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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Table of Contents



Introduction

The procedures in this directory are useful for accessing image files used by STSDAS (Space Telescope Science Data Analysis System), developed for the analysis of images from HST (Hubble Space Telescope). The appendix below gives more information about this data format. The use of the STSDAS format is being phased out; the second generation instruments on HST (STIS and NICMOS) now use the FITS data format.

The LaTex file idl_stsdas.tex in the /text directory gives a detailed description of the IDL interface with STSDAS images and tables. This file is actually a copy of Chapter 10 in "A User's Guide to the GHRS software" available as a postscript file or as .hmtl help from http://hrssun.gsfc.nasa.gov/daf/sw_docs.html.

The procedure STSUBIM (which calls STSUB) can be used to read a subset of an STSDAS image file or to read every nth pixel in a file.

The following procedures from the FITS directory can also be used to manipulate STSDAS headers: 

SXADDHIST - Add history records to a FITS or STSDAS header
SXADDPAR - Add a keyword and value to a FITS or STSDAS header
SXDELPAR -  Delete a keyword from a FITS or STSDAS header
SXPAR  -   Obtain the value of a keyword in a FITS or STSDAS header
The procedure STWRT can be used to quickly write simple STSDAS images. However, STWRT does not ensure that the STSDAS files are in REAL*4 and group format. Thus files written with STWRT may not be readable in the STSDAS environment. If one is to be switching between IDL and STSDAS then the use of the SX procedures (SXOPEN, SXWRITE) is indicated. Similarly, STRD can quickly read a simple STSDAS image file but cannot be used to access multiple groups (use SXOPEN and SXREAD instead). For most purposes, STRD is superseded by IMGREAD which can access multiple groups and recognizes certain WF/PC and FOC keywords.

STSDAS Format

Closely related to FITS format is the STSDAS (or SDAS, or GEIS,or modified FITS) format. This is data format used by the Space Telescope Science Data Analysis System, and it differs from standard FITS in several ways:
  1. The FITS header is kept as a separate ASCII '.hhh' file. This allows the header to be read outside of program control.
  2. The data is stored in 512 byte records in an '.hhd' file, rather than the FITS standard of 2880 bytes. On a VMS machine, this makes disk I/O much faster.
  3. The binary data is stored in a format appropriate to the host machine, i.e. unlike a disk FITS file, an STSDAS file does not require byte swapping on a VMS machine.
  4. STSDAS data files is in a "group" format which differs somewhat from the standard FITS "group" format. In particular, the STSDAS group parameters do not have to be the same data type as the data, and the group parameter block follows rather than precedes the data. If the data is not actually going to be used with STSDAS, the we suggest dropping the requirement that the data be "group" format.
A bit of ancient history here. Especially because of (2) and (3), the STSDAS format became popular with users of IDL Version 1 in the mid 1980s. The Version 1 Users library (part of the standard IDL package) included many procedures, all beginning with 'ST' for accessing STSDAS files. The procedure STREAD was even an intrinsic IDL function. Unfortunately, in 1986 Space Telescope changed the position of their group parameters, and a new set of procedures (now beginning with 'SX') was needed, and is available in this directory. (Because STREAD was an intrinsic IDL function, a different prefix had to be used.) Some of the SX procedures are generally useful for FITS I/O, in particular, SXPAR to read the value of a FITS keyword, and SXADDPAR to add a FITS keyword to a header.

Index of Subroutines

AFhread Subroutine of WFPCREAD to read a GEIS header from an HST STSDAS image.
ASTRMFIX Calculate a rough HST WFPC or FOC astrometry solution
CONV_STSDAS Convert internal format of an STSDAS image to host machine architecture
EXTGRP Extract the group parameter information out of SXREAD output
IMGREAD Read a WFPC or FOC file into IDL image and data arrays
ST_DISK_GEIS Routine to construct GEIS files from ST FITS disk files.
STRD Open an STSDAS file and read into an image array and header.
STSUB Subroutine of STSUBIM to read a subset of a SDAS image file.
STSUBIM Open an STSDAS file and read a portion of the file into an array.
STWRT Write a STSDAS header and/or 2-D array to disk (without groups!)
SXGINFO Return information on all group parameters in an STSDAS header.
SXGPAR Obtain group parameter value in SDAS/FITS file
SXGREAD Read group parameters from a Space Telescope STSDAS image file
SXHCOPY Copies selected portions of one header to another
SXHEDIT Routine to interactively edit an STSDAS header on disk.
SXHMAKE Create a basic STSDAS header file from an IDL data array
SXHREAD Procedure to read a STSDAS header from disk.
SXHWRITE Procedure to write an STSDAS or FITS header to disk as a *.hhh file.
SXMAKE Create a basic ST header file from an IDL array prior to writing data.
SXOPEN Open a Space Telescope formatted (STSDAS) header file.
SXREAD Read a Space Telescope STSDAS image file
SXWRITE Write a group of data and parameters in ST format to a STSDAS data file.
WFPCREAD Read designated header and chip of a WFPC1 image
Last modified by pro2html on 2008 November 09 at 04:12 UTC

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Jörn Wilms (wilms@astro.uni-tuebingen.de)
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