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R.M. Cohen (University of Idaho), S.S. Murray (SAO)
In spring 1997, the Advanced X-ray Astrophysics Facility (AXAF) telescope system and Integrated Science Instrument Module (ISIM) underwent a series of performance tests at the X-ray Calibration Facility (XRCF) of NASA's Marshall Space Flight Center. The spatial linearity data taken for the High Resolution Camera (HRC), one of two detectors on board the ISIM, was found to show small nonlinearities that can be systematically corrected during data analysis of HRC images.
The HRC is a Micro Channel Plate (MCP) based detector that uses a Crossed Grid Charge Detector (CGCD) for a readout, of the same type as used on the High Resolution Imagers on the ROSAT and Einstein observatories. At XRCF, the HRC was mounted onto a device capable of moving along various axes while accurately recording its position as a function of time. We created a model from this data and compared our model with the observed images produced by the HRC detector, allowing us to see regions on the HRC detector where nonlinearities tainted the imaging response. Often, the regions where the XRCF data deviated greatest from our model were regions that corresponded to tap boundaries, or areas on the CGCD where the incident charge from the MCP is collected. These nonlinearities result in gaps in the images produced by the HRC, and various techniques have been employed, e.g., flat-field images, to determine what correction factors to apply to the data to degap the HRC images.
Murray and Chappell (1985) showed that a constant linear correction factor applied over the entire field of the detector works quite well to degap the data. We found that this degap method can be improved upon by applying specific correction factors to particular regions of the detector, especially when the distance between neighboring events is of the same magnitude as the correction factors applied. This method produces more accurate images of the x-ray objects under study.
The author(s) of this abstract have provided an email address for comments about the abstract: cohe9592@uidaho.edu