You need to put the following into your .bashrc file for IDL to work
properly: 
#setup sloan stuff
source /u/dss/products/eups/bin/setups.sh
setup -v idlutils
#setup -v photoop v1_9_11
setup -v idl v7_1

export
IDL_PATH=${IDL_PATH}:/usr/linux/common/peyton/licensed/idl71/lib 
export IDL_PATH=${IDL_PATH}:/u/schlegel/dustpub/CodeIDL #Schlegel's dust maps
export IDLSPEC2D_DIR=/u/schlegel/idlspec2d

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 I'd like to make a few requests about IDL in the homework.  I don't
know if I will require all of the following, but even if not, I would
still very much appreciate you following them.

 FIrst, please write any IDL code (particularly something you hand in)
in a text file.  You can run this file from within IDL with the
command
 @file
 if it is a batch file, or 
 .run file
 if it is a program file.  This way, you can edit your program (in
 emacs, for example) and then call it when you're done.  This is
 especially useful if you have an error in your program, since you
 don't have to retype the rest of the code.

 Second, please make sure to comment your file.  Don't over-comment by
explaining obvious things, like
 c = a + b  ; set c to be the sum of a and b
 You'll find a happy medium, and I'll write on your programs if I
 think the commenting is grossly over or under-done.

 Finally, please hand in nice-looking code by using the postscript
print option from emacs.  First, make sure you are printing to a color
printer.  Change the printer to ps in your .bashrc file, for example. 
Then select File -> Postscript Print Buffer from within emacs.  This
will make it much easier for me to read your code.