Readout Streak Events

The ACIS CCDs transfer the current exposure into a readout buffer. This transfer happens in parallel; all 1024 columns transfer data into the buffer at the same time. It takes 40us per row or 0.04096secs to transfer a full frame exposure. During that time, the ACIS CCDs are still exposed to the sky. Any photons hitting the detector will be read out as normal but will have incorrect chip coordinates.

For bright sources, this can lead to a line of events along the readout direction (CHIPY) through the source. The events along the readout "streak" belong to the bright source but since they were detected at incorrect CHIPY locations they have bad sky coordinates too.

The tool acisreadcorr may be useful to re-assign these readout streak events (also known as "Out of Time" events) to the source they belong to.

Typically readout streaks are discussed in the context of pileup, since any source with a clearly visible readout streak has significant pileup.