Old IRAF Format

A dataset, typically an image or spectrum, in the Old IRAF Format (OIF) format consists of two files: a header file which has file-type .imh and a pixel file which has file-type .pix. These files are usually stored in standard locations in your IRAF directories. The .pix files are large and can be stored in either the same directory as the header files or in a different directory. The header file contains the dimensions of the data array, the name of the pixel file and optionally any auxiliary information describing the image, expressed as a series of keywords and their values. These OIF keywords deliberately have a syntax similar to FITS keywords. The image dimensions and name of the pixel file are stored in a binary format, but the keywords are stored as simple ASCII characters. Thus, you can check the values of the keywords by listing a header file on the terminal.

There are several points about the OIF format which you should be aware (and beware) of.

1. IRAF version 2.10 supports only the original OIF (that is, version 1 of the format). However, version 2.11 also supports an enhanced version of the format (version 2).

2. In version 1 of the format the .pix file differs on different types of computer. That is, if you are using IRAF version 2.10 then a data file written on a SUN workstation cannot be read on a Digital Alpha or vice versa. Thus, in this case, it is sensible to carry out all your IRAF work on the same sort of machine.

3. Version 2 of the format is independent of the host operating system. That is, if you are using IRAF version 2.11 then a data file written on one type of computer can be read on another type.

4. This change is backwards but not forwards compatible. IRAF version 2.11 will read files in either version of the format, but version 2.10 only reads files in version 1 of the format. If you need to write a data file with version 2.11 and subsequently read it with version 2.10 then one approach is to write it as a FITS file. Alternatively, version 2.11 can be made to write files in version 1 of the format by setting an appropriate environment variable:

   set  oifversion=1

5. In either version of the format, the header file contains the full name and directory specification of the pixel file. Consequently, IRAF data files cannot be manipulated (copied, renamed or deleted) using standard Unix operating system commands; there are special commands within IRAF (imcopy, imrename and imdelete respectively) which must be used for these tasks. This point is covered fully in the Beginner's Guide[1].