Introduction to IRAF
by Andrew Walker
Setting up IRAF (Image Reduction and Analysis Facility)
In your home directory just type mkiraf and respond to the prompt
with xterm. This will create a file login.cl and two
sub-directories, uparm and images, for you.
Starting IRAF
To start up IRAF just type cl (for command language) in
your home directory. You should make sure that you run IRAF from an
xterm window, else the display may prove erratic.
The nature of IRAF
IRAF is hierarchical in nature with packages, tasks, and
parameters. Packages contain tasks or further packages, and tasks
contain parameters. As the name suggests it is the tasks that will
perform some operation on an image or spectrum, the exact operation being
determined by the way you set the parameters for that task.
To load a package or operate a task you simply type its name, and to obtain
help on it simply type help [package/task]. When running a task you
will be prompted for several parameters if you don't specify them in the
command line.
You should be aware that IRAF stores the last values for all parameters that
you used for a given task, and will use these as the default the next time
the task is run. Each task may also have hidden parameters, which will not be
prompted for each time the task is run, but can be set using
epar [taskname], and then :g to run the task and save the
parameters, or :q to just save the changed parameters but not run the
task.
Pre-processing data in IRAF
At the most basic level, the pre-processing can be achieved by using the one task
images.imarith, where images is a package name, and imarith the
task name. This task allows you to add, subtract, divide, and multiply images
together by setting various parameter values. e.g.
| operand1 | = | image1 |
| op | = | / |
| operand2 | = | image2 |
| result | = | imageout |
| title | = | |
| divzero | = | 0. |
| hparams | = | |
| pixtype | = | real |
| calctype | = | real |
| verbose | = | no |
| noact | = | no |
Only the first 4 parameters will be prompted for and the rest must
be set using epar. It is important that you set both
pixtype and calctype to real else the calculation
will give integer results, which will be of little use to you.
To pre-process your data you must take at very least the following steps:
- bias subtraction: object - dark
- bias subtraction: lamp - dark
- flat-fielding: (object - dark) / (lamp-dark)
The output from this last step should give you a pre-processed image, which
should account for the signature of the detector with which you took your
observations.
Other Tasks
Other tasks which you may find useful when using IRAF to look at images
are briefly summarised below:
- images.imstatistics: Compute and print stat's for a list of images
- images.imcopy: Copy an image or image section
- images.imdelete: Delete a list of images
- images.imheader: Print an image header
- images.tv.display: Load an image or image section into the
display
- images.tv.imexamine: Examine images using image display,
graphics, and text
- dataio.rfits: Convert a FITS image into an IRAF image
- dataio.wfits: Convert an IRAF image into a FITS image
- plot.pcol: Plot a column of an image
- plot.prow: Plot a line (row) of an image
These are only a small fraction of all the available tasks within IRAF.
Browse through the manual, or load up different packages, to get some
idea of what else is available. The chances are that if you can think of something
that it would be useful to have IRAF do, it already does it.
Image Display
The image display of choice is SAOimage. To start it up just type
saoimage & from an xterm or command tool window. From within IRAF you
will then need to type something like set stdimage=imt1024, where the
numeric suffix will depend on the exact size of your image.
To actually display an image you can use the images.tv.display task.