The fundamental strategy for calibration of the ACIS instrument is to make measurements which constrain free parameters of mathematical models of the system. This approach was followed in calibration of the CCD detectors themselves, and the reader must therefore understand the detector models in order to understand the detector calibration.
In this section we describe some principles of X-ray CCD detector operation and describe the parameters of the (simplified) CCD model which we have fit to the data. In section 4.2, we present an overview of the measurements made to constrain those parameters.