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Session 77 - AXAF Calibration.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
In order to measure the absolute x-ray flux from objects observed by AXAF, the absolute effective area of the telescope must be known. Because there are no standard candles in the x-ray sky, a prelaunch calibration must be done. The overall effective area of the combined optic and focal plane detectors requires a knowledge of the absolute flux in the incident x-ray beam that illuminates them in the X-ray Calibration Facility at Marshall Space Flight Center. In order to calibrate the beam, a standard detector is needed. We report on the approach being developed to calibrate standard detectors to 1% in effective area. Basic data on physical properties of materials, such as the Henke tables, are not adequate. This is true both because of imprecision in the data, and difficulties with designing detectors whose overall efficiency can be synthesized from a knowledge of constituent properties. Therefore, we resort to calibrating the detector against a standard source, the PTB beam lines at the BESSY synchrotron. The PTB white beam line is calibrated to 0.3%, and the monochromatic line to 1% over the lower portion of the AXAF energy range, and to a few percent over the higher energy range. Special precautions are taken to assure that the detectors are stable following calibration at BESSY. These include dealing with condensation of frozen water vapor on a germanium detector, and providing a built-in radioactive source to check stability. The AXAF effective area is sensitive to small changes in particulate and molecular contamination on its x-ray optics. Therefore, the absolute standard established on the ground must be transferred to orbit. To accomplish this, AXAF employs radioactive sources mounted forward of the optic. X-rays from these sources reflected from the optic are detected by a flight focal plane detector. The counting rates from before and after launch can be used to measure any change in the effective area, establishing the on-orbit absolute effective area.
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