Thermal excitation of electrons is a major source of noise in Charge-Coupled Device (CCD) imagers. Those electrons are generated even in the absence of light, hence the name dark current. Dark current is particularly important for long exposure times and elevated temperatures. The standard procedure to correct for dark current is to take several pictures under the same condition as the real image, except with the shutter closed. The resulting dark frame is later subtracted from the exposed image. We address the question of whether the dark current produced in an image taken with a closed shutter is identical to the dark current produced in an exposure in the presence of light. In our investigation, we illuminated two different CCD chips to different intensities of light and measured the dark current generation. A surprising conclusion of this study is that some pixels produce a different amount of dark current under illumination. Finally, we discuss the implications that this has for dark frame image correction.
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