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FOS has its own standard flatfield which observers should be able to use and take home with them. Most observers do like to take their own however. Use a slit width similar to that used for your observations, and expose the tungsten lamp, taking care not to saturate. Use the GG495 filter for first order, take several flatfields, use COMBINE to produce an average flatfield, then use DCSUB to remove the bias from this image. finally use FFNORM to remove the strong wavelength dependence. This produces an image which has exactly 20000 counts where no light fell, and an average of 20000 counts where light fell, independent of wavelength.
To apply a flatfield, use DOFF which automatically removes the dc bias, and handles the 20000 count normalisation.