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In integration mode, the source flux is intentionally increased to the point where multiple photons are hitting the CCD during a single integration time. The charge from the photons accumulates (integrates) in each pixel. This mode can be used to quickly determine centroid locations, but is not applicable for spectral measurements where we need to know the energy of every single photon.
The problem with this mode is that normal event detection fails. There is typically no local maximum except at the very center of the image, so all the photons on the gradient leading up to the peak would be lost. A different way of extracting data is therefore needed.
Integration mode data is extracted in two ways: event list and postage stamps. The two methods were chosen to satisfy differing requirements of the ACIS team and the ASC. Both methods have advantages and disadvantages which will be discussed below.