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The ACIS CCDs are usually operated in timed exposure mode. Data are accumulated for a fixed period of time, the frame time (often 3.1 or 3.2 s) before being read out. The read-out process happens quickly, with a row-to-row transfer time of 0.04 ms. This sequence is repeated for each successive frame of data. Conversely, for continuous-clocking mode observations, data are always being read out, but with a row-to-row transfer time of 2.85 ms instead of 0.04 ms. An advantage of continuous-clocking mode is that it yields a much better time resolution. It is also possible to reduce the effects of pileup for bright sources. However, these benefits come at the cost of a loss of one spatial dimension. The location along a column of a CCD where an event occurs is unknown. The CHIPY coordinate does not represent a position, it represents a time.

Further resources:

• Continuous Clocking Mode section of the Proposers' Observatory Guide (POG)
• Why topic: Continuous Clocking Mode

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