ATLAS Muon Endcap Alignment

To align the endcap muon chambers of the ATLAS experiment, an optical grid is set up between aluminum “alignment bars” nested in each layer of chambers. Optical lines are made of laser diodes and CCD cameras that form an alignment grid. The alignment bars are self-aligning. They are then carefully measured using a large coordinate measuring machine (CMM). The subsequent shape changes of the bar are determined by calculations that are corrected by the readings of the internal monitors. The relationship between the bars is then established by a network of sensors that measure the bearing angle of light sources on the other parts of the system. The system is over-determined and the location and orientation of each bar is determined using a fitting program. Chambers are then referenced to the alignment grid using proximity sensors. This information is used to provide corrections to the nominal chamber positions before calculating track momentum. The performance of the system has been validated in a test beam at CERN. We have been able to track changes in muon sagitta's to an accuracy of about 15 microns under normal temperature variation and controlled movements. The system is stable over a period of months.