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Theoretical and measured spectral resolutions are shown in Figure 4.6.
Different relations are shown for a range in system noise.
The theoretical energy resolution for a single event entirely absorbed in one pixel is described by
Figure 4.7 shows minimum detectable equivalent
widths, and Figure 4.8 shows model
spectra. Energy resolution depends critically
upon the efficiency of counting only single events (see
Operating Principles [Section 18.104.22.168]),
but also upon CTE, bias correction accuracy,
and thermal stability.
Because of the many factors affecting instrument performance it will be necessary to provide individual response models for at least every CCD chip and possibly every CCD output node. In practical use these models will take the form of response matrices, compatible with commonly used X-ray analysis packages such as XSPEC and PROS. Unfortunately there is no simple analytic conversion to be able to transform empirical calibration data into response matrices. Instead we utilize a simulation based on physical parameters of the CCD devices, filters, HRMA and other relevant parts of AXAF, and compare the results of the simulation to the calibration measurements.
Once acceptable agreement is found between the simulation (after physical parameters have been adjusted to reasonable levels and the simulation reaches an acceptable level of detail), the simulation is used to generate response matrices and predictions of ACIS performance on scales or parameter values which are impossible to empirically measure. In the next section we describe this simulation tool.