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The primary in-flight calibration datasets related to the PSF are the AR Lac pointings. AR Lac was chosen for this as it has no discernable extended emission, yet it is a bright X-ray source. The early AR Lac pointings were taken at various positions across the detector, these observations are contained in ROR sequences 110586-110602. Details of these sequences are listed in App. B. There was no successful early calibration observation with this source centered in the PSPC, thus in the PSF memo CAL/ROS 92-001 [Hasinger et al.1992] many proprietary sequences belonging to the authors were used as a test of the PSF model.
Other bright Galactic sources were similarly observed across the face of the PSPC, but most of these had some extended component to their X-ray emission. For example Cyg X-2 was observed over sequences 110001-110012, but is thought to have an extended X-ray emission component due to a dust-scattering halo.