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+Microscopes based on Fresnel zone plates will often suffer from "energy drift" where the position of an observed sample feature will vary linearly with respect to the photon energy. This drift is due to systematic positioning errors that occur when the zone plate Z-axis stage moves, and has two parts:
+- **Real Drift**: caused by the ZP stage not moving parallel to X-ray beam axis.
+- **Virtual Drift**: caused by the interferometer mirror surface not being parallel to the ZP stage movement.
+
+  
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