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Currently, our best flat-field images are generated from twilight images. To
create a twilight flat, we image high elevation fields at twilight, away from
the setting sun. We then take the median of these images to remove the stars. Because the
light in the twilit sky is dominated by scattered broadband sunlight, fringing
from strong night-sky lines is not evident.
A sample twilight flat is shown in Figure 7.1. Along with the vignetting
(on the order of 10%), other features can be seen that appear to be real.
A sample twilight flat. The
diagonal lines appear to be real variations in the flat.