GEIS format1 is the standard format for reducing data from FGS, FOC, FOS, GHRS, HSP, WF/PC-1, and WFPC2. Data from these instruments are distributed by the HDA in waivered FITS files and must be converted to GEIS format. Note that WFPC2 data is now available from the HDA in waivered FITS and MEF format files
2.
All HST images in GEIS format consist of two components: a
header file (with suffix ending in “h”), and a separate
binary data file (with suffix ending in “d”). Both files must reside in the same directory for processing.
GEIS header files (e.g., w0lo0105t.c1h), consist entirely of ASCII text in fixed-length records of 80 bytes. These records contain header keywords that specify the properties of the image itself and the parameters used in executing the observation and processing the data.
GEIS binary data files, (e.g., w0lo0105t.c1d), contain one or more
groups of binary data. Each group comprises a data array followed by an associated block of binary parameters called the Group Parameter Block (GPB). Each group of a GEIS file has identical array sizes, data types, and group parameters.
Figure 2.2 depicts the structure of a GEIS data file graphically.
Your data must be in GEIS format for you to use the STSDAS software tools developed specifically for analysis of these data. It is important to use the
strfits task found in
stsdas.fitsio or in
tables.fitsio to perform the conversion from waivered FITS format to the GEIS format. A special convention is used to map GEIS format to waivered FITS format. While other FITS readers may be able to read portions of the data correctly, they are unlikely to reconstruct the entire data file properly.
This command tells IRAF to write output files in GEIS format. You then need to set the
strfits parameters
xdimtogf and
oldirafname both to “yes”. For example, after you have set
imtype =
hhh, you can convert the FITS file
*_hhf.fits into the GEIS format files
*.hhh and
*.hhd by typing:
As another example, the waivered FITS WFPC2 dataset u6n20101m_clf.fits can be converted using
strfits to created two GEIS files:
u6n20101m.clh (a header file) and
u6n20101m.cld (a data file).
One of the original advantages of GEIS format was that it could accommodate multiple images within a single file. This feature is useful because a single
HST observation often produces multiple images or spectra. For example, a single WF/PC-1 or WFPC2 exposure generates four simultaneous images, one for each CCD chip. Likewise, a single FOS or GHRS dataset may comprise many spectra. The data corresponding to each CCD (for WF/PC-1 or WFPC2), or each readout (FOS) or bin (GHRS), are stored sequentially in the groups of a single GEIS binary data file. The header file corresponding to this data file contains the information that applies to the observation as a whole (i.e., to all the groups in the image), and the group-specific keyword information is stored in the group parameter block of each data group in the binary data file.
The number of groups produced by a given observation depends upon the instrument configuration, the observing mode, and the observing parameters.
Table 2.4 lists the
contents and the number of groups in the final calibrated image for the most commonly-used modes of each instrument that uses the GEIS data format.
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Group n contains accumulated counts from groups (subintegrations) 1, 2, ... n. The last group is the full exposure.
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Each group is an independent subintegration with exposure time given by group parameter EXPOSURE. If FP-SPLIT mode was used, the groups will be shifted in wavelength space. The independent subintegrations should be coadded prior to analysis.
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Group n represents CCD chip n, e.g., group 1 is chip 1 (unless not all chips were used). Group parameter DETECTOR always gives chip used.
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Group n is chip n + 4, e.g., group 1 is chip 5. If not all chips were used, see the DETECTOR parameter which always gives the chip used.
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Header keyword information relevant to each group of a GEIS file resides in two places, the primary header file and the parameter block associated with a group. Because GEIS header files are composed solely of ASCII text, they are easy to view or print using standard Unix text-handling facilities. However, the group parameters are stored in the binary data file. To access them you need to use an
IRAF task such as
imheader, as shown in the section titled
“Printing Header Information”.
You can use the IRAF hedit task to edit the keywords in GEIS headers. While it is possible to edit GEIS header files using standard Unix text editors, you must maintain their standard 80-character line length. The
hedit task automatically preserves this line length. If you need to add or delete group parameters, you can use the
STSDAS groupmod task in the
stsdas.hst_calib.ctools package. The
STSDAS chcalpar task is useful for updating header keywords containing calibration switches and calibration reference files.
Numerous IRAF/
STSDAS tasks exist for working with GEIS images. Most of these tasks operate on only one image at a time, so you usually need to specify the GEIS file group to be processed. If you do not specify a group, the task will operate on the first group by default.
To specify a particular group in a GEIS file, append the desired group number in square brackets to the file name (e.g.,
z2bd010ft.d0h[10]). For example, to apply the
imarith task to group 10 of a GEIS image, type the following:
The command above will add 77.0 to the data in group 10 of the file indata.hhh, and will write the output to a new single-group file called
outdata.hhh. Any operation performed on a single group of a multi-group GEIS file results in an output file containing a single group.
This command extracts a 100 by 200 pixel subsection of the image in the second group of the file
indata.hhh, multiplies it by a factor of 32.0, and stores the result in a new output file,
outdata.hhh, which is a 100 by 200 pixel single group GEIS file.
An image section of one group of a GEIS image may be overwritten or operated upon, leaving the rest of the image intact. For example, the following two commands will first create
outdata.hhh and then overwrite a section of it:
As for MEF files, the task imheader extracts and prints information about a GEIS image. This task prints the image name, dimensions (including the number of groups), pixel type, and title of the image when it is run in default mode. For example:
cl> imhead indata.hhh indata.hhh[1/64][500][real]: INDATA[1/64]
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The output line indicates that indata.hhh is a multi-group GEIS file which contains 64 groups of data, each consisting of an array 500 pixels in length. The data type of the values is real (floating point). Note that since no group designation was provided, the task defaulted to the first group. To reveal more information regarding group 10, you can type:
This will generate a long listing of both the ASCII header parameters in the *.hhh file and the specific group parameters for group 10 of the
*.hhd file.
Currently, IRAF tasks and many
STSDAS tasks cannot simultaneously process all the groups in an input image and write the results to corresponding groups in an output image in one step. However, there are several
STSDAS tasks, particularly in the
toolbox.imgtools and
hst_calib.ctools packages, written to support group format data. Please refer to the
STSDAS User’s Guide for more details about working with GEIS images.
File formats for the first and second generation HST instruments (FGS, FOC, FOS, HSP, WF/PC-1, GHRS, and WFPC2) were developed before the standardization of MEF format. The waivered FITS format was developed in response to the need for a machine independent storage format for these data and was based on the idea of stacking multi-group GEIS data as a new dimension in a FITS image.
For example, a WFPC2 science data GEIS file with four groups has four 800x800 pixel images in its data file. When this GEIS file is converted to a waivered FITS file (using the
IRAF task
stwfits), the resulting FITS file has the dimensions of 800x800x4 (a three-dimensional image!) in its primary HDU. Similarly, an FOS GEIS file may have 40 groups, each group being a one-dimensional image (spectrum) that is 2064 pixels in length. The waivered FITS file equivalent of this FOS GEIS file has one 2D image of the size 2064x40 as its primary HDU.
In the case of a 4-group WFPC2 image, the first extension of the waivered FITS file is a table containing four rows. Each row represents a group. Each column in the table will correspond to a group keyword. Each element in the table contains keyword values for a specific group. This can be viewed using the
tread command:
You can also display the values of specific keywords using a command like tdump, which in the example below, writes the values to a file called “params.txt”:
The data component of a multi-group GEIS file, when converted to waivered FITS, is stored in the primary HDU of the waivered FITS image as a multi-dimensional image. The task
display can be used to view one group image at a time. For instance, to view group 2 of a 4-group waivered FITS WFPC2 image, type:
The display task reads the image from the primary HDU, and specifies the image using three-dimensional syntax, where the “*” represents all pixels in
x and
y.
