The next step is to generate the pixel flat field and slit illumination images. The pixel flat corrects gain variations on the detector, and the illumination function corrects for the fact that the internal quartz lamp has a slight gradient in intensity across the slit (~5%). 
Firehose needs 4 pieces of information to perform this operation: (1) The flat field file itself (internal quartz), (2) A sky flat for illumination correction, (3) The file indicating slit positions and order numbers generated in the trace step, and (4) a data frame which helps firehose calculate the "tilt" of the slit on the detector, to facilitate its fitting out the wavelength dependence of the lamp and/or twilight sky.
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