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Browse around the image for a bit to make sure that the fit to the edges makes sense everywhere. About 1 out of 10 times, the fit is bad in the upper left corner of the array. If you find this condition, try another exposure - this usually works. Or, if you select multiple exposures the software will average them together and fit on the combined image.
You will know that the software worked correctly if: the slit boundaries are located correctly on the image, and a file is produced in the subdirectory redux/Flat with the name "Orders_0123.fits" where 0123 is the frame number of whatever file you used to trace the slits.
Generating Flat Fields
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%).
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Clicking in the "Flat" tab will allow you to enter these exposures in for the combine process:
In the "Flat Field Files" area, click the browse button to select the files you'd like to combine for the flat. Typically one should not combine together flats that were taken after moves of the slit wheel. Instead, you should generate a new composite flat for each time you moved the slit, and then the software will apply the appropriate flat one to your data. You may select multiple files in the dialog box; these will be combined using an avsigclip algorithm when generating the composite.
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For the Slit Tilt file, enter in the name of a long science exposure (to use OH lines to fit the slit tilt) or an Arc lamp file. We've found that the OH lines work slightly better if you have a suitably long frame (>300 sec).
Finally, enter in the name of the order mask file (i.e. Orders_*.fits) generated in the previous step.
Click "Make flat field" and the software will run. This process takes some time, roughly 10 minutes per flat field generated. At the end of the process, the illumination flat and pixel flat will be displayed for inspection.
You will know that the software worked correctly if: The pixel flat looks like a flat image centered around 1.00 with fairly little structure, and the illumination image shows a smooth gradient with amplitude of 1.0 +/- ~0.05. Also, two files should be created in the redux/Flat directory. One named Pixflat_0123to0223.fits where the numbers again represent the range of files used. And another named Illumflat_0.60_0122.fits where the 0.60 represents the slit used, and the 0122 is the frame number of the sky flat.
Generating the FIRE Structure
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