The Image Reduction and Analysis Facility (IRAF) is a UNIX oriented software package being developed at NOAO in Tucson. It has many features required to reduce and analyse data. This document is an introduction to IRAF for those who would like an understanding of the system and how it works. More information about IRAF can be obtained using the help pages, manuals or by request directly from Tucson.

When using IRAF, a login.cl file is used to set up the system environment variables, to define such things as the terminal type, printer, your home and image directories, and many others. Within IRAF you can examine these by simply typing show. This will list the many defined variables necessary to use the software. These values can be changed, for example to send output to a different printer, using set environment variable=new value.

IRAF is structured in a tree fashion such that "tasks" (programs) are arranged by topic into "packages". The packages are divided into two classes, system packages and scientific packages. It is a tree in the sense that packages have subpackages, which have subpackages and so on... If you wish to run a task in a certain subpackage, the packages in the path must be loaded. This is done by typing the name of the package and subpackages until the task is shown. For example, to use the task "combine", first type imred, and ccdred to load these packages. Now I can run the task combine. If you forget where you are in IRAF, the prompt will give the first two letters of the package you most recently loaded. To unload a package, type bye. In the example, it would be necessary to type "bye" two times to unload the two packages that were loaded. It is not necessary to unload the loaded packages in order to quit IRAF.

Running a task involves many parameters which can be set for the particular operation necessary. These parameters are displayed using "lpar" and can be edited using "epar". These are abreviations for "lparam" and "eparam". Many of the package and task names can be abreviated using enough letters to uniquely distinguish it from others which are similar. If you edit the task parameters, the keypad arrows will move the cursor up and down, type in the change on the parameter line and hit return. Typing ctrl-z will quit the edit mode and accept any changes you have made. If you wish to quit the menu without changing the parameters, use ctrl-c to exit. In the parameter list, there are two types of parameters, the ones with brackets (hidden parameters) and those without brackets (positional parameters). The positional parameters are required to run a task and will be queried for if not specified. The hidden parameters have default values that will be used if not changed by an epar, a direct assignment, or input on the command line (examples follow). If you want to keep the defaults and some of the parameters have been changed, then to reset all the defaults, type unlearn taskname. It is possible to run some tasks in the background which frees up the terminal to accept input. To do this just add a "&" at the end of the command line. There can be up to 4 jobs running in the background. To look at the jobs being run in background just type jobs. Examples of the parameter assignment:

      cc> combine.weight=yes                      (direct assignment)
      cc> combine image1,image2 outimage weigh=yes (running the task)

All image files are stored in two seperate files, the image header and pixel files. The image directory environment variable "imdir" is the storage area for the large pixel arrays. The image header files are stored in the directory where you are when the files are moved onto the computer via tape or other source. Because of their special status in IRAF, all commands involving images are specialized. Deleting, renaming or copying image files must be done using "imdelete", "imrename", or "imcopy", respectively. If for example you tried to delete an image header file in the current directory, using delete image.imh, you would leave a pixel file for that image in your image directory. An added feature now is that an error message will be output if this is attempted. There are many other image specific commands and tasks which reside in the "images" package. There are other formats for data also which will not be discussed, but more information about these can be obtained using the help utility within IRAF.

For simplicity, it is useful to use templates for the image names when working with many of them. The possiblities are:

\fI      CHAR      MEANING                             EXAMPLE\fR
      *       - Match 0 or more characters       -  c*.imh
      [...]   - Any character in range           -  [a-c].imh
      [^...]  - Any character not in range       -  [^d-z].imh
      ?       - Any single character             -  comp???.imh
      @       - Read filenames from a file list  -  @arclist

So any of these may be used in place of explicitly typing out the whole filename being operated upon. For example, if you have a large number of images called "obj003.imh, obj004.imh, obj005.imh,..." then using a template to get a simple header for each file is easy:

      im> imhead obj*.imh
      im> imhead obj???.imh
      im> imhead o*.imh

Using these templates with the image names is quite simple. If however you also have files called "obj003.0001.imh,..." then the first and last commands will include those as well. These templates are valid UNIX constructions and can be used for files as well as images.

Commands which are typed while in IRAF are remembered and can be called again using "ehistory". Typing e or ehist will give the last command which was used. If you want a previous command, page up through the list using the up arrow on the keypad. You can then edit this line using the left arrow and the backspace key. The cursor deletes to the left. Just type return to run the modified command. If you know the last time you used a command and no modifications are necessary, then use "^". For example, 10 commands ago you typed the command "imhead obj*.imh" and haven't used "imhead" since, type ^imhead. The last use of "imhead" will be found and executed. If you now want a listing of your comparison arcs instead of objects, type ^obj^comp. It will change the last command used and replace "obj" with "comp". This is particularly nice if you are using numbers and need to change a previous number.

Knowing what you now know about IRAF, it is important to note that IRAF does not have an editor. Therefore the VI editor is "imported" into the system. If you need to use an editor, a help page has been made for VI. This can be accessed using "help editor" while in IRAF.

Some help can be found in most circumstances if you get stuck. There are the general "help" and "apropos" commands. The "apropos" command is used with some topic. The many short help lines will be searched for the topic for which you have asked. Then you may use "help" to learn more about one of the tasks or packages refered to in the "apropos". The "help" command gives more detailed information about a given task or package. There have been some help pages written at CTIO which will be listed when you type help manuals. This gives a list of help pages written specifically for sending mail, using the SUN console, etc... Since help works even if the task or package is not loaded, it is a good way to find out where a task resides in IRAF. If you need information of another sort, ask Mario Haumy or Lisa Wells for assistance.

More information about IRAF can be found in the CTIO Reductions Manual. In it you will find a tutorial of IRAF structure and some of the important tasks. Help on programming can be obtained from Pedro Gigoux or directly from the computer group in Tucson.