The alignment procedure needs as a starting point a geometry which is as close as possible to the real geometry in the experiment. This geometry can be taken from the technical drawings, but in most cases it comes from an optical survey. The alignment procedure then improves the accuracy of the positions of the different volumes by using signals generated in the active parts of the detector. Depending on the experiment and the detector system these signals can be produced by a laser calibration system, cosmic particles or particles from collision events.
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