers assuming the
newly distributed reference files will be moved to in the standard
"oref$" directory, prior to execution of the "sx1d" task. If for some 
reason you cannot copy the new reference files to your "oref$" directory, 
then you can edit the script "sc2dref", changing the value of the "refdir" 
parameter from "oref$" to the directory containing the new reference 
files. This approach should be used only a last resort, since there are 
no tools to automatically change the reference file pointers back to 
"oref$" should that become appropriate. The "sc2dref" script can only 
be used to add new reference file pointers; it will not update existing 
pointers.

After invoking the "sc2d" package, the new two-dimensional scattered
light correction algorithm is selected by using "epar sx1d" to set the
"algorithm" parameter to "sc2d". The final scattered light model can be
saved to disk by using "algpars" to set the "sc2dimage" parameter to
the name of the file which is to contain the output image.

Note that the algorithm is rather memory intensive, requiring about 330
Mb of free memory, with at least 256 Mb of real memory required to
prevent excessive swapping. On a 360 MHz Sun 360, individual image
sets take 6-11 minutes to process, depending on echelle grating and
central wavelength.

ADDITIONAL HELP:

If you have questions or comments about the new scattered light
algorithm, please contact help@stsci.edu.

INSTALLATION:

The software is pre-built for the Sun Solaris architecture/OS. If
installing in a different architecture, the package must be built
from the source code, which is also provided in the distribution.

Installing on a Sun Solaris machine:

1) Select where you want the software to be installed. For the sake of
   exposition, we will assume in what follows that the software is to
   be installed in the directory /user/mysoftware/.

2) Download the distribution tar file into /user/mysoftware/.

3) Extract the contents of the tar file. This creates a subdirectory
   named "sc2d" containing the software and reference files.

4) Go to the "sc2d/reffiles" subdirectory and copy or move all files
   the your "oref$" directory. If you do not have write permission for
   that directory, ask your system manager to copy the files.

5) After starting IRAF, install the new package by issuing the
   commands:

    cl> set sc2d = "/user/mysoftware/sc2d/"
    cl> task sc2d = sc2d$sc2d.cl

    These two commands must be executed every time a new IRAF session
    is started, so if you are going to use the new software a lot, it
    might be more convenient to include these two commands as part of
    your login.cl startup script.

At this point the package is available for loading. Don't forget to
pre-process your STIS echelle files with the "sc2dref" script in order
to insert the new reference file pointers.

Installing on a different architecture/OS:

1) Execute steps 1 - 4 described above. 

2) Set the environment for compiling/linking under IRAF:

   3.1) Set the environment variable "iraf" to point to your local
        IRAF installation, such as:

        % setenv iraf /project/devcl/odosw1/ssg/build_4_1/iraf/

   3.2) Source the IRAF initialization file:

        % source $iraf/unix/hlib/irafuser.csh

3) Make sure your Unix environment variable IRAFARCH is set to the
   appropriate architecture (e.g "ssun", "alpha", "redhat", etc.). If
   not, set it explictly, as in:

   % setenv IRAFARCH alpha

4) Change to the directory /user/mysoftware/sc2d/cal/ and delete file
   calstis.a. Change to the subdirectory cs6/ and delete files cs6.o
   and cs6.e.

5) Start an IRAF session.

6) Move to the directory /user/mysoftware/sc2d/cal/ and issue the command:

    cl> mkpkg -p stsdas

    This should re-build the cs6.e executable file.

7) Execute step 5 above to define the package.