Twilight Flats

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 ${\sim}$10%), other features can be seen that appear to be real.

Figure 7.1: A sample twilight flat. The diagonal lines appear to be real variations in the flat.