The directory which is to contain the pixel files associated with bulk datasets (such as images or spectra) is specified by the value of environment variable imdir. For example, the sample loginuser.cl customisation file contains the line:
set imdir=HDR$pixels/
which specifies that the pixel files are to be kept in subdirectory pixels of your IRAF home directory. Note that the trailing `/' is important. Here HDR$ denotes your IRAF home directory, which contains the header files. If you wanted to keep the pixel files with the header files in your IRAF home directory you would simply specify:
set imdir=HDR$
An Introduction to IRAF