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When finished click on "Accept". Then on the main GUI choose the flux units desired and click on "Construct Telluric Spectra". 
3. Next, determine the relative velocity/wavelength shift between the telluric spectrum and the source spectrum. In general, this correction is small to zero, but worth examining when you are interested in data near the telluric bands. Click on "Get Shift" and a GUI will appear (below). The top panel shows the extracted Object spectrum and inverse of the telluric spectrum; the bottom panel shows the corrected object spectrum. If there is a significant shift in the dispersion between source and telluric spectrum, you will see "spikes"in the corrected spectrum around strong telluric features. You can manually set the shift (0.1 unit changes are usually a good step size); or you can choose a sharp telluric feature (e.g., 2.0 micron) typing "s" and then clicking on either side of the feature, and then click on "Auto Find" to get a shift that minimizes variations. Click on "Accept" when you are done. 
4. Finally, enter a base file name in the Object File box (e.g., "prism_mysource" - you do not need to add the ".fits" extension) and click on "Write File". A display window will appear showing the final spectrum. The output file format is a 2048 x 3 fits array containing wavelength in micron, and flux and uncertainty in the units chosen in step 2. 
Have fun with your FIRE data!