When extraction completes, the data are stored in two separate ways. One method used "object structures" similar to the echelle pipeline; these contain extra metadata associated with each observation.
However since many observers are familiar with the spextool pipeline used at the IRTF, we also save the output in SpeX format, in the file "Object/Spec_xxxx.fits" for each exposure. These files are 2048 x 3 element matrices, with the three rows representing wavelength, flux, and error. By saving in this format, we can use the spextool package routines to combine spectra and perform telluric correction.
To start this process, we first combine all exposures of a single object. At the IDL prompt, type:
IDL> fire_xcombspec
This will launch the xcombspec GUI. First, enter in your data path (which should be the "Object" directory in your reductions folder). Then specify the root filename (i.e. "Spec_") and the file numbers (range or comma delimited, e.g. "140-144,147"). Load the spectra and then follow the help routines to finish this step. This needs to be done for both your object and telluric. In many cases the SNR of the telluric is high enough to use a single exposure, but we still advise observers to combine dithered tellurics to avoid bad pixels.