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1.4 Reading HST Data Tapes

If you requested Exabyte or DAT tapes in your Phase II proposal, STScI will mail you one or more tapes containing your data within a few weeks of your HST observations. Your shipment of HST data ought to include:

At least one data tape, together with a tape log listing its contents.
Several printouts, called paper products, which are instrument-specific and are described in each instrument's Data Structures chapter. Each of the files on your tape is in FITS (Flexible Image Transport System) format.¹ Any FITS reader should be able to read these files; however, GEIS (Generic Edited Information Set) is the standard format for analyzing all HST data other than STIS and NICMOS data. Chapter 2 discusses the HST implementations of the FITS and GEIS formats in more detail, explains the various types of HST data files, and interprets their names.
To read FITS files from your tape and write them to your disk, you can use the strfits task in IRAF/STSDAS. If you are not familiar with IRAF, consult the IRAF Primer in Appendix A before proceeding further. Chapter 3 contains additional information on STSDAS, the STScI data analysis software package.

The STSDAS strfits FITS reader preserves the multigroup format of an HST image. This format must be retained if you plan to recalibrate your data in -STSDAS.

To read an HST data tape using strfits, you need to:

Start IRAF and load the stsdas and fitsio packages.
Mount the tape.
Set global parameters (e.g., imtype).
Set the strfits parameters and read the tape.

1.4.1 Loading Packages

Go to your IRAF home directory and start IRAF by typing:

cl

This action will start an IRAF session. Software in IRAF is organized into packages. To load a package, type its name. Once you are in IRAF, load the stsdas and fitsio packages by typing the following commands:

cl> stsdas

st> fitsio

The prompt (such as fi>) shows the first two letters of the most recently loaded package. The fitsio package contains tasks for handling the FITS format files used for HST images. You can use catfits (described on page 2-4) to produce a listing of the contents of your tape, and strfits to read the data onto disk. When you are done working with your data, you may choose to write it back to tape using stwfits.

1.4.2 Mounting the Tape

Mount the tape on your tape drive. Allocate the device within IRAF by typing:

fi> allocate device

where device is the IRAF name of the tape drive. If you are not sure how to mount tapes or don't know the IRAF names that match your tape drives, see your local system administrator for help.

1.4.3 Setting File Format

If you are reading FOC, FOS, FGS, GHRS, HSP, WF/PC-1, or WFPC2 data files, set the IRAF environment variable imtype to specify that your data files are to be written to your disk in GEIS format, for example:

fi> set  imtype="hhh"

Currently strfits will also copy STIS and NICMOS files properly with imtype set to this value.

Then go to the directory in which you want your files to be stored. For example:

fi> cd /nem/data1/hstdata-

1.4.4 Using strfits

Like most IRAF and STSDAS tasks, strfits has several parameters that control the task's behavior. You can either specify the appropriate parameters on the command line or edit the parameter set using the epar task described on page A-8. In STSDAS version 2.0, most of the default parameters are set correctly for reading files from HST data tapes, so you need to specify only the name of the tape drive, the numbers of the data files to be read, and the oldirafname parameter.

For example, to read files 1 through 10 from the data tape on drive mta, type:

fi> strfits mta 1-10 oldirafname=yes

The tape log you received with your tape will tell you the number of each data file. You can also use the catfits task as follows to print a list of all the files on the tape to the screen:

fi> catfits mta 1-999 | page

Contact the Archive help desk at archive@stsci.edu if you have trouble reading your tape.

Be sure to set oldirafname and to "yes" or else the tape will not be read correctly and you will not be able to manipulate the data. This step is vital if you plan to recalibrate your data. (The parameter xdimtogf should be "yes" by default.)

Once your data are written on your disk, you can deallocate your tape drive by typing:

fi> deallocate device

You are now ready to work with your HST data files.

Deallocating the drive is important if you want to remain friends with your coworkers. No one else can use the drive until you deallocate it.

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¹ A description of the FITS format and the various options and parameters that can be used in the FITS standard can be found in the document "Implementation of the Flexible Image Transport System (FITS)," by the NASA/OSSA Office of Standards and Technology. The document is available via FTP to -nssdca.gsfc.nasa.gov in the directory FITS. A listing of FITS standards and documentation is available from NRAO via the World Wide Web.

stevens@stsci.edu