... We are developing and integrating software to simulate the focal plane detectors, shutters, and gratings for the Advanced X-ray Astrophysical Facility (AXAF). ... The simulations are currently being used for planning calibration activities, on-orbit performance prediction and for testing the analysis and telemetry software. ... The models, being developed as part of the AXAF Science Center, are being used to aid in calibration planning and to characterize the performance of the AXAF observatory. ...