ACIS (for Advanced CCD Imaging Spectrometer) is an instrument on Chandra X-ray Observatory. It captures a spectrum at each pixel using CCD sensors that sense the amount of (photon) energy received, and makes use of the fact that for distant targets, photon receptions are relatively rare. Its basic operation is capturing photons for a short period of time (3.2 seconds), then quickly "cloning" the data, resetting the sensors and beginning photon-sensing again while unloading the copied data corresponding to the just-collected photon receptions. Given the relative rarity of photon receptions, a pixel is very likely to have received at most one photon in the 3.2-second interval, and the results of a long observation (thousands of seconds or more) yields a four-dimensional dataset: two spatial dimensions, a photon-energy (equivalent to frequency) dimension and a time dimension.
A use for the instrument is surveys of distant X-ray sources such as active galactic nuclei or galaxy clusters. With Chandra, the instrument has been in use since 1999 and is not at the end of its life.