How can I make a mask from an exposure file?

The tool dmimgthresh may be used to create a mask of an image, e.g. an exposure map.

Flatten all the values greater than 1 to 1:

unix% dmimgthresh infile=expfile.fits outfile=expfile_flat.fits cut=:1 value=1

Then cut the values less than 1 and set them to 0:

unix% dmimgthresh infile=expfile_flat.fits outfile=mask.fits cut=1 value=0 

A sample mask image.