The error is small, and usually unimportant, but could be significant for programs aimed at measuring the absolute level of the sky background. A duration of 13.6s is required both to clear the CCDs before the exposure begins, and to read each CCD after the exposure. We note that bias frames contain approximately seconds of dark current. This effect is unimportant for most observations, but could be significant if one averaged many undithered deep exposures of the same field, or if one is interested in measuring the absolute level of the sky background. However, the hotpixels vary on monthly timescales, so this simple correction is only partially successful.
Work is currently underway to improve the way bias calibration files are computed.
The timing of dark calibration frames is slightly different from that of external science exposures. Dark calibration frames currently used in the calibration pipeline are averages of ten on-orbit dark frames taken over the space of about 2 weeks. As of this writing there are plans to alter this method to both reduce the noise and provide better tracking of hotpixels.