Plot of the truncated mean of a time series of 10 000 consecutive images. Blue is the truncated mean of the image area, and green is the truncated mean of the overscan region. Red is the difference between the two curves. An absolute variation in bias of about  ± 3 ADU is seen over time (image index) for both the image area and the overscan region, whereas the variance of difference between them (red curve) is only about  ± 0.05 ADU.