Reviewer comments added.

Hi Katie,

Your IDL exercises are very well done!

I ran into a couple of hiccups:
   * I was unable to recreate your _find_ calls without knowing your inputs
   * Arrays should always be indexed with [] instead of () to avoid confusion with functions.  Parentheses work for backwards-compatibility with old IDL code.
   * It's nice to set the plotting device back to the original (in this case 'X') if you're going to share your code (set_plot, 'X').
   * The units in FITS files are usually stored in the header associated with the extension of interest (i.e. ext1 had the physical units of ELECTRONS instead of ext0).

I will post some in-line comments after submitting this pull-request.

-Sean