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The rate of change for the two instruments is quite different. The front-illuminated device XIS3, even with charge injection turned on, has a centroid decrease that is roughly 5 times larger than ACIS-I3, while the back-illuminated device XIS1, with charge injection, has a centroid decrease that is 9 times larger than ACIS-S3. This difference may be due to the much warmer temperatures on XIS (-90C versus --120C), or to the different radiation environment encounter in low and high-Earth orbit.
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