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If you are not happy with your location, press "C" to clear and start over. However if you are happy, press "O" to extend your trace location to all wavelengths/slits. FIREHOSE uses the map of the order boundaries to define the shape of the trace. A black line indicates where the center of the trace has been defined, as shown below. 
Now you must define the extent of the extraction aperture. For boxcar extraction this is simply the width of the boxcar function. For optimal extraction we approximate the user-defined width as +/- 2 sigma for a Gaussian weighting function.
Press "A" or select "Apertures -> Define Center/Width" and the following dialog box should appear: