Exposure Time Dependence of Dark Current in CCD Imagers

In this paper, we present a systematic study on the rate of dark current generation of two scientific charge-coupled device imagers. The dark current in both imagers was measured for exposure times from 5 to 7200 s at a constant temperature. As one would expect, the majority of pixels show a linear increase in dark count with exposure time. However, we found distinct groups of pixels that show a nonlinear dark current dependence versus exposure time well below saturation. Since the dark count is often assumed to scale linearly with exposure time, these pixels can pose a problem during dark current correction. We also discuss what could cause some pixels to produce a dark count that is linear versus exposure time whereas others do not.