This corrects for thermal noise, or "hot pixels" (the occasional white dot you see in the image above). When the camera is hot, there are stray electrons flowing around everywhere. Unfortunately the CCD chip detects electrons emitted by the silicone when photons hit it, and it has no way of telling whether it came from the silicone or the electronics. Often times, CCD cameras are cooled, which dramatically reduces this effect. However, our camera is not cooled and hot pixels are a significant source of noise. Fortunately, it is consistent, linear in time (that is to say, twice the exposure time means there is twice the dark current), and can be characterized by taking a long exposure with the lens cap on. This image is called the dark frame. After subtracting the bias, the dark frame can be scaled to the exposure time subtracted from the object image.
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