Hi John, You can use the Astrolib library to read and write FITS files. This is a third party library and it not officially supported by Exelis VIS. You can download the library from here: http://idlastro.gsfc.nasa.gov/ and some of the functions are documented on our website. A few of these functions are WRITEFITS (http://www.exelisvis.com/docs/writefits.html) and READFITS (http://www.exelisvis.com/docs/readfits.html). For your second question, if you mean to return an array of data from your FITS files, then I think that is what the READFITS function does automatically. Specifically, this is from the above documentation page for READFITS and the result returned from the function: "Result = FITS data array constructed from designated record. If the specified file was not found, then Result = -1"