When was the spatial gain correction fixed?
The spatial gain correction is applied to PSPC photons by the SASS subroutine DCORG. Its purpose is to correct the detected photon energy bins for variations caused by differences in the separation of the PSPC anode wires. This correction depends on the electronically-corrected Y coordinate of the detected photons. In early versions SASS had been applying the spatial-gain correction based upon the electronically-corrected X coordinate, which introduced spurious variations in the PI channel assigned to each photon. (See the discussion of the spatial gain correction in cal_ros_95_010 for more information.) The subroutine DCORG was corrected to use the electronically-corrected Y coordinate in PSPC7_4, which was implemented in SASS7_8 (first used at GSFC in Oct, 1995). Users can check the value of the PROC_SYS keyword in their RDF files; if the value of this keyword is 'SASS7_8' or later, then the correct version of DCORG has been applied. For earlier datasets, users can run the ftool pcsasscor to correct the PI values.
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