  We have discovered an anomalous behavior of CCD readout electronics that
affects their use in many astronomical applications. An offset in the
digitization of the CCD output voltage that depends on the binary encoding of
one pixel is added to pixels that are read out one, two and/or three pixels
later. One result of this effect is the introduction of a differential offset
in the background when comparing regions with and without flux from science
targets. Conventional data reduction methods do not correct for this offset. We
find this effect in 16 of 22 instruments investigated, covering a variety of
telescopes and many different front-end electronics systems. The affected
instruments include LRIS and DEIMOS on the Keck telescopes, WFC3-UVIS and STIS
on HST, MegaCam on CFHT, SNIFS on the UH88 telescope, GMOS on the Gemini
telescopes, HSC on Subaru, and FORS on VLT. The amplitude of the introduced
offset is up to 4.5 ADU per pixel, and it is not directly proportional to the
measured ADU level. We have developed a model that can be used to detect this
"binary offset effect" in data and correct for it. Understanding how data are
affected and applying a correction for the effect is essential for precise
astronomical measurements.
