  We describe the flattening of scientific CCD imaging data using a dome flat
field system. The system uses light emitting diodes (LEDs) to illuminate a
carefully constructed dome flat field screen. LEDs have several advantages over
more traditional illumination sources: they are available in a wide range of
output wavelengths, are inexpensive, have a very long source lifetime, and are
straightforward to control digitally. The circular dome screen is made of a
material with Lambertian scattering properties that efficiently reflects light
of a wide range of wavelengths and incident angles. We compare flat fields
obtained using this new system with two types of traditionally-constructed flat
fields: twilight sky flats and nighttime sky flats. Using photometric standard
stars as illumination sources, we test the quality of each flat field by
applying it to a set of standard star observations. We find that the dome flat
field system produces flat fields that are superior to twilight or nighttime
sky flats, particularly for photometric calibration. We note that a ratio of
the twilight sky flat to the nighttime sky flat is flat to within the expected
uncertainty; but since both of these flat fields are inferior to the dome flat,
this common test is not an appropriate metric for testing a flat field. Rather,
the only feasible and correct method for determining the appropriateness of a
flat field is to use standard stars to measure the reproducibility of known
magnitudes across the detector.
