If the program is a command line program, and its not printing to the screen, then it is most likely not writing to stdout to begin with. There may be verbose options that will turn on additional print-out, however I'm not familiar with the 'IDL' program off the top of my head.
If it _is_ printing a bunch of stuff to screen, but you can't capture it, the problem may be that its using stderr, in which case you can redirect stderr to a logfile like so:
http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-3.html